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Exogenous carbonaceous microstructures in Early Archaean cherts and BIFs from the Isua Greenstone Belt: Implications for the search for life in ancient rocks
Abstract
The microstructure of HF-etched samples of Early Archaean banded iron formations (BIFs) and cherts from the >3.7 b.y.-old Isua Greenstone Belt (southwestern Greenland) was investigated using high resolution scanning electron microscopy equipped with an electron diffraction system, capable of analysing light elements. The rocks contain both endogenous (of internal origin) and exogenous (of external origin) carbonaceous microstructures. The former consist of inclusions of graphite and, possibly, small, amorphous carbonaceous particles, both embedded in metacherts (however, further in situ TEM studies are needed to verify the endogeneity of the amorphous particles). Moreover, these rocks also contain endolithic microorganisms (i.e. inhabiting cracks in rocks), as well as undifferentiated carbonaceous matter, that occur in fractures and cracks between grains. The microorganisms include cyanobacteria, filamentous microorganisms such as fungal hyphae and possibly bacteria, as well as large, unidentified cells or spores. Most of the microorganisms appear to have been fossilised. The endoliths are evidently younger than the host rock, but must have infiltrated at different periods, most likely after the Inland Ice retreated (∼8000 years ago).The presence of endolithic carbonaceous matter in cracks and microfissures in these rocks will affect any analyses of bulk samples, such as carbon isotopes and chemical biomarkers, as well as analyses of acid-macerated residues. Thus, previous isotope measurements made on BIFs and cherts from Isua may reflect younger contamination rather than an endogenous (original) signal. Likewise, some of the previously described Isuan microorganisms probably represent recent, endolithic contamination.
... First, the microbodies are primary and preserved in-situ. It was made sure that the samples used for analysis were not contaminated by new microorganisms (Westall and Folk 2003). Careful observation of all samples shows that these microbodies are closely integrated with their surrounding matrix; some were embedded in holes and cracks of the host rock, while others were interbedded with their surrounding rocks. ...
... 2) A large number of spherical carbonate aggregates were found in carbonates of a cold spring, located in the northern part of the South China Sea. Magnified, the sphere surfaces exhibit worm-like and filamentous structures (similar to Fig. 4l), which are all microbial structures ). 3) Some spherical (or walnut-shaped) microbial fossils found in hydrothermal plume fluids may reflect spherical bacterial cells which were completely replaced by minerals (Westall and Folk 2003). 4) Microbial spherical structures, the size of a micron, were found well preserved in dark layers of Neogene stromatolites of the Qaidam Basin of northwestern China. ...
... According to morphological taxonomy, rhabditiform bacteria should be classified as sulphur-and metal-oxidizing bacteria, which can promote mineral crystallization and aggregation (Westall and Folk 2003). Based on size, shape, cell division, and other physical features, four types of filaments have been identified in the Black Smoker samples from the Okinawa Trough, and it has been speculated that these filaments may be some fungi and chemoautotrophic bacteria formed by sulfur or iron oxidation . ...
Many kinds of ichnofossil Zoophycos occur commonly in the carbonate rocks of Pennsylvanian to Cisuralian Taiyuan Formation in North China. In this study, carbonate microbodies types were identified in four differently-colored fillings of Zoophycos using scanning electron microscopy (SEM) and energy dispersive spectroscopy (EDS). Based on the morphologic characteristics, these carbonate microbodies can be divided into three groups, i.e., spheroids, framboids and rhabditiforms. According to the structural features of surface and individual or aggregate morphologies, the three groups can be further subdivided into thirteen types: (1) smooth spheroids; (2) spheroids with tiny thorns; (3) spheroids with a finely granulated surface; (4) spheroids with a flocculent surface; (5) spheroids with a vermiform surface; (6) framboid monomers; (7) framboid colonies; (8) linear smooth rhabditiform bodies; (9) smooth rhabditiform bodies with expanding ends; (10) biserial rhabditiform bodies; (11) spiral rhabditiform bodies; (12) thorny rhabditiform bodies; and (13) branched rhabditiform bodies. This paper not only describes the morphology, composition and occurrence of the various carbonate microbodies, but also discusses their possible microbial genesis, as follows: (1) carbonate spherical microbodies most likely were generated after globular bacterial cells had been fully displaced by minerals; (2) framboid monomers and colonies corresponding to the morphology of biogenic strawberry (or raspberry) pyrite, with their appearance and internal structure possibly inheriting the morphology of microbial cells, were indirectly generated by some microenvironmental changes due to microbial activity; (3) the morphological features, size, occurrences and preservation of filamentous and rhabditiform microbodies indicate that they may be biogenic structures, and possibly mineralized microbial fossils; and (4) some kind of symbiotic relationship exists between microbial action and the Zoophycos trace-makers. Besides, the differently-colored fillings of Zoophycos are most likely closely related to differences in the composition of microbial taxa, which in turn reflect different microenvironmental conditions.
... Scale bars: (a) 50 μm, (b) 1 μm, (c) 10 μm, (d) 10 μm, (e) 20 μm, (f) 20 μm. Credits-images used with permission from (a) Elsevier Science Publishers (Westall and Folk 2003). (c) Geological Society of London (Wacey et al. 2008). ...
... To demonstrate indigenousness of the putative fossils, one has to demonstrate that they are embedded in the prehistoric rock matrix. Contamination by modern biota within the rock can arise by percolation through cracks and microfissures or during sampling, but it can also be comprised of modern endolithic organisms inhabiting pores and fissures beneath the rock surface (e.g., recent endoliths inhabiting the cracks and fissures of the 3.7 Ga rocks at Isua, Greenland, along with carbonaceous remains washed into cracks by rainwater; Westall and Folk 2003) (Fig. 3.4a). Because of these, many authors recommend use of fresh samples from beneath the weathering front of outcrops and use of petrographic thin sections to ascertain microscopically that the putative fossils do not occur on fissures Buick 1990;Morris et al. 1999). ...
... Further, some of the carbon in the Isua rocks was found to originate from recent endolithic organisms infiltrating cracks and fissures in the rock. This indicates that studies using bulk-sampling methods may have detected carbon from these nonindigenous sources (Westall and Folk 2003). Although the evidence discussed above is inconsistent with an ancient biological origin for the Isua graphite, none of these studies have been able to firmly reject biogenicity. ...
The study of microbial fossils involves a broad array of disciplines and covers a vast diversity of topics, of which we review a select few, summarizing the state of the art. Microbes are found as body fossils preserved in different modes and have also produced recognizable structures in the rock record (microbialites, microborings). Study of the microbial fossil record and controversies arising from it have provided the impetus for the assembly and refining of powerful sets of criteria for recognition of bona fide microbial fossils. Different types of fossil evidence concur in demonstrating that microbial life was present in the Archean, close to 3.5 billion years ago. Early eukaryotes also fall within the microbial realm and criteria developed for their recognition date the oldest unequivocal evidence close to 2.0 billion years ago (Paleoproterozoic), but Archean microfossils >3 billion years old are strong contenders for earliest eukaryotes. In another dimension of their contribution to the fossil record, microbes play ubiquitous roles in fossil preservation, from facilitating authigenic mineralization to replicating soft tissue with extracellular polymeric substances, forming biofilms that inhibit decay of biological material, or stabilizing sediment interfaces. Finally, studies of the microbial fossil record are relevant to profound, perennial questions that have puzzled humanity and science—they provide the only direct window onto the beginnings and early evolution of life; and the methods and criteria developed for recognizing ancient, inconspicuous traces of life have yielded an approach directly applicable to the search for traces of life on other worlds.
... Primary signatures preserved in rocks might also be obscured by the formation of secondary minerals or the intrusion of organic matter during much later stages (e.g., Rasmussen et al., 2008;Summons et al., 2021;van Zuilen et al., 2002;Westall & Folk, 2003). For these reasons, a robust knowledge of biosignature formation and preservation (i.e., taphonomy*) is critical to studies concerned with Precambrian geobiology and astrobiology, and the geological context of target records must always be considered. ...
... Endolithic micro-organisms can inhabit a rock any time after its formation, even billions of years after deposition (Hoshino et al., 2014;McLoughlin et al., 2007;Westall & Folk, 2003). Moreover, it has been shown for other rock types (e.g., pillow basalts, seafloor volcanic glasses, and chert) that microbial trace fossils can be confused with abiotic post-depositional features such as ambient inclusion trails or metamorphic titanite microtubes (e.g., Grosch & McLoughlin, 2014;Knoll & Barghoorn, 1974;Lepot et al., 2011;McCollom & Donaldson, 2019). ...
Deep- sea hydrothermal systems provide ideal conditions for prebiotic reactions and an-cient metabolic pathways and, therefore, might have played a pivotal role in the emer-gence of life. To understand this role better, it is paramount to examine fundamental interactions between hydrothermal processes, non- living matter, and microbial life in deep time. However, the distribution and diversity of microbial communities in ancient deep- sea hydrothermal systems are still poorly constrained, so evolutionary, and ecolog-ical relationships remain unclear. One important reason is an insufficient understanding of the formation of diagnostic microbial biosignatures in such settings and their preser-vation through geological time. This contribution centers around microbial biosignatures in Precambrian deep- sea hydrothermal sulfide deposits. Intending to provide a valuable resource for scientists from across the natural sciences whose research is concerned with the origins of life, we first introduce different types of biosignatures that can be preserved over geological timescales (rock fabrics and textures, microfossils, mineral precipitates, carbonaceous matter, trace metal, and isotope geochemical signatures). We then review selected reports of biosignatures from Precambrian deep- sea hydrothermal sulfide deposits and discuss their geobiological significance. Our survey highlights that Precambrian hydrothermal sulfide deposits potentially encode valuable information on environmental conditions, the presence and nature of microbial life, and the complex in-teractions between fluids, micro- organisms, and minerals. It further emphasizes that the geobiological interpretation of these records is challenging and requires the concerted application of analytical and experimental methods from various fields, including geol-ogy, mineralogy, geochemistry, and microbiology. Well- orchestrated multidisciplinary studies allow us to understand the formation and preservation of microbial biosigna-tures in deep- sea hydrothermal sulfide systems and thus help unravel the fundamental geobiology of ancient settings. This, in turn, is critical for reconstructing life's emergence and early evolution on Earth and the search for life elsewhere in the univers
... Step-combustion experiments revealed that non-graphitic carbonaceous matter exists in Isua rocks that could explain the low 13 C org compositions recorded in some bulk-rock analyses of graphite-poor samples (van Zuilen et al., 2002), consistent with microscopic observation of recent microbes in Isua rocks (Westall and Folk, 2003). However, δ 13 C org values lower than -20‰ have been found at the microscale in graphite inclusions as well as in bulk rocks of the Saglek block displaying values as high as 0.6 wt% organic carbon (compared to contaminant levels of less than 0.01% carbon measured in van Zuilen et al., 2002). ...
... However, it remains difficult to confirm that these structures are indigenous to the studied metamorphic rocks as they have only been reported in ultrathin sections made for TEM, and evidence for mature graphitizing structures-which can be obtained by observation of dark color and high reflectance under the optical microscope, by high-resolution TEM or by Raman spectomicroscopy-are still lacking. Such a demonstration of the indigenous nature of candidate microfossils is particularly necessary in surface-exposed rocks, as demonstrated by the observation of endolithic microbial contamination in Eoarchean rocks (Westall and Folk, 2003). Schopf et al., 2007). ...
The Archean era (4 to 2.5 billion years ago, Ga) yielded rocks that include the oldest conclusive traces of life as well as many controversial occurrences. Carbonaceous matter is found in rocks as old as 3.95 Ga, but the oldest (graphitic) forms may be abiogenic. Due to the metamorphism that altered the molecular composition of all Archean organic matter, non-biological carbonaceous compounds such as those that could have formed in seafloor hydrothermal systems are difficult to rule out. Benthic microbial mats as old as 3.47 Ga are supported by the record of organic laminae in stromatolitic (layered) carbonates, in some stromatolitic siliceous sinters, and in some siliciclastic sediments. In these deposits, organic matter rarely preserved fossil cellular structures (e.g. cell walls) or ultrastructures (e.g. external sheaths) and its simple textures are difficult to attribute to either microfossils or coatings of cell-mimicking mineral templates. This distinction will require future nanoscale studies. Filamentous-sheath microfossils occur in 2.52 Ga rocks, and may have altered counterparts as old as 3.47 Ga. Surprisingly large spheres and complex organic lenses occur in rocks as old as 3.22 Ga and ~ 3.4 Ga, respectively, and represent the best candidates for the oldest microfossils. Titaniferous microtubes in volcanic or volcanoclastic rocks inferred as microbial trace fossils have been reevaluated as metamorphic or magmatic textures. Microbially-induced mineralization is supported by CaCO3 nanostructures in 2.72 Ga stromatolites. Sulfides 3.48 Ga and younger bear S-isotope ratios indicative of microbial sulfate reduction. Ferruginous conditions may have fueled primary production via anoxygenic photosynthesis–as suggested by Fe-isotope ratios–possibly as early as 3.77 Ga. Microbial methanogenesis and (likely anaerobic) methane oxidation are indicated by C-isotope ratios as early as 3.0 Ga and ~ 2.72 Ga, respectively. Photosynthetic production of O2 most likely started between 3.2 and 2.8 Ga, i.e. well before the Great Oxidation Event (2.45–2.31 Ga), as indicated by various inorganic tracers of oxidation reactions and consistent with morphology of benthic deposits and evidence for aerobic N metabolism in N-isotope ratios at ~ 2.7 Ga.
This picture of a wide diversification of the microbial biosphere during the Archean has largely been derived of bulk-rock geochemistry and petrography, supported by a recent increase in studied sample numbers and in constraints on their environments of deposition. Use of high-resolution microscopy and micro- to nanoscale analyses opens avenues to (re)assess and decipher the most ancient traces of life.
... Morphology is one of the primary criteria used to establish the biogenicity and taxonomic affinities of a potential microfossil (Brasier et al., 2005;Buick, 1990;Hofmann, 2004;Schopf & Walter, 1983;Kenichiro Sugitani et al., 2007;Westall & Folk, 2003). And yet morphology of single cells and cell aggregates can be conservative and indistinguishable among phylogenetically diverse taxa. ...
... F I G U R E 4 Additional biomorphic features. (a) Experimentally produced bowl-shaped precipitates, similar to collapsed cells, a′ (Westall & Folk, 2003); (b) multisphere precipitate similar to ornamented sphere b′ (Sugitani et al., 2015). Scale bars: a, b′: 10 μm; a′: 1 μm; b: 100 μm F I G U R E 5 Microfossil features not observed in our pseudofossils: (a) Ornamented envelope, separate from, but encasing, apparent discontinuous blastomeres (Yin & Zhu, 2012); (b) internal filaments interpreted as organic (Xiao & Knoll, 1999); (c) matryoshkas (arrows) (Chen et al., 2014); (d) acanthomorphic acritarch (Yin et al., 2007); and (e) early Cambrian lobed embryo (Steiner, Qian, Li, Hagadorn, & Zhu, 2014); scale bars: 100 μm ...
Certain phosphatic grains preserved in the rock record are interpreted as microfossils representing a diversity of microorganisms from bacteria to fossil embryos. In addition to bona fide primary biological features, phosphatic microfossils and fossil embryos commonly exhibit features that result from abiotic precipitation or diagenetic alteration. Distinguishing between abiotic and primary biological features can be difficult, and some features thought to represent biological tissue could instead be artifacts that are unrelated to the original morphology of a preserved organism. Here, we present experimentally generated, abiotically produced mineral precipitates that morphologically resemble biologically produced features, some of which may be observed in the rock record or noted in extant organisms, including embryos. These findings extend the diversity of biomorphic features known to result from abiotic precipitation.
... Some of fossillike biological structures were found scattering or embedding in the bitumen. Most of them had a smooth surface, but some were winkled, with a size range of 0.5∼5 m, a characteristic of prokaryotes [27,28]. Energy Dispersive Spectra (EDS) analysis shows that the carbon content of biological structures could be up to >30%, much higher than the carbon content in regular carbonate minerals, suggesting these fossil-like substances might be bacterial fossils rather than mineral crystals [29,30]. ...
... As prokaryotes have likely undergone little morphological evolution through Earth history, morphological comparison with modern bacteria provides the general reference guiding Geofluids 9 our ancient bacterial fossil identification. Thus, in drawing conclusions as to whether these fossil microbes from the Xiamaling Formation are from the Mesoproterozoic Era, we appeal to the criteria used during similar interrogations of possible Precambrian prokaryote record [28,38]. Based on above observations and discussion, we emphasize the observed structures are indigenously syngenetic microbes. ...
Prokaryotes, often generally referred to as “bacteria,” are the original and thus oldest life on Earth. They have shaped the chemical environment of the Earth, but they are difficult to find as ancient fossils due to their subtle structure. Here we report well-preserved fossilized microbial communities in silicified bitumen concretions from unit 3 of the Xiamaling Formation (1.39 Ga) in northern China. The numerous silicified bitumen concretions are in a variety of forms including ellipsoidal, spindle, and pancake ones, with diameters of 1~16 cm and thicknesses of 0.5~3 cm. The principal planes of the concretions are at low angle or directly parallel to the depositional plane level, showing obvious depositional characteristics. The concretions are silicified with abundant bitumen inside. Many different kinds of microbial fossils are found in the bitumen, including spherical forms, rods, and filaments, and some of the microbes are aggregated together in the forms of multicellular structures. These concretions preserve a delicate Mesoproterozoic biotic community.
... These microstructures consist of small (1-5 lm) spherical and rod-shaped cell-like structures (Fig. 11A), and branching filamentous constructions (,60 lm long) (Fig. 11C); they are interpreted to be fossil coccoid and bacillary bacteria cell colonies, or filamentous microbes, embedded within extracellular polymeric substance (EPS) remnants, and/or phenomena associated with microbes (Fig. 11). This is chiefly based on the identification criteria of Westall and Folk (2003): (1) Geological plausibility: Shallow-marine/tidal and sunlit seafloor venting hydrothermal systems like Cape Vani are ideal environments for growth of bacteria under extreme conditions (Reysenbach andCady 2001, Noffke 2010). (2) Size: Most of the microstructures fall within the size range of modern bacteria and cyanobacteria (Westall and Folk 2003). ...
... This is chiefly based on the identification criteria of Westall and Folk (2003): (1) Geological plausibility: Shallow-marine/tidal and sunlit seafloor venting hydrothermal systems like Cape Vani are ideal environments for growth of bacteria under extreme conditions (Reysenbach andCady 2001, Noffke 2010). (2) Size: Most of the microstructures fall within the size range of modern bacteria and cyanobacteria (Westall and Folk 2003). Energy dispersive X-ray spectroscopy (EDS) was performed in locations comprising the bacteriomorphous structures; they reveal clear signals for the elements C, O, Mn, Ba, K, and Si (Fig. 11B). ...
Microbial mat–related sedimentary structures are present in Lower Pleistocene mixed epiclastic-volcaniclastic sediments that host the Cape Vani manganese-oxide (-barite) deposit on NW Milos Island, Greece. Milos Island is a dormant and recently emergent 2 Ma volcano of the active Southern Aegean volcanic arc. The deposit occurs in a 1-km-long marine rift basin floored by a dacite dome. Basin fill is a .60-m-thick sequence of epiclastic glauconite-bearing sediments sandwiched between lower and upper mixed volcaniclastic sandy tuffs and epiclastic sandstones. Host siliciclastics consist of glass shards, lithic fragments, plagioclase, K-feldspar, biotite, pyroxene, and silica and clay cements, overprinted by a barite–
silica–K-feldspar–illite assemblage. Manganese (IV)–oxide minerals include dominantly d-MnO2 (vernadite), hollandite group minerals, pyrolusite, ramsdellite, and nanocrystalline todorokite. Microbially induced structures occur in a specific lithofacies referred to as upper ‘‘ferruginous and white volcaniclastic sandy tuffs/sandstones’’ and are characterized by: (1) planar and herringbone cross-bedding, (2) small-scale, vertical fining-upward sequences, (3) flaser, wavy, and lenticular bedding, (4) marine trace fossils similar to Skolithos, and (5) beveling of ripple marks and desiccated silicified mudstone beds. These features, together with the microbially induced structures and the widespread presence of glauconite, reflect a littoral to tidal-flat
paleoenvironment. The microbial mat–related sedimentary structures developed in the Mn-oxide ore formation are recognized as: (1) mat-layer structures, (2) growth bedding structures and nodules, (3) wrinkle structures and exfoliating sand laminae, (4) cracks with upturned and curled margins, (5) roll-up structures, (6) fossil gas domes, (7) mat fragments and chips, and (8) mat slump structures, suggesting photoautotrophic, possibly cyanobacterial, mats. The ubiquitous presence of barite, in the host sediments, in the mat-related structures, in feeder-vein and bedding conformable
layers, and in the gravel unit that caps the Cape Vani sedimentary rocks, suggests that microbial mats were developed in association with white smokers acting as Mn(II) suppliers, in a sunlit shallow-water or tidal-flat paleogeothermal system. The intimate relationship of Mn(IV)-oxide ore mineralization with the microbial mat–related sedimentary structures, coupled with the presence of Mn mineralized microbial fossils in the ore, strongly suggests the possible role of bacterial photosynthesis in Mn(II) bio-oxidation and Mn(IV)-oxide biomineralization at Cape Vani. It is envisaged that most Mn(IV)-
oxide mineralization was synsedimentary and syngenetic and formed due to an interplay among shallow-marine/tidal-flat sedimentation, hydrothermal seafloor to subaerial hot spring activity, which provided Mn(II), and active, possibly photosynthetic, microbial activity. Chemotrophic influence on Mn(IV)-oxide biomineralization cannot be excluded.
... Contamination of the rock by younger organisms can also produce an erroneous biosignature. Westall and Folk (2003), for instance, identified Holocene fossilized endolithic microorganisms in the Isua banded iron formation that was used for Schidlowski's (1988) isotopic studies. ...
... Study of these organic remains is derived from palynological methods in which the detrital silicates encasing the cells are eliminated by acid-digestion of the rock. A major problem with this method is that microorganisms commonly colonize fractures in rocks, thus introducing younger contamination (e.g., Westall and Folk, 2003;Walsh and Westall, 2008). Such contamination led Pflug (1979) to identify eukaryotic yeast cells in a 3.8 Ga-old rock from Isua as in situ microfossils. ...
... Despite the fact that the microbial signatures in these terrestrial sediments were particularly well preserved at the microscopic scale due to very rapid silicification, identification of the biosignatures in these rocks is not without controversy, even given the availability of sophisticated analytical capabilities. Two main questions always need to be asked: Is the potential feature a bona fide biosignature and not an abiogenic look-alike and, if it is biogenic, did it form at the same time as the rocks, that is, is it syngenetic (e.g., Westall and Folk, 2003;Rasmussen et al., 2008)? ...
... For instance, water-lain sediments may contain the biosignatures of colonies of chemotrophic organisms that inhabited the volcanic sediments and/or associated subaqueous hydrothermal environments, or they may contain detrital or dissolved organic molecules chemically bonded to phyllosilicates, or even eroded detrital fragments of rocks that contained preserved biosignatures. Once lithified, these same sediments and mineral deposits could have hosted endolithic species in cracks or intergrain spaces (Cockell et al., 2002;Westall and Folk, 2003). On Earth, endolithic microorganisms tend to be phototrophic cyanobacteria and eukaryotic fungal species, presumably with associated prokaryotic chemoorganotrophs feeding off the OM of the primary producers. ...
The search for traces of life is one of the principal objectives of Mars exploration. Central to this objective is the concept of habitability, the set of conditions that allows the appearance of life and successful establishment of microorganisms in any one location. While environmental conditions may have been conducive to the appearance of life early in martian history, habitable conditions were always heterogeneous on a spatial scale and in a geological time frame. This "punctuated" scenario of habitability would have had important consequences for the evolution of martian life, as well as for the presence and preservation of traces of life at a specific landing site. We hypothesize that, given the lack of long-term, continuous habitability, if martian life developed, it was (and may still be) chemotrophic and anaerobic. Obtaining nutrition from the same kinds of sources as early terrestrial chemotrophic life and living in the same kinds of environments, the fossilized traces of the latter serve as useful proxies for understanding the potential distribution of martian chemotrophs and their fossilized traces. Thus, comparison with analog, anaerobic, volcanic terrestrial environments (Early Archean >3.5-3.33 Ga) shows that the fossil remains of chemotrophs in such environments were common, although sparsely distributed, except in the vicinity of hydrothermal activity where nutrients were readily available. Moreover, the traces of these kinds of microorganisms can be well preserved, provided that they are rapidly mineralized and that the sediments in which they occur are rapidly cemented. We evaluate the biogenicity of these signatures by comparing them to possible abiotic features. Finally, we discuss the implications of different scenarios for life on Mars for detection by in situ exploration, ranging from its non-appearance, through preserved traces of life, to the presence of living microorganisms.
... Contamination of the rock by younger organisms can also produce an erroneous biosignature. Westall and Folk (2003), for instance, identified Holocene fossilized endolithic microorganisms in the Isua banded iron formation that was used for Schidlowski's (1988) isotopic studies. ...
... Study of these organic remains is derived from palynological methods in which the detrital silicates encasing the cells are eliminated by acid-digestion of the rock. A major problem with this method is that microorganisms commonly colonize fractures in rocks, thus introducing younger contamination (e.g., Westall and Folk, 2003;Walsh and Westall, 2008). Such contamination led Pflug (1979) to identify eukaryotic yeast cells in a 3.8 Ga-old rock from Isua as in situ microfossils. ...
The interplay between Geology and Biology has shaped the Earth from the early Precambrian, 4 billion years ago. Moving beyond the borders of the classical core disciplines, Geobiology strives to identify cause-and-effect chains and synergisms between the geo- and the biospheres that have been driving evolution of life in modern and ancient environments. Combining modern methods, geobiological information can be extracted not only from visible remains of organisms, but also from organic molecules, rock fabrics, minerals, isotopes and other tracers. Exploring these processes and their signatures also creates enormous applied potentials with respect to issues of environment protection, public health, energy and resource management. The Encyclopedia of Geobiology is designed as a key reference for students, researchers, teachers, and the informed public to provide basic, but comprehensible knowledge on this rapidly expanding discipline at the interface between modern geo- and biosciences.
... Syngenetic components are found within early phases in the host material and do not occur in late-stage alteration products or crosscutting veins, and this can be tested by microscopic examination of thin section. Thus, for example, microbial remains found in superficial cracks, late-stage veins, or metastable minerals are unlikely to be syngenetic and rather, more likely, to be derived from subrecent organisms (e.g., Westall and Folk 2003). Syngenetic components should also have experienced the same degree of deformation and alteration as the host rock. ...
... Nevertheless, organic matter displays functionalized polycyclic aromatics , as well as heteroatoms ( However, abiotic carbonaceous matter can be formed in such hydrothermally-influenced environments (Brasier et al., 2002;McCollom & Seewald, 2006) that may bear heteroatomic functional groups (Lepot, 2020), increasing the challenge of assessing biogenicity. Contaminations by younger endolithic microbes (Westall & Folk, 2003) or migrated oil must also be evaluated. For instance, a recent study proposed that anatomically shaped cavities left by the 1.9 Ga Gunflint microfossils may have been filled with oil (Rasmussen, Muhling, & Fischer, 2021). ...
The morphogenesis of most carbonaceous microstructures that resemble microfossils in Archean (4–2.5 Ga old) rocks remains debated. The associated carbonaceous matter may even—in some cases—derive from abiotic organic molecules. Mineral growths associated with organic matter migration may mimic microbial cells, some anatomical features, and known microfossils—in particular those with simple spheroid shapes. Here, spheroid microstructures from a chert of the ca. 3.4 Ga Strelley Pool Formation (SPF) of the Pilbara Craton (Western Australia) were imaged and analyzed with a combination of high‐resolution in situ techniques. This provides new insights into carbonaceous matter distributions and their relationships with the crystallographic textures of associated quartz. Thus, we describe five new types of spheroids and discuss their morphogenesis. In at least three types of microstructures, wall coalescence argues for migration of carbonaceous matter onto abiotic siliceous spherulites or diffusion in poorly crystalline silica. The nanoparticulate walls of these coalescent structures often cut across multiple quartz crystals, consistent with migration in/on silica prior to quartz recrystallization. Sub‐continuous walls lying at quartz boundaries occur in some coalescent vesicles. This weakens the “continuous carbonaceous wall” criterion proposed to support cellular inferences. In contrast, some clustered spheroids display wrinkled sub‐continuous double walls, and a large sphere shows a thick sub‐continuous wall with pustules and depressions. These features appear consistent with post‐mortem cell alteration, although abiotic morphogenesis remains difficult to rule out. We compared these siliceous and carbonaceous microstructures to coalescent pyritic spheroids from the same sample, which likely formed as “colloidal” structures in hydrothermal context. The pyrites display a smaller size and only limited carbonaceous coatings, arguing that they could not have acted as precursors to siliceous spheroids. This study revealed new textural features arguing for abiotic morphogenesis of some Archean spheroids. The absence of these features in distinct types of spheroids leaves open the microfossil hypothesis in the same rock. Distinction of such characteristics could help addressing further the origin of other candidate microfossils. This study calls for similar investigations of metamorphosed microfossiliferous rocks and of the products of in vitro growth of cell‐mimicking structures in presence of organics and silica.
... Some Archean and Paleoproterozoic sedimentary rocks are composed of high concentrations of silica and iron oxides due to depositional waters enriched in Si and Fe, like some modern acid brines. These cherts and banded iron formations host some of the first signs of life on Earth, in the form of stromatolites, microbial cellular morphologies, and carbonized matter (i.e., Konhauser and Ferris, 1996;Westall and Folk, 2003). Our study of water activities in modern acid brines may have implications for better understanding the links between geochemistry and microbial life for Precambrian Earth and perhaps on Mars. ...
Water activity is an important characteristic for describing unusual waters and is a determinant of habitability for microorganisms. However, few empirical studies of water activity have been done for natural waters exhibiting an extreme chemistry. Here, we investigate water activity for acid brines from Western Australia and Chile with pH as low as 1.4, salinities as high as 32% total dissolved solids, and complex chemical compositions. These acid brines host diverse communities of extremophilic microorganisms, including archaea, bacteria, algae, and fungi, according to metagenomic analyses. For the most extreme brine, its water activity (0.714) was considerably lower than that of saturated (pure) NaCl brine. This study provides a thermodynamic insight into life within end-member natural waters that lie at, or possibly beyond, the very edge of habitable space on Earth.
... Fossil biofilms in marine paleoenvironments are common appearances (e.g., Schieber, 1986Schieber, , 1989Walsh and Lowe, 1999;Westall and Folk, 2003;Tice andLowe, 2004, 2006;Tice, 2009;Noffke, 2000Noffke, , 2010Heubeck, 2009;Gamper et al., 2012). Occurrences in phosphorites have been documented by Banerjee (1971a,b), who described phosphatic stromatolites in the Paleoproterozoic rocks of Jhamarkotra region of Rajasthan, India. ...
The phosphatic bands of the Halkal Shale of the Neoproterozoic Bhima Basin, record an anoxic paleoenviron-ment, where only a few episodic depositional events introduced oxygen. In thin-sections, five distinct micro-facies, caused by biofilms colonizing the ancient sea floor, can be distinguished. The microfacies document that low sedimentation rate allowed growth of biomass-rich laminae sets, whereas episodic influx of sand led to stacks of organic laminae alternating with fine sandy interlayers. Such depositional events may have also caused local rupture of biofilms and the release of small biofilm roll-ups and clasts. The ancient biofilms led to precipitation of phosphorous and iron-rich minerals and likely contributed to phosphorite formation. In modern equivalent environments, sea floor-colonizing biofilms include a high abundance of sulfide oxidizing and sul-phate reducing bacteria.
... These rocks were all metamorphosed and no hydrocarbons or other biogenic organic compounds could have survived. And, while isotopically light carbon is a signature of life (Schidlowski 2001), abiotic processes may complicate interpretation (Westall and Folk 2003). In some cases, questions have arisen whether the carbon is syngenetic or transported into the rock at a younger time (Papineau et al. 2010). ...
Certain lipids and biopolymers retain their original carbon backbone structure through sedimentary diagenesis and catagenesis and can be assigned to a specific biological origin. These “taxon-specific biomarkers” (TSBs) can serve as chemical fossils that trace the evolution of life. TSBs in Precambrian rocks reveal the early evolution of archaea, cyanobacteria, and eukarya and the development of atmospheric free oxygen. However, improved criteria for assessing syngeneticity have questioned their proposed earliest occurrence in Archean rocks. Steroidal TSBs document the changing assemblages of marine phytoplankton from Neoproterozoic organic-walled acritarchs to present-day predominance of diatoms. Terpanoid TSBs reveal the evolution of higher land plants. TSBs used in conjunction with isotopic analysis can identify the taxa of enigmatic fossils, provide important clues to the causes of mass extinctions, and describe the global changes in biotic diversity and Earth’s conditions as the biosphere recovers from them. Biomarkers record the evolutionary history of life on Earth and, perhaps, other planets.
... In extreme terrestrial settings, discarding contamination means foremost proving the indigeneity of the biosignature (i.e., whether the feature has originated where it is measured/sampled, e.g., Santelli et al., 2010). For ancient life, beyond its indigeneity (Westall and Folk, 2003), the syngenicity of the biosignature-whether the feature originated at the same time as its host rock-must also be ensured to discard contamination (e.g., van Zuilen et al., 2007;Rasmussen et al., 2008;Javaux et al., 2010). ...
The search for signs of life in the ancient rock record, extreme terrestrial environments, and other planetary bodies requires a well-established, universal, and unambiguous test of biogenicity. This is notably true for cellular remnants of microbial life, since their relatively simple morphologies resemble various abiogenic microstructures that occur in nature. Although lists of qualitative biogenicity criteria have been devised, debates regarding the biogenicity of many ancient microfossils persist to this day. We propose here an alternative quantitative approach for assessing the biogenicity of putative microfossils. In this theoretical approach, different hypotheses-involving biology or not and depending on the geologic setting-are put forward to explain the observed objects. These hypotheses correspond to specific types of microstructures/systems. Using test samples, the morphology and/or chemistry of these systems are then characterized at the scale of populations. Morphologic parameters include, for example, circularity, aspect ratio, and solidity, while chemical parameters could include elementary ratios (e.g., N/C ratio), isotopic enrichments (e.g., d13C), or chirality (e.g., molar proportion of stereoisomers), among others. Statistic trends distinguishing the different systems are then searched for empirically. The trends found are translated into ''decision spaces'' where the different systems are quantitatively discriminated and where the potential microfossil population can be located as a single point. This approach, which is formulated here on a theoretical level, will solve several problems associated with the classical qualitative criteria of biogenicity. Most importantly, it could be applied to reveal the existence of cellular life on other planets, for which characteristics of morphology and chemical composition are difficult to predict.
... Microbes may infiltrate cracks and fissures in rocks of various ages (as chasmoliths or endoliths) and can become fossilised in their endo-/chasmolithic habitats. Westall and Folk (2003), for example, demonstrated that organisms previously considered syngenetic within ∼3.8 Ga rocks from the Isua supracrustal belt are in fact Holocene endolithic cyanobacteria. The case for syngenicity in carbonaceous microfossils on Earth is often strengthened by Raman spectroscopy demonstrating that the carbonaceous material and its host rock have equivalent thermal histories (e.g. ...
The icy moons of the outer Solar System harbor potentially habitable environments for life, however, compared to the terrestrial biosphere, these environments are characterized by extremes in temperature, pressure, pH, and other physico-chemical conditions. Therefore, the search for life on these icy worlds is anchored on the study of terrestrial extreme environments (termed “analogue sites”), which harbor microorganisms at the frontiers of polyextremophily. These so-called extremophiles have been found in areas previously considered sterile: hot springs, hydrothermal vents, acidic or alkaline lakes, hypersaline environments, deep sea sediments, glaciers, and arid areas, amongst others. Such model systems and communities in extreme terrestrial environments may provide important information relevant to the astrobiology of icy bodies, including the composition of potential biological communities and the identification of biosignatures that they may produce.
Extremophiles can use either sunlight (phototrophs) or chemical energy (chemotrophs) as energy sources, and different chemical compounds as electron donors or acceptors. Aerobic microorganisms use oxygen (O2) as a terminal electron acceptor, whereas anaerobic microorganisms may use nitrate (NO3−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\mathrm{NO}_{3} ^{-}$\end{document}), sulfate (SO42−\documentclass[12pt]{minimal} \usepackage{amsmath} \usepackage{wasysym} \usepackage{amsfonts} \usepackage{amssymb} \usepackage{amsbsy} \usepackage{mathrsfs} \usepackage{upgreek} \setlength{\oddsidemargin}{-69pt} \begin{document}$\mathrm{SO}_{4} ^{2-}$\end{document}), carbon dioxide (CO2), Fe(III), or other organic or inorganic molecules during respiration. The phylogenetic diversity of extremophiles is very high, leading to their broad dispersal across the phylogenetic tree of life together with a wide variety in metabolic diversity.
Some metabolisms are specific to archaea, for example, methanogenesis, an anaerobic respiration during which methane (CH4) is produced. Also sulfur-reduction performed by some bacteria and archaea is considered as a primitive metabolism which is restricted to anoxic sulfur-rich habitats in nature.
Methanogenesis and sulfur reduction are of specific interest for icy moon research as it might be one of the few known terrestrial metabolisms possible on these celestial bodies.
Therefore, the adaptation of these intriguing microorganisms to extreme conditions will be highlighted within this review.
... Microbes today happily infiltrate cracks and fissures in rocks of various ages (as chasmoliths or endoliths) and can even become fossilized in their endo-/chasmolithic habitats. For instance, Westall and Folk (2003) described silicified endolithic cyanobacteria (< 8000 years old) within rocks ~3.8 billion years from the Isua complex in Greenland. Without conducting tests for syngenicity, younger microorganisms could be mistaken for ancient fossils and, indeed, were (Pflug and Jaeschke-Boyer 1979). ...
On a volcanic and anaerobic planet characterized by abundant hydrothermal activity, physicochemical gradients and disequilibria at the local scale would have been fundamental for the emergence of life on Earth. Unfortunately, the early rock record pertaining to this existential process no longer exists, and, while chemists attempt to recreate life in a test tube, two other approaches can provide some information about early life and its metabolic processes. In the first place, phylogenetic, geological, thermodynamic, and microbiological settings suggest that disproportionation of reduced sulfurous compounds might have been essential for microbial evolution by delivering both sulfide and sulfate on Earth's surface. These processes would have been fueled by serpentinization reactions in hydrothermal systems. Another approach is to study ancient and modern microbialites in order to better understand primitive microbial metabolic traits that occurred more than 3 billion years ago. The combination of all of these approaches to understanding early terrestrial life is of relevance to the emergence of life on other planets and satellites in the solar system, especially, for example, Mars.
... Fossil biofilms in marine paleoenvironments are common appearances (e.g., Schieber, 1986Schieber, , 1989Walsh and Lowe, 1999;Westall and Folk, 2003;Tice andLowe, 2004, 2006;Tice, 2009;Noffke, 2000Noffke, , 2010Heubeck, 2009;Gamper et al., 2012). Occurrences in phosphorites have been documented by Banerjee (1971a,b), who described phosphatic stromatolites in the Paleoproterozoic rocks of Jhamarkotra region of Rajasthan, India. ...
The phosphatic bands of the Halkal Shale of the Neoproterozoic Bhima Basin, record an anoxic paleoenvironment, where only a few episodic depositional events introduced oxygen. In thin-sections, five distinct microfacies, caused by biofilms colonizing the ancient sea floor, can be distinguished. The microfacies document that low sedimentation rate allowed growth of biomass-rich laminae sets, whereas episodic influx of sand led to stacks of organic laminae alternating with fine sandy interlayers. Such depositional events may have also caused local rupture of biofilms and the release of small biofilm roll-ups and clasts. The ancient biofilms led to precipitation of phosphorous and iron-rich minerals and likely contributed to phosphorite formation. In modern equivalent environments, sea floor-colonizing biofilms include a high abundance of sulfide oxidizing and sulphate reducing bacteria.
... Microbes today happily infiltrate cracks and fissures in rocks of various ages (as chasmoliths or endoliths) and can even become fossilized in their endo-/chasmolithic habitats. For instance, Westall and Folk (2003) described silicified endolithic cyanobacteria (< 8000 years old) within rocks ~3.8 billion years from the Isua complex in Greenland. Without conducting tests for syngenicity, younger microorganisms could be mistaken for ancient fossils and, indeed, were (Pflug and Jaeschke-Boyer 1979). ...
... The isotopic data in question, which are within the range expected for biological fractionation, were obtained from analyses of bulk carbonaceous matter extracted from rock samples by demineralization with acids. Yet, in situ spatially resolved investigations using scanning electron microscopy (SEM) in rock samples from Isua, have shown the presence of organic matter produced by recent endolithic microorganisms [16]. Therefore, the antiquity of all the organic carbon cannot be demonstrated for such old and altered rocks from the bulk analyses alone. ...
Here we discuss the early geological record of preserved organic carbon and the criteria that must be applied to distinguish biological from non-biological origins. Sedimentary graphite, irrespective of its isotopic composition, does not constitute a reliable biosignature because the rocks in which it is found are generally metamorphosed to the point where convincing signs of life have been erased. Rather, multiple lines of evidence, including sedimentary textures, microfossils, large accumulations of organic matter and isotopic data for co-existing carbon, nitrogen and sulfur are required before biological origin can be convincingly demonstrated.
... Microbes today happily infiltrate cracks and fissures in rocks of various ages (as chasmoliths or endoliths) and can even become fossilized in their endo-/chasmolithic habitats. For instance, Westall and Folk (2003) described silicified endolithic cyanobacteria (< 8000 years old) within rocks ~3.8 billion years from the Isua complex in Greenland. Without conducting tests for syngenicity, younger microorganisms could be mistaken for ancient fossils and, indeed, were (Pflug and Jaeschke-Boyer 1979). ...
On a volcanic and anaerobic planet characterized by abundant hydrothermal activity, physicochemical gradients and disequilibria at the local scale would have been fundamental for the emergence of life on Earth. Unfortunately, the early rock record pertaining to this existential process no longer exists, and, while chemists attempt to recreate life in a test tube, two other approaches can provide some information about early life and its metabolic processes. In the first place, phylogenetic, geological, thermodynamic, and microbiological settings suggest that disproportionation of reduced sulfurous compounds might have been essential for microbial evolution by delivering both sulfide and sulfate on Earth’s surface. These processes would have been fueled by serpentinization reactions in hydrothermal systems. Another approach is to study ancient and modern microbialites in order to better understand primitive microbial metabolic traits that occurred more than 3 billion years ago. The combination of all of these approaches to understanding early terrestrial life is of relevance to the emergence of life on other planets and satellites in the solar system, especially, for example, Mars.
... On Earth, groundbreaking work on isotopic signatures and their preservation in ancient rocks was made by Manfred Schidlowski (1988) and ever since, the δ 13 C signature is considered as a useful accompanying biosignature. Schidlowski (1988) documented isotopically light carbon in 3.8 Ga rocks containing carbon from the Isua Greenstone Belt in Greenland, although Westall and Folk (2003) noted the presence of recent (<8000 year-old) fossilised endolithic microorganisms in the rocks analysed. More recent studies of Isua rocks using the graphite combustion method do indeed confirm the presence of isotopically light carbon with δ 13 C values between À10 and À20‰ (Ohtomo et al. 2014). ...
Demonstrating the existence of simple life forms (past or present) on a cosmic body other than Earth is exceedingly challenging: (1) A naturally sceptic scientific community expects the evidence to be convincing—for example, several independent lines of analyses performed on a feature where the results can only be explained by a biological process. (2) Most bodies are difficult to explore in situ, just about the only way to achieve the above goal, and even then, typically, several missions are required to understand where to go and what to study. (3) Planets and moons that can only be observed remotely (e.g. exoplanets) or from orbit can at best provide some indirect hints of life potential. The actual verification of life would require studying samples containing biosignatures. With the exception of some active moons where jets and plumes may provide the means for satellites to analyse surface sourced material, most other cases require landing, exploring, collecting samples, and analysing them in situ—or bringing them back to Earth.
... Sandstones represent ancient siliciclastic environments including tidal flats, lakebeds, and sabkhas, where early benthic cyanobacterial mat communities likely thrived (Altermann 2001;Westall 2005;Noffke et al. 2006; Walsh 2010). In contrast to chemical deposits like cherts where biomaterial is preserved in situ and individual microfossils are abundant, siliciclastic sediments are susceptible to local environmental influences such as exposure to air and circulation of water throughout much of the early lithification process ( Knoll and Barghoorn 1977;Walsh 1992;Brasier et al. 2002;Westall and Folk 2003;Allwood et al. 2006; Tice and Lowe 2006; Schopf et al. 2007;Van Zuilen et al. 2007;Gamper et al. 2011;Westall et al. 2011). This means that any carbonaceous or other labile materials in the sediments are easily broken down, altered, or lost. ...
Cyanobacteria are ubiquitous in a variety of modern habitats, and siliciclastic sediments in particular are home to a wide diversity of microbial communities. Benthic microbial mats, typically established by cyanobacteria on modern Earth, were likely prevalent on Archean Earth, yet explicit traces of their ancestors in Archean siliciclastic rocks are difficult to detect. To understand the taphonomy of benthic microbial mats in sandy, subaquatic environments, cyanobacterial mats were incubated for five months under a range of temperatures representative of ambient (258C) and eogenetic conditions (378C, 708C, and 1008C). Cyanobacterial materials including trichomes, sheaths, and extracellular polymeric substances (EPS) were analyzed using scanning electron microscopy-energy dispersive X-ray spectroscopy (SEM-EDS) and micro Raman spectroscopy. Textures were permineralized in all temperature regimes with phases that included mixed silicates, Na-carbonate, clays, gypsum-anhydrite, pyrrhotite, anatase, akaganeite, magnetite, natrojarosite, and ankerite. Pigments including chlorophyll, b-carotene, and scytonemin were identified in the lower temperature regimes, but were not easily detected in the samples incubated at 1008C. The morphological characteristics of trichomes and sheaths were maintained to some degree in all temperature regimes, but there was a higher relative abundance of EPS as temperatures increased. The profusion of EPS obscured the absolute differentiation between individual trichomes and sheaths at higher temperatures. The results indicate that over time, morphological, mineralogical, and carbonaceous features that formed at the end of these incubation experiments could collectively create the laminations characteristic of fossilized microbial mats found in sandstones throughout the geologic record. In Archean sandstones, where very little is preserved, these collective features may prove to be especially important in the detection of ancient life.
... Alleon et al., 2016b;Javaux et al., 2010). Microscopy is crucial to distinguish i) fossilization of biogenic/anatomical structures, ii) late endolithic contamination (Westall and Folk, 2003) and iii) mineral microstructures associated with migrated organic matter mimicking microfossils (Brasier et al., 2015;Brasier et al., 2005;Lepot et al., 2009b;Van Zuilen et al., 2002). Methods of microscopy include multiplane optical imaging (Brasier et al., 2005), scanning and transmitted electron microscopy on acid-extracted microfossils (Cloud and Hagen, 1965;Javaux et al., 2004;Moczydłowska and Willman, 2009), microscale Raman imaging (Schopf and Kudryavtsev, 2009), Transmission Electron Microscopy performed on Focused Ion Beam sections Moreau and Sharp, 2004;She et al., 2013;Wacey et al., 2012), and nanotomography using Focused Ion Beam ablation coupled to electron microscopy (Wacey et al., 2016). ...
The Paleoproterozoic (2.5–1.6 Ga) Era is a decisive time in Earth and life history. The paleobiological record (microfossils, stromatolites, biomarkers and isotopes) illustrates the biosphere evolution during a time of transitional oceanic and atmosphere chemistries. Benthic microfossil assemblages are recorded in a variety of oxygenated, sulfidic, and ferruginous environments representative of the spatial heterogeneities and temporal variations characteristic of this Era. The microfossil assemblages include iron-metabolizing and/or iron-tolerant prokaryotes, sulfur-metabolizing prokaryotes, cyanobacteria, other undetermined prokaryotes, and eukaryotes. The undetermined microfossils represent a majority of the assemblages and thus raise a challenge to determine the nature and role of microorganisms in these changing environments. Despite the early evolution of the eukaryotic cellular toolkit, early eukaryotic crown group diversification may have been restrained in the Paleoproterozoic by ocean chemistry conditions, but it increased during the late Mesoproterozoic–early Neoproterozoic despite the continuation of similar conditions through the (miscalled) “boring billion”, then amplified significantly (but perhaps within lower taxonomic levels), with the demise of euxinic conditions and increase in ecological complexity.
The emerging picture is one of a changing and more complex biosphere in which the three domains of life, Archaea, Bacteria and Eukarya, were diversifying in various ecological niches marked by the diversification of identified microfossils, stromatolites, increasing abundance of preserved biomarkers, and appearance of macroscopic problematic fossils or trace fossils.
... Brasier et al. (2002) vs. Schopf (1993) and Schopf et al. (2002)], as well as their syngenicity [e.g. Westall & Folk (2003) vs. Pflug & Jaeschke-Boyer (1979)], recent multidisciplinary investigations have revealed morphological, geochemical and isotopic traces that clearly originate from micro-organisms living in and on the Early Archaean sediments. For instance, Westall et al. (2006a) documented silicified colonies of small (<1 lm) coccoidal micro-organisms associated with 3.466 Ga-old volcanic sands and silts deposited in an intertidal setting from the Pilbara Craton in Australia (Fig. 1). ...
... Los asteriscos señalan las glaciaciones más importantes registradas. Datos procedentes de Eriksson, et al. (2004), Knoll (2003) (Westall & Folk, 2003). Sin embargo, estos problemas pueden salvarse cuando se usa la asociación de diferentes criterios, que incluyan información relevante sobre el ambiente sedimentario, morfología e información bioquímica de los fósiles (Schopf, 2004;Westal, 2005), lo que no deja de ser una llamada al estudio integral del sistema Tafosfera. ...
Life has deeply modified the Earth, making it to differ from any other planet of the Solar System. Biosphere and Lithosphere interaction has changed the surface of our planet leading to the formation of a layer different from any other of the Earth. We propose the term Taphosphere to name this layer, which emphasizes the process of burial responsible of the transference of biologically originated materials from the realm of the Biosphere to the Lithosphere. The Taphosphere aims to be a palaeontological and sedimentological concept establishing a framework for the comprehension of the taphonomical processes recorded throughout the history of the Earth. According to its definition, the Taphosphere is limited, on the one hand, by its interphase with the Biosphere, and, on the other hand, by those layers of the earth crust that have not Suffer any direct biologically-mediated alteration. The concept of Taphosphere provides a more precise and coherent framework for the study of the interaction between Life and Lithosphere than previous terms like Fossil Record or Earth Crust. In fact, Taphosphere incorporates the whole Fossil Record plus all the Geological Record formed under the biological conditions of the earth surface. Taphosphere concept also allows distinguishing between two different geneses for the Earth Crust, an abiotic one, formed by basalts of Mantle origin, and a biotic crust including materials that have been part of the Biosphere in a given moment of their history.
... Yet, the rock itself was shown to have experienced intense strain, which should have caused any spherical shape to deform into ellipsoids or more likely rod-shaped objects (Appel et al., 2003). These spheres are therefore clearly epigenetic, and indeed have been interpreted as limonitestained fluid inclusions, cavities, or post-metamorphic endolithic contamination (Westall and Folk, 2003) (Figure 7.2a). In order to rule out simple shapes such as fluid inclusions or cavities, it has been suggested that putative microfossils should be of organic character. ...
... On Earth, the identification of biosignatures in very ancient rocks is particularly controversial, not only because of the difficulty in correctly identifying the biosignature but also in terms of demonstrating its syngenicity with the formation of the rock (e.g. Westall and Folk, 2003;Rasmussen et al., 2008). On Mars, although any evidence of life will be of primary importance, whatever its age and relationship with the host rock, it will be equally important to understand the timing of the formation and emplacement of the biosignature in order to better interpret it. ...
In order to prepare for the next in situ missions to Mars (MSL-2011 and ExoMars-2018), the objective of mythesis is to create a collection of relevant analogue rocks and minerals for calibrating and testing future (and existing) space flight instruments, in accordance with the geology of Mars. They were characterized using standard laboratory instrumentation (optical microscopy, Raman, IR, XRD, SEM, electron microprobe and ICP-MS), as well as by flight instrumentation in development (Mössbauer MIMOSII, ExoMars Raman and IR (MicrOmega) spectrometers). All the samples are described in an online database in the following web site: www.isar.cnrs-orleans.fr. A part of this thesis is dedicated to the development of a cathodoluminescence (CL) instrument that could potentially be adapted for space flight. Study of alteration processes of basalts on Earth that show some similarities to surface and subsurface processes occurring on Mars may help understand and interpret martian features. Therefore, to complete the collection, samples of hydrothermal and acidic weathered basalts were collected from the Skouriotissa mine in Cyprus. The mineralogical evolution of the basalt through different alteration facies was studied. Because terrestrial basalts are poorer in Fe and Mg than martian basalts, I synthesized two artificial martian basalts. The two artificial basalts are different in terms of cooling rate (~110°C/h and drop-quenched, >1200°C/h). Interestingly, the more slowly-cooled sample exhibits a spinifex texture, similar to that of komatiites. If similar basalts occur on Mars, such rocks when altered by aqueous processes may have astrobiological implications.
... C. FERNÁNDEZ REMOLAR, J. F. GARCÍA-HIDALGO y E. MORENO-EIRIS lía ·y Sudáff'ÍClil (RASMUSSEN, 2001;TICE & LowE, 2004), lo cual sugiere que la aparición de la vida tuvo lugar al menos poco después de Ja formación y estabilización de los primeros hábitats en la Tierra (NISBET, 2000) ( Fig. 1). Por otra parte, estudios recientes sobre los restos encontrados en los depósitos de Isua (Groenlandia) como /suasphaera isua Pflug (PFLUG, 1978) sugieren que proceden de la alteración precuaternaria de los materiales arcaicos (APPEL et al., 2003;VAN ZUILEN et al., 2003;WESTALL & FOLK, 2003). Aunque estos datos desestimarían el registro más antiguo de la aparición de la vida jamás encontrado, el debate sobre la presencia de trazas de actividad biótica en los metasedimentos del cinturón supracortical de Isua mantiene las expectativas de revelar la existencia de los restos orgánicos más antiguos preservados en la Tierra (Mmz-SIS et al., 1996;UENO et al., 2002). ...
Microfossils identification, especially in very old rocks, is extremely challenging because morphological and chemical signatures of microfossils are subtle and inevitably altered with aging. Chert nodules from the Doushantuo Formation in the Yangtze Gorges area of South China have captured significant interest due to their remarkable preservation of Ediacaran microfossils. To understand the taphonomic processes leading to an exceptional preservation of microbes in these rocks, we studied the morphological and chemical features of microfossils within the lower Doushantuo Formation chert nodules down to atomic scales via advanced microscopy techniques. Our results align with previous studies, confirming that most microfossils there are preserved by silicification. Further analysis of a representative filamentous microfossil, Polytrichoides lineatus, reveals that both the cell wall (or extracellular sheath) and cytoplasm are preserved by loose aggregates of subrounded or irregular quartz grains, along with patchy organic residuals, which are distinctive from the closely packed and angular-shaped quartz grains in the non-fossil matrix of the chert nodule. The cell wall or extracellular sheath likely provided narrower spaces and more nucleation sites for silicification, resulting in smaller quartz grains (i.e., 115 ± 42 nm) compared to those formed in the cytoplasm (i.e., 1150 ± 258 nm). The permeation and precipitation of quartz grains within the cell wall or extracellular sheath could contribute to an exceptional preservation of subcellular structures. This study offers valuable insights into the preservation of microbes in the Doushantuo Formation chert nodules and even older siliceous sedimentary rocks.
The evolutionary history of early prokaryotes is recorded in Paleoproterozoic sedimentary rocks. The ca. 1.88 Ga Gunflint Formation is considered key to constrain the course of Paleoproterozoic microbial evolution. However, whether the multicellularity of prokaryote and eukaryote was already present by the Gunflint age remains uncertain. Here, we report novel morphotypes of prokaryotes including colonial, ellipsoidal, spherical, with intracellular inclusions (ICIs), spinous-type, and tail-bearing type, in the Gunflint stromatolitic chert. Biogenicity of such morphotypes was indicated based on their unique microstructures with the parallel C, N, and S distributions and lack of evidence of their post-depositional artifact origin. The new finding of colonial-type microbes in the Gunflint Formation indicates global flourishment of the colonial-type in this age. Moreover, unknown spherical cell-like structures with ICIs were identified, along with microfossils bearing strong similarities to cyanobacterial akinetes. ICIs were more enriched in N-bearing organic compounds than cell wall organic matter. Those ICIs were interpreted as biological contracted protoplasts. These new findings suggest that Paleoproterozoic prokaryotes were more diverse and complex than previously considered and had already acquired adaptability to survive drastic environmental changes. Furthermore, the protruding appendages in the novel spine- and tail-bearing type microfossils likely provided them with advantages in nutrient access and motility respectively, resulting in the promotion of the intercellular interactions. This suggests that functional evolution toward eukaryotes had already started in the Gunflint age.
The oldest carbonaceous matter in the solar system, aged at 4.5 billion years old, can be found trapped in meteorites. On Earth, the oldest carbonaceous matter of biological origin is fossilised in cherts dated at 3.5 billion years old. The EPR study of samples of this primitive carbonaceous matter provides information on the nature, the environment and the means by which carbon-based radicals were formed. This information is precious to develop scenarios for the formation of organic matter in the solar system and the emergence of life on Earth. The same methods could be used to analyse samples collected on Mars.
Earth has been habitable for 4.3 billion years, and the earliest rock record indicates the presence of a microbial biosphere by at least 3.4 billion years ago—and disputably earlier. Possible traces of life can be morphological or chemical but abiotic processes that mimic or alter them, or subsequent contamination, may challenge their interpretation. Advances in micro- and nanoscale analyses, as well as experimental approaches, are improving the characterization of these biosignatures and constraining abiotic processes, when combined with the geological context. Reassessing the evidence of early life is challenging, but essential and timely in the quest to understand the origin and evolution of life, both on Earth and beyond. Abiotic processes can mimic or alter the biogenic traces of early life but advances in micro- and nanoscale analyses provide evidence that—with geological contextualization—improves our ability to address this issue.
A recent claim to have found traces of Earth's earliest life (>3.95 Ga) utilising isotopically light carbon in graphite-bearing metapelites from the Saglek Block of northern Labrador, Canada, is re-evaluated applying rigorous geological and geochronological criteria. The establishment of these criteria in previous evaluations of early life claims from southern West Greenland and northern Canada is reviewed in order to provide a backdrop to discussion of the Saglek claim. In particular, we emphasise the importance of the scale of lithological continuity in determining the veracity of such claims, which are considerably easier to demonstrate from large, relatively less tectonised supracrustal remnants like the Isua Greenstone Belt than they are from smaller, isolated enclaves of the kind found on Akilia or the highly tectonised and imbricated unit that is found in the Saglek Block. Unambiguous field relationships between ca. 3.9 Ga tonalitic gneiss and the graphite-bearing metasediments have not been demonstrated in the literature that the Saglek claim relies upon, and earlier U-Pb-Hf isotopic studies on zircon from metasediments at one of the localities used in the claim indicate a Mesoarchean to Neoarchean time of deposition. We conclude that, irrespective of the validity of the carbon isotopic evidence, field relationships and geochronological evidence fail to demonstrate an age of >3.95 Ga for the potential traces of life.
A lens of black schist within 3.7 Ga quartzites of Greenland may be Earth's oldest known alluvial paleosol. The suspect metamorphic rock is a lens in orthoquartzite of berthierine schist with crystals of ripidolite, but it has a truncated top above dark gray grading down to gray color, ptygmatically folded surface cracks filled with silt grains, and large sand crystals, unusual for sedimentary or metamorphic rocks. The paleosol hypothesis was tested with thin sections showing plausible mineral weathering trends, and by chemical analysis showing molar weathering ratios and REE distribution like those of soils. The schist is deeply weathered and at the culmination of weathering trends from analysis of other metasediments of the Isukasia area. The protolith can be reconstructed as a saponite clay with a salt-rich horizon of kieserite, like other acid-sulfate paleosols of the early Earth. Models for proton and electron consumption of paleosols applied to the profile reveal an atmosphere with only 36 ± 510 ppm O2 and 820 ± 201 ppm CO2, and humid, cool temperate paleoclimate. The profile has organic δ¹³CPDB consistently of −24.2 to −27.4‰ and modest Raleigh distillation near the top. Similar consistent values and trends are produced by decay of organic matter in living soils, but biotic carbon isotopic composition of sediments is erratic from bed to bed, and abiotic carbon compounds of meteorites differ dramatically for each kerogen particle. Thus life in this very ancient soil is not precluded by our analyses, but ultrastructural and geochemical testing of carbon particles would further test this hypothesis.
Approaches from the sciences, philosophy and theology, including the emerging field of astrobiology, can provide fresh perspectives to the age-old question 'What is Life?'. Has the secret of life been unveiled and is it nothing more than physical chemistry? Modern philosophers will ask if we can even define life at all, as we still don't know much about its origins here on Earth. Others regard life as something that cannot simply be reduced to just physics and chemistry, while biologists emphasize the historical component intrinsic to life on Earth. How can theology constructively interpret scientific findings? Can it contribute constructively to scientific discussions? Written for a broad interdisciplinary audience, this probing volume discusses life, intelligence and more against the background of contemporary biology and the wider contexts of astrobiology and cosmology. It also considers the challenging implications for science and theology if extraterrestrial life is discovered in the future.
Analysis of the solvent insoluble residue of a cyanobacterium (Oscillatoria sp.) subjected to simulated diagenesis under hydrothermal conditions (350 and 260 °C and hydrostatic pressure of 700 atmospheres) affords a macromolecule with significant aliphatic content that is similar to that found in hydrogen rich kerogen. The aliphatic component is primarily derived from incorporation of the original fatty acids into a solvent insoluble polymer, as demonstrated by performing similar experiments with model lipid compounds. Bacterial biomass can, therefore, contribute significantly to the insoluble organic inventory in ancient sediments.
Habitability can be defined on many scales. The early Earth was globally habitable because of its global ocean, but early Mars was not. Relatively dry conditions appear to have reigned on Mars throughout its history, but, from a microbial point of view, the necessary conditions for the appearance of life were still theoretically possible. The lack of connectivity between potential habitats in time and space may have resulted in life appearing and disappearing simultaneously in different geographical locations. The absence of habitable environments on geologically long timescales of 100s My together with the likelihood that lakes and seas were covered by ice are inhibiting factors for the evolution of photosynthesis. Martian life thus probably remained in a primitive chemotrophic form. Nevertheless, established life could have colonized newly formed habitats, even on an ephemeral basis, providing that viable cells could be transported into the habitats. For in situ missions and the search for Martian life, its heterogeneous distribution implies that the search for past traces of life will be challenging, but such environments do exist.
Astrobiology is an exciting interdisciplinary field that seeks to answer one of the most important and profound questions: are we alone? In this volume, leading international experts explore the frontiers of astrobiology, investigating the latest research questions that will fascinate a wide interdisciplinary audience at all levels. What is the earliest evidence for life on Earth? Where are the most likely sites for life in the Solar System? Could life have evolved elsewhere in the Galaxy? What are the best strategies for detecting intelligent extraterrestrial life? How many habitable or Earth-like exoplanets are there? Progress in astrobiology over the past decade has been rapid and, with evidence accumulating that Mars once hosted standing bodies of liquid water, the discovery of over 500 exoplanets and new insights into how life began on Earth, the scientific search for our origins and place in the cosmos continues.
Cavities are considered as plausible and favorable habitats for life on early Earth. In such microenvironments, organisms may have found an adequate protection against the intense ultraviolet (UV) radiation that characterized the Archean ozone-free atmosphere. However, while there is clear evidence that benthic life existed in the Paleoarchean, the oldest traces of cavity-dwelling microbes (coelobionts) have been found thus far in Neoarchean rocks. Here we present the results of a detailed investigation of early-silicified cavities occurring in the oldest well-preserved siliciclastic tidal deposits, the 3.22 Ga-old Moodies Group of the Barberton Greenstone Belt (South Africa). Downward-growing microstromatolitic columns, comprised of kerogenous laminae, are commonly present in planar, bedding-parallel, now silica-filled cavities that formed in sediments of the peritidal zone. In situ d13CPDB measurements of the kerogen range from -32.3‰ to -21.3‰ and are consistent with a biogenic origin. Scanning electron microscopy (SEM) analysis of the silicified cavities show well-preserved chains of cell-sized molds that are interpreted as fossil filamentous microorganisms. The geological context, the morphology of the microstromatolites, the d13C composition of the kerogen, and the presence of microfossils all suggest that a microbial community inhabited the cavities. These results extend the geological record of coelobionts by ~500 Ma, supporting the view that cavities were among the first ecological niches to have been occupied by early microorganisms.
The earliest preserved rock record, although fragmentary, provides us with unique evidence for testing models of when and where life first appeared on Earth. It is widely agreed that life emerged on our planet prior to 3,000 million years ago, but there currently exists no consensus as to the earliest fossil evidence of life on Earth. In order for researchers to be able to work to a consistent baseline, we must first define “ what is life?” This in itself is not a simple task (see Cleland and Chyba, 2002). For example, the current NASA definition of life as: “ a system which is self-sustained by utilising external energy or nutrients owing to its internal process of component production and coupled to the medium via adaptive change that persist during the time history of the system” (Luisi, 1998) is both vague and awkward. For the purpose of this book a clearer, if more restrictive, definition is required which is tailored towards evidence that may feasibly be retrieved from the rock record. Namely that fossil life is “ a complex structure that encodes evidence of biological behaviour and processing (e.g., growth, decay, and community tiering), and who's distribution and abundance is controlled by biologically significant variables such as light levels, temperature and nutrient gradients.”
Microorganisms were the sole inhabitants of our planet for almost 3 billion years. They have survived the intense geological upheavals that have marked the history of the Earth. They profoundly shaped their environment, thus participating in a true co-evolution between the biosphere and the geosphere. Through their activity, they also created favourable conditions for the emergence of multicellular aerobic organisms (particularly with an intense production of oxygen released into the atmosphere).
Among past microorganisms, LUCA occupied a central position in the evolutionary history of life. The possible origin and the large uncertainties about the nature of LUCA are discussed: where and when did LUCA live? Was it a hyperthermophilic, thermophilic or mesophilic organism? How did its genome look like?
Scenarios and hypotheses regarding the emergence and the relationships of the three domains of life – Archaea, Bacteria and Eucarya – as well as the transition from a prokaryotic to eukaryotic cell organisation are discussed in the light of the most recent data. Possible major steps in the evolution of microorganisms are deduced from genomic investigations and from the geological record (fossils, isotopic ratios, biomarkers). Although the early steps of microbial metabolic evolution are still hotly debated, it is possible to speculate on the occurrence of the first living entities, from the primordial metabolisms to the advent of photosynthesis.
Since the earliest life forms known to date (> 3 Gyr) were preserved due to the precipitation of dissolved silica on cellular structures (silicification), we undertook an experiment to silicify several microbial species (the Archaea Methanocaldococcus jannaschii and Pyrococcus abyssi, and the Bacteria Chloroflexus aurantiacus and Geobacillus sp.), representative of anaerobic, thermophilic microorganisms that could have existed in the environmental conditions of early Earth and early Mars. This is the first time that Archaea have been used in a simulated fossilisation experiment and one of the very first fossilisations of thermophilic microorganisms.
The experimental fossilisation was monitored by electron microscopy (SEM, TEM, Cryo-SEM) for the morphological study, and by chemical analysis (GC, GC-MS, HPLC) for the study of the preservation or degradation of organic matter during silicification.
This experiment demonstrated that not all microorganisms silicify under the same conditions. M. jannaschii cells lysed rapidly, although the EPS (extracellular polymeric substances) were preserved, as opposed to P. abyssi, Geobacillus sp. and C. aurantiacus where the cells were preserved and fossilized with differing degrees of silicification between species. The microorganisms apparently used active mechanisms to protect themselves temporarily from silicification, such as EPS production or silica repulsion. These results suggest that differences between species have a strong influence on the potential for different microorganisms to be preserved by fossilisation.
This study provides valuable insight into the silicification and preservation processes of the kind of microorganisms that could have existed on the early Earth. Knowledge of these mechanisms can be helpful for the search and the identification of microfossils in both terrestrial and extraterrestrials rocks, and in the particular case of Mars.
Among the important catalytic processes for precipitating iron and silica in today's aqueous environment are those performed by iron-bacteria and siliceous algae. Not only do iron-fixing bacteria and silica-fixing algae strip these undersaturated elements out of the water, but in doing so, they leave distinct mineralogical and textural evidence of their former presence.
A study using methods similar to those used for modern bacteria, cyanobacteria, and algae was applied to Precambrian iron formations to see if there might be evidence that bacteria and algae utilized similar catalytic mechanisms in the Precambrian and left similar mineral and textural evidence. This sections and acid residues of Archean and Proterozoic iron formations from North America, Australia, and Greenland were studied for their possible microfossil and mineralogical content. The morphology of micrometer-size hematite rods and laths, hematite spherules, magnetite crystals, siderite spherules, chalcedony spherules, and gelatinous granules in these rocks indicate that ancient iron-and silica-fixing bacteria and algae could have played a major role in the precipitation of iron formation.
In view of this high abundance of silicate minerals in the near-surface crustal regime, it is perhaps not surprising that organisms . . . have found numerous ways of interacting with siliceous materials" (Heinen and Oehler, 1979). ABSTRACT A number of neogenic opaline structures, not previously reported in the literature, as well as other neogenic phases are described from four Oligocene to Pliocene biosiliceous sediment samples from Hole 699A. The possible influence of microbes on the formation or the morphology of some of them is discussed. The samples, which are early Pliocene, early to middle Miocene, and late Oligocene (two) in age, were histologically fixed aboard ship upon retrieval. Investigations of the samples used SEM (with Edax/Tracor) and XRD methods. Diagenesis has affected all four samples, but the most extensive development of neoformed structures occurs in the Miocene and uppermost Oligocene samples, where microbial filaments (0.05 to 10 µm long), microbial colonies, and siliceous microhemi-spheroids (0.2 to 0.7 µm diameter) were observed. The latter encrust filaments, diatoms, and detrital grains to varying degrees. Other neoformed structures include (1) flakes formed by coalesced microhemispheroids, some of which are guided by short, stubby filaments, which occur only in the Miocene and uppermost Oligocene samples, and (2) flakes characterized by smooth or microfissured surfaces, which grow on diatom frustules and in pore spaces and have a more widespread distribution. The XRD data indicate possible cristobalite formation in the Miocene and uppermost Oligocene samples; we believe that the neoformed opaline structures (encrusted filaments and microhemispheroids) may represent an early phase of opal-CT. The timing of neoformation of most of these features appears to have been fairly recent, continuing even at the time of sampling. There appears to be no direct correlation of this incipient, lower Miocene-uppermost Oligocene diagenetic layer and the pore-water chemistry profiles; a massive increase in shear strength in these sediments, however, may indicate some cementation. Smectite was identified by XRD as the most prominent clay mineral in these generally clay-poor sediments. Honeycombed minerals with filamentous edges, which could correspond to smectite, were observed with SEM in the pore spaces.
Secondary minerals near and within fractures in Columbia River basalts contain objects the size and shape of bacteria. These bacteriomorphs are most commonly rods or ellipses but also include cocci and diplococci forms, vibrioids and club-shaped rods, and associated pairs of objects that suggest cellular division by binary fission. Secondary minerals associated with, enclosing, and making up bacteriomorphs include iron oxyhydroxides, sulfides, and smectites containing ferrous iron. The secondary minerals are intimately intermixed with kerogen. Moreover, bacteriomorphs in the pyrite consist of kerogen. Careful consideration of mineral associations, the occurrence of organic carbon, and the spatial context of bacteriomorphs indicate that they are microfossils. The association of microfossils with minerals formed in reducing environments suggests an ancient ecosystem dominated at least in part by sulfate-reducing bacteria, similar to communities within these basalts today.
The oldest sedimentary rocks on Earth, the 3.8‐Ga Isua Iron‐Formation in southwestern Greenland, are metamorphosed past the point where organic‐walled fossils would remain. Acid residues and thin sections of these rocks reveal ferric microstructures that have filamentous, hollow rod, and spherical shapes not characteristic of crystalline minerals. Instead, they resemble ferric‐coated remains of bacteria. Modern so‐called iron bacteria were therefore studied to enhance a search image for oxide minerals precipitated by early bacteria. Iron bacteria become coated with ferrihydrite, a metastable mineral that converts to hematite, which is stable under high temperatures. If these unusual morphotypes are mineral remains of microfossils, then life must have evolved somewhat earlier than 3.8 Ga, and may have involved the interaction of sediments and molecular oxygen in water, with iron as a catalyst. Timing is constrained by the early in fall of planetary materials that would have heated the planet's surface.Because there are no earlier sedimentary rocks to study on Earth, it may be necessary to expand the search elsewhere in the solar system for clues to any biotic precursors or other types of early life. Evidence from Mars shows geophysical and geochemical differentiation at a very early stage, which makes it an important candidate for such a search if sedimentation is an important process in life's origins. Not only does Mars have iron oxide‐rich soils, but its oldest regions have river channels where surface water and sediment may have been carried, and seepage areas where groundwater may have discharged. Mars may have had an atmosphere and liquid water in the crucial time frame of 3.9–4.0 Ga. A study of morphologies of iron oxide minerals collected in the southern highlands during a Mars sample return mission may therefore help to fill in important gaps in the history of Earth's earliest biosphere.
Turbiditic and pelagic sedimentary rocks from the Isua supracrustal belt in west Greenland [more than 3700 million years ago
(Ma)] contain reduced carbon that is likely biogenic. The carbon is present as 2- to 5-micrometer graphite globules and has
an isotopic composition of δ13C that is about –19 per mil (Pee Dee belemnite standard). These data and the mode of occurrence indicate that the reduced
carbon represents biogenic detritus, which was perhaps derived from planktonic organisms.
Organic polymeric substances are a fundamental component of microbial biofilms. Microorganisms, especially bacteria, secrete extracellular polymeric substances (EPS) to form slime layers in which they reproduce. In the sedimentary environment, biofilms commonly contain the products of degraded bacteria as well as allochthonous and autochthonous mineral components. They are complex structures which serve as protection for the colonies of microorganisms living in them and also act as nutrient traps. Biofilms are almost ubiquitous wherever there is an interface and moisture (liquid/liquid, liquid/solid, liquid/gas, solid/gas). In sedimentary rocks they are commonly recognized as stromatolites. We also discuss the distinction between bacterial biofilms and prebiotic films. The EPS and cell components of the microbial biofilms contain many cation chelation sites which are implicated in the mineralization of the films. EPS, biofilms, and their related components thus have strong preservation potential in the rock record. Fossilized microbial polymeric substances (FPS) and biofilms appear to retain the same morphological characteristics as the unfossilized material and have been recognized in rock formations dating back to the Early Archaean (3.5 b.y.). We describe FPS and biofilms from hot spring, deep-sea, volcanic lake, and shallow marine/littoral environments ranging up to 3.5 b.y. in age. FPS and biofilms are more commonly observed than fossil bacteria themselves, especially in the older part of the terrestrial record. The widespread distribution of microbial biofilms and their great survival potential makes their fossilized remains a useful biomarker as a proxy for life with obvious application to the search for life in extraterrestrial materials.
Cryptoendolithic microbial communities living within Antarctic rocks are an example of survival in an extremely cold and dry environment. The extinction of these micro-organisms formerly colonizing sandstone in the Mount Fleming area (Ross Desert), was probably provoked by the hostile environment. This is considered to be a good terrestrial analogue of the first stage of the disappearance of possible life on early Mars. To date, only macroscopically observed indirect biomarkers of the past activity of cryptoendoliths in Antarctic rocks have been described. The present paper confirms, for the first time, the existence of cryptoendolith microbial fossils within these sandstone rocks. The novel in
situ application of scanning electron microscopy with backscattered electron imaging and simultaneous use of X-ray energy dispersive spectroscopy allowed the clear detection of microfossils left behind by Antarctic endoliths. Careful interpretation of the morphological features of cells, such as preserved cell walls in algae, fungi and bacteria, cytoplasm elements such as chloroplast membranes in algae and organic matter traces, mineral associations, and the spatial context of these structures all point to their identification as cryptoendolith microfossils. This type of investigation will prompt the development of research strategies aimed at locating and identifying the signs that Martian microbiota, probably only bacteria if they existed, may have been left for us to see.
SEM imaging of HF-etched, 3.3–3.5 Ga cherts from the Onverwacht Group, South Africa reveals small spherical (1 μm diameter) and rod-shaped structures (2–3.8 μm in length) which are interpreted as probable fossil coccoid and bacillar bacteria (prokaryotes), respectively, preserved by mineral replacement. Other, possibly biogenic structures include smaller rod-shaped bacteriomorphs (<2 μm in length) and bacteriomorph moulds. The identification of these structures as fossil bacteria is based on size, shape, cell division, distribution in colonies and occurrence in biolaminated sediments. The exceptionally fine conservation has preserved textures such as wrinkled outer walls on the coccoid fossils, while the bacillar fossils are turgid. Carbon isotope analyses support the presence of bacteria in these cherts with one δ13C value around −27 per mil. The cherts are characterised by fine, wavy laminae created by granular to smooth or ropy-textured films coating bedding planes, interpreted as probable bacterial biofilms, which have also been pseudomorphed by minerals. Although most of the Onverwacht Group was deposited in relatively deep water (>900 m), textures in the sediments in which these biogenic structures occur suggest that they were probably deposited in a shallow water environment which was subjected to intermittent subaerial exposure. Pervasive hydrothermal activity is evidenced by oxygen isotope studies as well as the penecontemporaneous silicification of all rock types by low temperature (⩽220°C) hydrothermal solutions.
Life on Earth, as the only example that we have, provides the only available vehicle of study for potential extraterrestrial life. Given the similarities in the early histories of the terrestrial planets, life could have arisen on Mars or Venus, as well as elsewhere in our solar system. The most suitable candidate in the search for extraterrestrial life is Mars. Since Mars has not been subjected to the destructive plate tectonic forces which have eliminated the early crustal record from the Earth, there is also the possibility that Mars may harbour important evidence relating to the origin of life in which is lacking on Earth. Nevertheless, the remains of the early terrestrial record document a planetary context for the origin of life and its early evolution which was very different to that of the modern day Earth. Submerged protocontinental platforms provided shallow water environments in a volcanically and hydrothermally dynamic environment, subject to bolide impacts. Early life was abundant in these environments. Early Mars differed in that it was not water covered. However, this may not have been relevant for the surface-bound processes leading to the origin of life and its distribution.
Recent reports have described 'yeast-like microfossils' (Isuasphaera isua Pflug) in 3,800-million year old metaquartzites from the Isua supracrustal belt of south-west Greenland. A biogenic interpretation of these objects is inconsistent with the tectonic history of the Isua region, with the petrology of the metaquartzites, and with the morphology of the microstructures themselves. The putative microfossils are indistinguishable from limonite-stained fluid inclusions: microstructures which are demonstrably inorganic and post-depositional in origin. As such, it is contended that these objects should not be regarded as evidence of early Archaean life forms.
It is unknown when life first appeared on Earth. The earliest known microfossils (approximately 3,500 Myr before present) are structurally complex, and if it is assumed that the associated organisms required a long time to develop this degree of complexity, then the existence of life much earlier than this can be argued. But the known examples of crustal rocks older than 3,500 Myr have experienced intense metamorphism, which would have obliterated any fragile microfossils contained therein. It is therefore necessary to search for geochemical evidence of past biotic activity that has been preserved within minerals that are resistant to metamorphism. Here we report ion-microprobe measurements of the carbon-isotope composition of carbonaceous inclusions within grains of apatite (basic calcium phosphate) from the oldest known sediment sequences--a approximately 3,800-Myr-old banded iron formation from the Isua supracrustal belt, West Greenland, and a similar formation from the nearby Akilia island that is possibly older than 3,850 Myr. The carbon in the carbonaceous inclusions is isotopically light, indicative of biological activity; no known abiotic process can explain the data. Unless some unknown abiotic process exists which is able both to create such isotopically light carbon and then selectively incorporate it into apatite grains, our results provide evidence for the emergence of life on Earth by at least 3,800 Myr before present.
Revised terminology is proposed that describes the ecological niches of microorganisms within hard, mineral substrates. Organisms attached to the external surfaces of the rock are termed epiliths, while those in the interior of the rock are all termed endoliths. The latter are called chasmoendoliths if they inhabit fissures in rocks, cryptoendoliths if they dwell within structural cavities, and euendoliths if they actively penetrate calcareous substrates. -Authors
Endolithic microorganisms (those living inside rocks) occur in hot and cold deserts and exist under extreme environmental conditions. These conditions are discussed on a comparative basis. Quantitative estimates of biomass are comparable in hot and cold deserts. Despite the obvious differences between the hot and cold desert environment, survival strategies show some common features. These endolithic organisms are able to ‘switch’ rapidly their metabolic activities on and off in response to changes in the environment. Conditions in hot deserts impose a more severe environmental stress on the organisms than in the cold Antarctic desert. This is reflected in the composition of the microbial flora which in hot desert rocks consist entirely of prokaryotic microorganisms, while under cold desert conditions eukaryotes predominate.
Revised terminology is proposed that describes the ecological niches of microorganisms within hard, mineral substrates. Organisms attached to the external surfaces of the rock are termed epiliths, while those in the interior of the rock are all termed endoliths. The latter are called chasmoendoliths if they inhabit fissures in rocks, cryptoendoliths if they dwell within structural cavities, and euendoliths if they actively penetrate calcareous substrates.
The Isua greenstone belt (Fig. 1) contains the oldest known, relatively well preserved, metavolcanic and metasedimentary rocks on Earth. The rocks are all deformed and many were substantially altered by metasomatism, but both the deformation and metasomatism were heterogeneous. Transitional stages can be seen from relatively well preserved primary volcanic and sedimentary structures to schists in which all primary features have been obliterated. Likewise different kinds, and different episodes, of metasomatic alteration can be seen that produced a diversity of different compositions and metamorphic mineral assemblages from similar protoliths. New geological mapping has traced out gradations between the best preserved protoliths and their diverse deformed and metasomatised equivalents. By this means, the primary nature of the schists that make up most of the Isua greenstone belt was reinterpreted, and a new map that better portrays the primary nature of the rocks has been produced. The previously mapped stratigraphy was found to be of little value in understanding the geology. Stratigraphic units were defined by different and diverse criteria, such as current composition, structure, metamorphic texture, and inferred protoliths. Much of this stratigraphy represents a misinterpretation of the primary nature of the rocks. The new work indicates that most of the Isua greenstone belt consists of fault-bounded rock packages, mainly derived from basaltic and high-Mg basaltic pillow lava and pillow lava breccia, chert–BIF, and a minor component of clastic sedimentary rocks derived from chert and basaltic volcanic rocks. A previously mapped, extensive, unit of felsic volcanic rocks was found to be derived from metasomatised basaltic pillow lava and pillow breccia intruded by numerous sheets of tonalite.
A review is presented of the currently available evidence of life in the Precambrian, with special reference to microfossils of the size range 0.1–3 μm. The particles are spotted in thin sections of the rock under high apertures of the light microscope, and have been examined in demineralized thick sections under the transmission electron microscope (TEM). They have been chemically analyzed utilizing microprobe and spectrophotometer microscopic techniques.On the basis of such studies, the interaction of microorganisms with the formation of minerals can be traced back to early Archean times, 3800 million years ago. There is no indication supporting the assumption that some kind of prebiotic evolution took place in the recorded history of the Earth. The origin of life is open to alternative explanations, including extraterrestrial phenomena.More information may be obtained from meteorites. Under high magnifications of the TEM, a portion of the carbonaceous matter in the Murchison, Orgueil and Allende meteorites appears to be structured. Particles of various morphology can be distinguished. Microprobe techniques are applied to confirm that the microstructures are organic and indigenous to the rock. Possible origins by self-assembly and morphogenesis are discussed.
In natural ecosystems, bacteria, unicellular algae, filamentous and yeast-like fungi are often organized in thin films attached to or entrenched in substrata such as surfaces of solid rocks, minerals or larger organisms. Frequently the formation of a biofilm is the most successful survival strategy. Especially within endolithic biofilms micro-organisms actively create a safe niche to avoid extreme and thus harmful environmental conditions such as electromagnetic radiation, mechanical abrasion, water and temperature stress and hazardous chemical agents. Exemplary survival strategies are presented for bacteria, ascomycetes and green algae. On substrata without organic carbon sources, biofilms are composed of chemolithotrophic or phototrophic primary producers and heterotrophic organisms (including destruents).
In an attempt to establish reliable criteria for the identification of potential fossil life in extraterrestrial materials, the fossilizable characteristics of bacteria, namely, size, shape, cell wall texture, association, and colony formation, are described, and an overview is given of the ways in which fossil bacteria are preserved (as compressions in fine-grained sediments; preservation in amber; permineralized by silica; replacement by minerals such as silica, pyrite, Fe/Mn oxides, calcite, phosphate, and siderite; or as molds in minerals). The problem of confounding minerally replaced bacteria with non biological structures having a bacterial morphology is addressed. Examples of fossilized bacteria from the Early Archaean through to the Recent are used to illustrate the various modes of preservation and the morphology of fossil bacteria.
The ability of the Gram-positive bacterium B. subtilis to bind and nucleate precipitates from silicate anions has been studied over 24 weeks in the presence of Fe and A1 at concentrations close to those levels in soils, and at slightly acid (5.5) and basic (8.0) pH. In all cases formation of silicate crystallites (quasi-crystalline precipitates) on the bacterial surfaces was observed. Bacterially-mediated minerals were more diverse in composition and morphology, less crystalline, smaller and (sometimes) more abundant than those that were abiotically formed. Fe pretreatment of the bacterial cells enhanced the binding of silicate at pH 8.0. Walls which were not pretreated with Fe, bound silicate more favourably at acid values. When heavy metals (Pb, Cd, Zn, Cr, Ni, Cu) were added to the mixture at pH 4.5, silicate retention was greatly favoured, giving greater retention of either Si or metals than was seen in abiotic controls. Experiments with only heavy metals showed a high affinity of the bacterial walls for the metals, even at low temperatures (4°C). It is postulated that a cationic bridging mechanism is involved in the binding of silicate anions by bacterial cell walls.
Bridgwater et al.1 issued a `cautionary note' concerning several reports published by Pflug and co-workers2-5 describing objects called yeast-like microfossils (Isuasphaera isua Pflug) from a metamorphosed quartzite of the 3,800-Myr-old Isua supracrustal belt of south-west Greenland; Bridgwater et al. believe that the objects described by Pflug et al.2-5 are `indistinguishable from limonite-stained fluid inclusions' and hence are non-biogenic. I show here that the objects are neither limonite-stained fluid inclusions nor microfossils, but are limonite-stained cavities from the otherwise complete dissolution by weathering of ferruginous dolomite grains in these rocks. Several supporting arguments presented by both sides are believed to be invalid, and others are ambiguous. In view of the extensive research on the earliest life forms, and then significance to evolution, to early geochemical cycles and to the origin of the atmosphere and some ore deposits, the exact nature of the Isua objects, and particularly the validity of the evidence either for or against a biological origin, are of considerable importance. A careful evaluation of the evidence from Isua is particularly pertinent, as bona fide Precambrian fossils are also found in chemically similar (but much younger) silica-rich environments.
Holocene ooids from Joulters Ooid Shoal (Bahamas) are bored in various
ways by blue-green algae that groove along the grain surface, reside
just beneath the grain surface, and tunnel extensively a few tens of
microns within the grain. The microborings, morphologically distinctive,
are documented with scanning electron micrographs of open borings and
resin casts. Gentle dissolution of ooid aragonite permits identification
of several algal genera by light microscopy and enables comparison with
the microboring casts. Pleistocene ooids from the Miami Limestone
(Florida) contain natural casts of microborings, some of which are
similar in form to Holocene examples. Significantly, these aragonite
casts are more resistant to solution than surrounding ooid aragonite.
They remain after most of the ooid is leached away and survive
replacement of the ooid by low-Mg calcite. Dissolution or precipitation
may occur along the walls of microborings, causing morphological
alteration during their preservation. This points out a difficulty in
the specific identification of endoliths on the basis of fossilized
microborings in ancient rocks composed of original aragonite grains.
*Present address: Gulf Research and Development Company, P.O. Box 36506,
Houston, Texas 77036
CELL-like inclusions detected in the cherty layers of a quartzite, which is part of the Isua series in South-west Greenland, consist of biological materials, according to analyses by Raman laser molecular microprobe. The available radiometric data place the age of the sequence at around 3,800 Myr1. Thin sections of our specimens were taken in their primary positions within the rock matrix. The material used was compact and unweathered. No maceration, etching, impregnation or other methods were applied which might have produced artefacts.
An increased ratio of 12C to 13C, an indicator of the principal carbon-fixing reaction of photosynthesis, is found in sedimentary organic matter dating back to almost four thousand million years ago-a sign of prolific microbial life not long after the Earth's formation. Partial biological control of the terrestrial carbon cycle must have been established very early and was in full operation when the oldest sediments were formed.
Fourier-transform laser Raman spectroscopy in the near infrared (1064 nm) has been used to characterize a variety of key pigments and biomolecules produced by cyanobacteria and other stresstolerant microbes in material from extreme Antarctic cold deserts analogous to martian habitats. These compounds include photosynthetic pigments and sunscreens to protect against harmful UV radiation in the light zone (chlorophyll, scytonemin, β-carotene) and photoprotective minerals, such as silica containing iron (III) oxide. Calcium oxalate mono- and dihydrate produced as a result of the biological weathering processes and stress-protective compounds, necessary to protect organisms against desiccation, freezing temperatures, and hypersalinity, such as water-replacement molecules (trehalose), are also monitored. From the results obtained using Antarctic samples, it is shown that a laser-based system can be used to characterize biomolecules in their natural state within their mineral microhabitats. Because of the similarities between the Antarctic cold desert ecosystems, which represent some of the most extreme terrestrial environmental habitats, and putative martian analogs, the laser-Raman spectrosocopic approach is proposed for the detection of former life on Mars analogs to terrestrial cyanobacteria under stress, such as stromatolites, evaporites, and endolithic communities. To this end, the spectral database that is being accumulated from laser-Raman studies of these Antarctic communities will provide a resource of potential biomarkers for future remote laser-Raman analysis on future Mars missions.
N and Ar elemental and isotopic analyses were conducted on Archean metasediments of Isukasia, West Greenland and Pilbara Craton, Western Australia, in order to investigate the N isotopic evolution during the first half of Earth’s history. The selected samples are deep-sea sediments and hydrothermal deposits having ages from 3.8 to 2.8 Ga and affected by different degrees of metamorphism. The release patterns of N and Ar obtained by high-resolution stepped combustion show the occurrence of at least two trapped components. The first is released at 600°C and it is likely contained in fluid inclusions. N is released together with primordial 36Ar and shows a δ15N value of −1.3 ± 1.0‰, close to that of modern atmospheric N2 (δ15N = 0‰). This component is well preserved in hydrothermal-vent silica deposits of North Pole, Pilbara Craton, and nitrogen may represent ammonium salt dissolved in deep-sea hydrothermal fluids. The second N component, released at temperatures higher than 1000°C, is accompanied by radiogenic 40Ar∗, and shows a δ15N value of −7.4 ± 1.0‰ in a kerogen-rich chert from North Pole, Pilbara Craton. This N is likely biogenic and negative 15N values may reflect a metabolic isotopic fractionation induced by chemosynthetic bacteria using inorganic NH4+ contained in hydrothermal fluids. This 15N-depleted biogenic component may occur in Isukasia Banded Iron Formation (δ15N ∼ −1.7‰), but further data are needed to confirm such a hypothesis. In all other samples, metamorphic-induced Rayleigh distillation has altered the pristine N isotopic signature.
Rare earth element (REE) abundances in individual apatite crystals in banded iron formations (BIFs), metacherts, metacarbonates and mafic dykes in the Isua supracrustal belt (ISB) have been determined by laser ablation inductively coupled plasma mass spectrometry. The results together with petrographic observations on the distribution of graphite have been used to track the origin of the different compositional types of apatite and to evaluate the potential, proposed in earlier studies, for use of the apatite-graphite association as a biomarker. The chondrite-normalized distribution patterns of apatite in metasedimentary BIFs and metacherts fall into three groups. Relatively flat profiles with distinct positive Eu anomaly are interpreted as characterizing sedimentary (diagenetic) apatite that carry the REE signature of the Archean ocean. Secondary apatite in Isua metasdiments with either middle REE enriched profiles or with light REE depleted profiles is interpreted to have crystallized from percolating carbonate-rich metasomatic fluids or from fluids derived from cross-cutting mafic dykes, respectively. The occurrence together of these different genetic types of apatite with distinct REE signatures within cm-scale samples shows the immobility of REE in preexisting apatite during metamorphic episodes. Apatite crystals in Isua rocks of uncontested chemical sedimentary origin (BIF and metachert samples) do not have graphite inclusions or coatings. Graphite inclusions and coatings on the other hand characterize apatite in secondary metacarbonate rocks. In these rocks graphite is produced by thermal-metamorphic reduction of carbonate ion, derived from dissociation of the metasomatic ferrous carbonate where iron serves as electron donor, oxidizing to form magnetite. In view of the non-sedimentary, metasomatic origin of Isua metacarbonates and the abiogenic source of graphite, the apatite–graphite assemblage can not be considered as a biomarker and does not provide information on early Archean life in the ISB.
In processes of biological incorporation and subsequent biochemical processing sizable isotope effects occur as a result of both thermodynamic and kinetic fractionations which take place during metabolic and biosynthetic reactions. In this chapter a review is provided of earlier work and recent studies on isotope fractionations in the biogeochemical cycles of carbon, sulfur, hydrogen, and nitrogen. Attention is given to the biochemistry of carbon isotope fractionation, carbon isotope fractionation in extant plants and microorganisms, isotope fractionation in the terrestrial carbon cycle, the effects of diagenesis and metamorphism on the isotopic composition of sedimentary carbon, the isotopic composition of sedimentary carbon through time, implications of the sedimentary carbon isotope record, the biochemistry of sulfur isotope fractionation, pathways of the biogeochemical cycle of nitrogen, and the D/H ratio in naturally occurring materials.
Endolithic microorganisms (those living inside rocks) occur in hot and cold deserts and exist under extreme environmental conditions. These conditions are discussed on a comparative basis. Quantitative estimates of biomass are comparable in hot and cold deserts. Despite the obvious differences between the hot and cold desert environment, survival strategies show some commom features. These endolithic organisms are able to 'switch' rapidly their metabolic activities on and off in response to changes in the environment. Conditions in hot deserts impose a more severe environmental stress on the organisms than in the cold. Antarctic desert. This is reflected in the composition of the microbial flora which in hot desert rocks consist entirely of prokaryotic microorganisms, while under cold desert conditions eukaryotes predominate.
Many microorganisms ("oligotrophs") grow in distilled water: Pseudomonas spp., Caulobacter spp., Hyphomicrobium spp., Arthrobacter spp., Seliberia spp., Bactoderma alba, Corynebacterium spp., Amycolata (Nocardia) autotrophica, Mycobacterium spp., yeasts, and Chlorella spp. Also, certain lower fungi can be found here. In the laboratory, these organisms thrive on contaminations of the air (CO, hydrocarbons, H2, alcohols etc.). All are euryosmotic and often grow also in higher concentrations of salts and nutrients. Natural locations with extremely low nutrient levels (snow, rain water pools, springs, free ocean water, Antarctic rocks and soils) do not contain more than 1-5 mg/l of organic carbon. Oligotrophs found here are especially adapted to constant famine: they frequently live attached to surfaces, form polymers and storage products even while starving, and often aggregate. Many of these oligotrophs alter their morphology (surface to volume ratio) with changing nutrient concentrations. Extreme oligotrophs also occur in generally nutrient-rich environments such as sewage aeration tanks or compost soil. Here they are thought to survive in nutrient-depauperate microhabitats.