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Pattern of cell division in ∼3.4 Ga-old microbes from South Africa
... The fossilized spheroidal cells in Fig. 16B-D consist of two to three subcells, which may indicate the division of "mother" cells. The reproductions of coccoidal cells have similar division patterns that have been defined in previous studies, e.g., binary (symmetrical and asymmetrical) and multiple (coupled and/or sequential) cell fission from the Kromberg Formation in South Africa (Kremer and Kaźmierczak, 2017;Kaźmierczak and Kremer, 2019); moreover, new cells formed by division of "mother cell" can also form "lima bean-shaped" cells with flat sides (Schopf and Kudryavtsev, 2009). The fossilized dividing cells of spheroidal cyanobacteria can also be found in the Mesoproterozoic Gaoyuzhuang Formation in northern China (Guo et al., 2018). ...
... In terms of host rock, most fossilized spheroidal cells have been found in chert or cherty rocks, e.g., chert within stromatolites in an evaporate bed (Calça and Fairchild, 2012;Calça et al., 2016), silicified carbonates (Butterfield, 2001), bedded chert (Cui et al., 2020), carbonaceous chert (Kremer and Kaźmierczak, 2017;Hickman-Lewis et al., 2018;Kaźmierczak and Kremer, 2019), cherty stratiform stromatolites (Joo et al., 1999;Guo et al., 2018), thick chert bands in massive dolomite, and stromatolitic chert (Igisu et al., 2009;Schopf and Kudryavtsev, 2009 (She et al., 2013;She et al., 2014), nodular chert in limestones and dolomites (Sergeev et al., 1997), black radiolarian chert with intercalations of black siliceous shales (Kremer and Kazmierczak, 2005;Kremer, 2009;Kremer, 2020), silicified microbial mats (Knoll et al., 2013;Manning-Berg and Kah, 2017). ...
... In terms of depositional environments, most fossilized spheroidal cells-bearing chert reported in previous studies have been formed in restricted marine environments, e.g., warm intertidal to supratidal carbonate facies (Butterfield, 2001), shallow-water intertidal to supratidal environments (Kazmierczak et al., 2009;Lekele Baghekema et al., 2017), peritidal environment (Sergeev et al., 1997), tidal flats with high carbonate saturation (Sharma, 2006), restricted to peritidal marine environment (coastal environments) (Joo et al., 1999;Knoll et al., 2013;Manning-Berg and Kah, 2017;Guo et al., 2018), shallow marine environment where benthic microbial mats developed (Kremer and Kaźmierczak, 2017;Hickman-Lewis et al., 2018;Kaźmierczak and Kremer, 2019), sea bottoms with a moderate depth where benthic cyanobacteria mats occur (Kremer and Kazmierczak, 2005;Kremer, 2009;Kremer, 2020), marginal environments in a marine platform (Cui et al., 2020), restricted basins on a rimmed carbonate shelf (She et al., 2013;She et al., 2014), and open shelf facies (Sergeev and Schopf, 2010). ...
The Permian Fengcheng Formation is composed of alkaline saline lake deposits in NW China and is tectonically part of the Central Asian Orogenic Belt (CAOB). Cherts are common in this formation and contain abundant fossilized microorganisms. These microfossils occur as individual spheres and clots in chert layers (laterally continuous and commonly parallel to the laminae of the host rocks) and nodules (chert grains formed by shrinkage and desiccation of chert layers). Carbonaceous cell walls have been well preserved in chert layers. Based on petrological and geochemical analyses of chert and their host rocks, the mechanisms of cyanobacterial cell silicification and their significance for hydrocarbon exploration have been discussed. A high concentration of dissolved inorganic silica in brine and high pH values are the premise for microorganism blooms and chert deposition. Hot springs may be an important silica source for chert deposition. Hot springs also create extreme conditions for algal blooms in brine with high temperatures, high salinities and toxic and anoxic conditions. Spheroidal cyanobacterial cells were rapidly silicified, and cell structures, e.g., cell walls and inner division patterns (reproduction of cells), were well preserved. Chert with fossilized spheroidal microorganisms can be considered hydrocarbon source 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.
This chapter includes a discussion of the origin of life, early evidences for life, the origin of photosynthesis, possibilities of extraterrestrial life, major events in the biosphere in the last 2 Gyr, and the origin of eukaryotes and multicellular organisms. It briefly summarizes the Cambrian Explosion of life and the evolution of Phanerozoic life forms. A final topic is mass extinctions including a review of causes and extinction mechanisms.
The ∼3.48 billion-year-old Dresser Formation, Pilbara Craton, Western Australia, is a key geological unit for the study of Earth's earliest life and the habitats it occupied. Here, we describe a new suite of spheroidal to lenticular microstructures that morphologically resemble some previously reported Archean microfossils. Correlative microscopy shows that these objects have a size distribution, wall ultrastructure, and chemistry that are incompatible with a microfossil origin and instead are interpreted as pyritized and silicified fragments of vesicular volcanic glass. Organic kerogenous material is associated with much of the altered volcanic glass; variable quantities of organic carbon line or fill the insides of some individual vesicles, while relatively large, tufted organic-rich laminae envelop multiple vesicles.
The microstructures reported herein constitute a new type of abiogenic artifact (pseudo-fossil) that must be considered when evaluating potential signs of early life on Earth or elsewhere. In the sample studied here, where hundreds of these microstructures are present, the combined evidence permits a relatively straightforward interpretation as vesicular volcanic glass. However, reworked, isolated, and silicified microstructures of this type may prove particularly problematic in early or extraterrestrial life studies since they adsorb carbon onto their surfaces and are readily pyritized, mimicking a common preservation mechanism for bona fide microfossils. In those cases, nanoscale analysis of wall ultrastructure would be required to firmly exclude a biological origin. Key Words: Microfossils—Pseudo-fossils—Volcanic vesicles—Archean life—Pilbara Craton—Dresser Formation. Astrobiology 18, 539–555.
In a modern peritidal microbial mat from Qatar, both biomediated carbonates and Mg-rich clay minerals (palygorskite) were identified. The mat, ca 5 cm thick, shows a clear lamination reflecting different microbial communities. The initial precipitates within the top millimetres of the mat are composed of Ca–Mg–Si–Al–S amorphous nanoparticles (few tens of nanometres) that replace the ultrastructure of extracellular polymeric substances. The extracellular polymeric substances are enriched in the same cations and act as a substrate for mineral nucleation. Successively crystallites of palygorskite fibres associated with carbonate nanocrystals develop, commonly surrounding bacterial bodies. Micron-sized crystals of low-Mg calcite are the most common precipitates, together with subordinate aragonite, very high-Mg calcite/dolomite and ankerite. Pyrite nanocrystals and framboids are present in the deeper layers of the mat. Calcite crystallites form conical structures, circular to triangular/hexagonal in cross-section, evolving to crystals with rhombohedral terminations; some crystallite bundles develop into dumb-bell and stellate forms. Spheroidal organo-mineral structures are also common within the mat. Nanospheres, a few tens of nanometres in diameter, occur attached to coccoid bacteria and within their cells; these are interpreted as permineralized viruses, and could be significant as nuclei for crystallite-crystal precipitation. Microspheres, 1 to 10 μm in diameter, result from intracellular permineralization within bacteria or the mineralization of the bacteria themselves. Carbonates and clay minerals are commonly aggregated to form peloids, tens of microns in size, surrounded by residual organic matter. Magnesium-silicate and carbonate precipitation are likely to have been driven by pH – saturation index – redox changes within the mat, related to micro-environmental chemical changes induced by the microbes – extracellular polymeric substances – viruses and their degradation. This article is protected by copyright. All rights reserved.
Many studies have investigated the various genetic and environmental factors regulating cyanobacterial growth. Here, we investigated the growth and metabolism of Synechocystis sp. PCC 6803, under different nitrogen sources, light intensities and CO2 concentrations. Cells grown on urea showed the highest growth rates. However, for all the conditions, the daily growth rates in batch cultures decreased steadily over time, and stationary phase was obtained with similar cell densities. Unexpectedly, metabolic and physiological analyses showed that growth rates during log phase were not primarily controlled by the availability of photoassimilates. Further physiological investigations indicated that nutrient limitation, quorum sensing, light quality and intensity (self-shading) were not the main factors responsible for the decrease in the growth rate and for the onset of the stationary phase. Moreover, cell division rates in fed-batch cultures were positively correlated with the dilution rates. Hence, not only light, CO2 and nutrients can affect growth, but also a cell-to-cell interaction. Accordingly, we propose that cell-cell interaction may be a factor responsible for the gradual decrease of growth rates in batch cultures during log phase, culminating with the onset of stationary phase.
The evolution of Photosystem II changed the history of life by oxygenating the Earth's atmosphere. However, there is currently no consensus on when and how oxygenic photosynthesis originated. Here we present a new perspective on the evolution of oxygenic photosynthesis by studying the evolution of D1 and D2, the core subunits of Photosystem II, as a function of time. A Bayesian relaxed molecular clock approach was applied to the phylogeny of Type II reaction center proteins using geochemical constraints and calibrations derived from the fossil record of Cyanobacteria and photosynthetic eukaryotes. The resulting age estimates were interpreted in the context of the structure and function of photochemical reaction centers. Firstly, we point out it is likely that the ancestral protein to D1 and D2 gave rise to a photosystem that was capable of water oxidation. Secondly, our results indicate that the gene duplication event that led to the divergence of D1 and D2 is likely to have occurred more than a billion years before the emergence of the last common ancestor of extant Cyanobacteria. Thirdly, we show that it is unlikely that Cyanobacteria obtained photosynthesis via horizontal gene transfer. Furthermore, the data strongly suggest that the origin of photosynthesis in the early Archaean was necessarily followed by a rapid diversification of reaction center proteins, which included the divergence of D1 and D2. It is therefore concluded that primordial forms of water oxidation could have originated relatively soon after the emergence of photosynthesis.
Microfossils, stromatolites, preserved lipids and biologically informative isotopic ratios provide a substantial record of bacterial diversity and biogeochemical cycles in Proterozoic (2500–541 Ma) oceans that can be interpreted, at least broadly, in terms of present-day organisms and metabolic processes. Archean (more than 2500 Ma) sedimentary rocks add at least a billion years to the recorded history of life, with sedimentological and biogeochemical evidence for life at 3500 Ma, and possibly earlier; phylogenetic and functional details, however, are limited. Geochemistry provides a major constraint on early evolution, indicating that the first bacteria were shaped by anoxic environments, with distinct patterns of major and micronutrient availability. Archean rocks appear to record the Earth's first iron age, with reduced Fe as the principal electron donor for photosynthesis, oxidized Fe the most abundant terminal electron acceptor for respiration, and Fe a key cofactor in proteins. With the permanent oxygenation of the atmosphere and surface ocean ca 2400 Ma, photic zone O 2 limited the access of photosynthetic bacteria to electron donors other than water, while expanding the inventory of oxidants available for respiration and chemoautotrophy. Thus, halfway through Earth history, the microbial underpinnings of modern marine ecosystems began to take shape.
This article is part of the themed issue ‘The new bacteriology’.
Most ancient organic microfossils delicately preserved in 3D have been found in cherts. Although entombment within silica has been shown to promote morphological preservation, the impact of early silicification on the molecular evolution of fossilized microorganisms during burial remains poorly constrained. Here, we report results of advanced fossilization experiments performed under pressure (250 bars) and temperature (250 °C) conditions typical of sub-greenschist facies metamorphism for different durations up to 100 days on microorganisms experimentally entombed (or not) within a silica gel. The experimental residues have been characterized using XRD and XANES spectroscopy. The present study demonstrates that entombment within silica limits the degradation of microorganism molecular signatures, likely through specific chemical interactions, despite the progressive conversion of silica into quartz during the experiments. Extrapolation of the present results suggests that such protection may persist during geological timescales. The present experimental study provides molecular evidence that, in addition to morphologies, cherts may support the chemical preservation of remains of ancient life. The present results thus constitute a step forward towards the reconstruction of the original chemistry of putative fossilized microorganisms.
Microbialites are organo-sedimentary rocks found in abundance throughout the geological record back to ~3.5 Ga. Interpretations of the biological and environmental conditions under which they formed rely on comparisons with modern microbialites. Therefore, a better characterization of diverse modern microbialites is crucial to improve such interpretations. Here, we studied modern microbialites from three Mexican alkaline crater lakes: Quechulac, La Preciosa and Atexcac. The geochemical analyses of water solutions showed that they were supersaturated to varying extents with several mineral phases, including aragonite, calcite, hydromagnesite, as well as hydrated Mg-silicates. Consistently, X-ray diffraction (XRD) and Fourier transform infrared spectroscopy (FTIR) analyses revealed that microbialites are composed of a diversity of mineral phases including aragonite and sometimes calcite, hydromagnesite, and more interestingly, a poorly-crystalline hydrated silicate phase. Coupling of scanning electron microscopy (SEM) with energy dispersive X-ray spectrometry microanalyses on polished sections showed that this latter phase is abundant, authigenic, magnesium-rich and sometimes associated with iron and manganese. This mineral phase is similar to kerolite, a hydrated poorly crystalline talc-like phase (Mg3Si4O10(OH)2·nH2O). Diverse microfossils were permineralized by this silicate phase. Some of them were imaged in 3D by FIB-tomography showing that their morphology was exquisitely preserved down to the few nm-scale. The structural and chemical features of these fossils were further studied using a combination of transmission electron microscopy and scanning transmission X-ray microscopy (STXM) at the carbon and magnesium K-edges and iron L2,3-edges. These results showed that organic carbon is pervasively associated with kerolite. Overall, it is suggested that the poorly-crystalline hydrated magnesium-rich silicate forms in many alkaline lakes and has a strong potential for fossilization of microbes and organic matter. Moreover, the frequent occurrence of such an authigenic silicate phase in modern lacustrine microbialites calls for a reappraisal of its potential presence in ancient rocks.
Photosystem II, the water oxidizing enzyme, altered the course of evolution by filling the atmosphere with oxygen. Here, we reconstruct the origin and evolution of water oxidation at an unprecedented level of detail by studying the phylogeny of all D1 subunits, the main protein coordinating the water oxidizing cluster (Mn4CaO5) of Photosystem II. We show that D1 exists in several forms making well-defined clades, some of which could have evolved before the origin of water oxidation and presenting many atypical characteristics. The most ancient form is found in the genome of Gloeobacter kilaueensis JS-1 and this has a C-terminus with a higher sequence identity to D2 than to any other D1. Two other groups of early evolving D1 correspond to those expressed under prolonged far-red illumination and in darkness. These atypical D1 forms are characterized by a dramatically different Mn4CaO5 binding site and a Photosystem II containing such a site may assemble an unconventional metal cluster. The first D1 forms with a full set of ligands to the Mn4CaO5 cluster are grouped with D1 proteins expressed only under low oxygen concentrations and the latest evolving form is the dominant type of D1 found in all cyanobacteria and plastids. In addition, we show that the plastid ancestor had a D1 more similar to those in early branching Synechococcus. We suggest each one of these forms of D1 originated from transitional forms at different stages towards the innovation and optimization of water oxidation before the last common ancestor of all known cyanobacteria.
© The Author(s) 2015. Published by Oxford University Press on behalf of the Society for Molecular Biology and Evolution.
In this review, I reexamine the origin and diversification of photochemical reaction centers based on the known phylogenetic relations of the core subunits, and with the aid of sequence and structural alignments. I show, for example, that the protein folds at the C-terminus of the D1 and D2 subunits of Photosystem II, which are essential for the coordination of the water-oxidizing complex, were already in place in the most ancestral Type II reaction center subunit. I then evaluate the evolution of reaction centers in the context of the rise and expansion of the different groups of bacteria based on recent large-scale phylogenetic analyses. I find that the Heliobacteriaceae family of Firmicutes appears to be the earliest branching of the known groups of phototrophic bacteria; however, the origin of photochemical reaction centers and chlorophyll synthesis cannot be placed in this group. Moreover, it becomes evident that the Acidobacteria and the Proteobacteria shared a more recent common phototrophic ancestor, and this is also likely for the Chloroflexi and the Cyanobacteria. Finally, I argue that the discrepancies among the phylogenies of the reaction center proteins, chlorophyll synthesis enzymes, and the species tree of bacteria are best explained if both types of photochemical reaction centers evolved before the diversification of the known phyla of phototrophic bacteria. The primordial phototrophic ancestor must have had both Type I and Type II reaction centers.
The past decade has seen an explosion of new technologies for assessment of biogenicity and syngeneity of carbonaceous material within sedimentary rocks. Advances have been made in techniques for analysis of in situ organic matter as well as for extracted bulk samples of soluble and insoluble (kerogen) organic fractions. The in situ techniques allow analysis of micrometer-to-sub-micrometer-scale organic residues within their host rocks and include Raman and fluorescence spectroscopy/imagery, confocal laser scanning microscopy, and forms of secondary ion/laser-based mass spectrometry, analytical transmission electron microscopy, and X-ray absorption microscopy/spectroscopy. Analyses can be made for chemical, molecular, and isotopic composition coupled with assessment of spatial relationships to surrounding minerals, veins, and fractures. The bulk analyses include improved methods for minimizing contamination and recognizing syngenetic constituents of soluble organic fractions as well as enhanced spectroscopic and pyrolytic techniques for unlocking syngenetic molecular signatures in kerogen. Together, these technologies provide vital tools for the study of some of the oldest and problematic carbonaceous residues and for advancing our understanding of the earliest stages of biological evolution on Earth and the search for evidence of life beyond Earth. We discuss each of these new technologies, emphasizing their advantages and disadvantages, applications, and likely future directions.
Microbialites form the earliest macroscopic evidence of life, and have always been important in particular aquatic ecosystems. They demonstrate the remarkable ability of microorganisms to provide the foundation for structures that can rival coral reefs in size. Microbialites are generally assumed to form by microbial trapping and binding of detrital grains, by carbonate organomineralization of microbial biofilms, or by inorganic mineralization around microbial templates. Here we present a significant discovery that modern thrombolitic microbialites in Lake Clifton, Western Australia, gain their initial structural rigidity from biofilm mineralization by the trioctahedral smectite mineral stevensite. This nucleates in and around microbial filament walls when biological processes suppress carbon and Ca activities, leaving Mg to bind with silica and form a microporous framework that replaces and infills the filament web. After microbial materials are entombed, local carbon and Ca activities rise sufficiently for aragonite microcrystals to grow within the stevensite matrix and perhaps replace it entirely, with eradication of biogenic textural features. This may explain why many ancient microbialite carbonates lack clear evidence for biogenicity. Stevensite may provide the missing link between microbial organomineralization and subsequent abiotic calcification.
Cyanobacteria contribute a significant fraction of global primary production and are therefore of great ecological significance. An individual cyanobacteria cell has four potential fates: to divide, perhaps after a dormant period, to be eaten, to undergo viral lysis, or to undergo cell death. In some studies, cyanobacteria cell death has been classified as programmed cell death, borrowing a concept more widely known in metazoan cells, and there are various biochemical parallels to support such a categorisation. However, at the same time there is a growing awareness of asymmetric division as a fundamental process in bacterial division which can result in non-equal daughter cells with differing fitness. Thanks to recent theoretical and experimental advances it is now possible to explore cyanobacteria cell death in the light of asymmetric division and to test hypotheses on the ultimate causes of cyanobacterial cell death. Assessing the degree of protein damage within individual cells during population growth is a sensible initial research target as is the application of techniques which allow the tracking of cell lineages. The existence of asymmetric division in cyanobacteria is likely given its suggested ubiquity across the bacterial domain of life. It will be technically difficult to test the interaction of asymmetric division with environmental variability, and how that leads to individual cell death via differing susceptibilities to environmental stress. However, testing such ideas could confirm asymmetric division as the ultimate cause of cell death in cyanobacteria and thereby allow a better understanding of the patterns of cell death in natural populations.
A morphologically variable assemblage of stromatolites has been discovered in thin chert layers within the Fig Tree Group of the Swaziland Supergroup, South Africa. They are commonly low-relief, nearly stratiform, laterally linked domes. Rarer forms include pseudocolumns and crinkly stratiform stromatolites. The stromatolites grew on a substrate of altered komatiitic lava and sediments deposited on the lava surface, and in most places are covered by later komatiitic flows. Abundant fine-grained tourmaline included within the stromatolite laminae suggests that stromatolites formed in an environment dominated by boron-rich hot-spring emissions and evaporitic brines.
summaryChroococcidiopsis sp. cells from hot desert rock (Timna National Park, Israel) were grown in modified BG-11 medium which was either N-free or which contained different concentrations of nitrogen, provided as nitrate. The life cycle, ultrastructure and physiological activities of the cyanobacterium were investigated over a period of 4 months. No alterations were detected in cells grown in medium containing 250 or 125 μg N ml−1. In contrast, cells grown in medium with 62 μg N ml−1 or lacking nitrogen occurred only as isolated viable forms, after 3 months and 1 month of growth, respectively. In these cells a decrease in the content of cellular chlorophyll and phycocyanin paralleled undetectable O2 evolution and depressed O2 uptake. In addition, such isolated cells were characterized by a multilayered envelope, whereas, in the cytoplasm, vesiculated thylakoids and glycogen granules were observed. N-limited and N-starved cells recovered their cellular organization and physiological activities upon N repletion. These ‘survival’ cells had a spore-like form, and were functionally comparable to akinetes.
Mass occurrence of mats comprised of benthic coccoid cyanobacteria is reported from early Silurian black radiolarian cherts exposed at Żdanow village (Bardzkie Mountains; Sudetes, southwestern Poland). The cherts contain laminated organic matter representing degraded benthic coccoid cyanobacterial mats. The remains of cyanobacteria occur as laminated agglomerations of variously preserved subglobular colonies composed of spherical cells of variable size and numbers. The morphology of remnants of cells and their mucilaginous envelopes, structure of colonies, and particularly the presence of small granular structures resembling reproductive cells known in extant coccoid cyanobacteria as baeocytes, permit to compare the Silurian microbiota with modern cyanobacteria assigned to the genera Stanieria or Chroococcidiopsis.
Multicellularity has evolved in several eukaryotic lineages leading to plants, fungi, and animals. Theoretically, in each case, this involved (1) cell-to-cell adhesion with an alignment-of-fitness among cells, (2) cell-to-cell communication, cooperation, and specialization with an export-of-fitness to a multicellular organism, and (3) in some cases, a transition from "simple" to "complex" multicellularity. When mapped onto a matrix of morphologies based on developmental and physical rules for plants, these three phases help to identify a "unicellular ⇒ colonial ⇒ filamentous (unbranched ⇒ branched) ⇒ pseudoparenchymatous ⇒ parenchymatous" morphological transformation series that is consistent with trends observed within each of the three major plant clades. In contrast, a more direct "unicellular ⇒ colonial or siphonous ⇒ parenchymatous" series is observed in fungal and animal lineages. In these contexts, we discuss the roles played by the cooptation, expansion, and subsequent diversification of ancestral genomic toolkits and patterning modules during the evolution of multicellularity. We conclude that the extent to which multicellularity is achieved using the same toolkits and modules (and thus the extent to which multicellularity is homologous among different organisms) differs among clades and even among some closely related lineages.
Epilithic field and cultured materials of Cyanosarcina Κονac., Stanieria Kom. et
Anagn. and Pseudocapsa Erceg. (Chroococcales) were studied from Hellenic marine and aerophytic
calcareous substrates. A new marine species, Cyanosarcina thalassia Anagn. et Pant , and
a new aerophytic, chasmoendolithic species, Cyanosarcina parthenonensis Anagn. are described.
The marine species Dermocarpa sublitoralis L indst. and Cyanocystis sphaerica (Srtch. et
Gardn .) Kom. et Anagn. are transferred to Stanieria. Morphological features and the life cycle of
the debatable, aerophytic, epilithic type species of Pseudocapsa, P. dubia Erceg. are elucidated,
confirming the validity of this genus.
Sulphur isotope data from early Archaean rocks suggest that microbes with metabolisms based on sulphur existed almost 3.5 billion years ago, leading to suggestions that the earliest microbial ecosystems were sulphur-based. However, morphological evidence for these sulphur-metabolizing bacteria has been elusive. Here we report the presence of microstructures from the 3.4-billion-year-old Strelley Pool Formation in Western Australia that are associated with micrometre-sized pyrite crystals. The microstructures we identify exhibit indicators of biological affinity, including hollow cell lumens, carbonaceous cell walls enriched in nitrogen, taphonomic degradation, organization into chains and clusters, and delta13C values of -33 to -460/00 Vienna PeeDee Belemnite (VPDB). We therefore identify them as microfossils of spheroidal and ellipsoidal cells and tubular sheaths demonstrating the organization of multiple cells. The associated pyrite crystals have Delta33S values between -1.65 and +1.430/00 and delta34S values ranging from -12 to +60/00 Vienna Canyon Diablo Troilite (VCDT). We interpret the pyrite crystals as the metabolic by-products of these cells, which would have employed sulphate-reduction and sulphur-disproportionation pathways. These microfossils are about 200 million years older than previously described microfossils from Palaeoarchaean siliciclastic environments.
A synthesis of new and existing data reveals that at least seven major shear zones and an unconformity separate the rocks of the southern Barberton Greenstone Belt into seven units with different geological histories. Current views that these units constitute a continuous stratigraphic sequence and their subdivision into successive formations cannot be maintained. We argue here that the status of the traditional ‘formations’ of the Onverwacht Group of the Barberton Greenstone Belt be changed to ‘complexes’, and collectively these complexes be referred to as the Onverwacht Suite. The total age range and tectonostratigraphic thickness of the suite is about 120 million years and 15 km, respectively. The precise age range of the rocks present in each complex is largely unknown. The original spatial relationships between the complexes can in most cases only be inferred.
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.
Calcification and silicification processes of cyanobacterial mats that form stromatolites in two caldera lakes of Niuafo'ou Island (Vai Lahi and Vai Si'i) were evaluated, and their importance as analogues for interpreting the early fossil record are discussed. It has been shown that the potential for morphological preservation of Niuafo'ou cyanobacteria is highly dependent on the timing and type of mineral phase involved in the fossilization process. Four main modes of mineralization of cyanobacteria organic parts have been recognized: (i) primary early postmortem calcification by aragonite nanograins that transform quickly into larger needle-like crystals and almost totally destroy the cellular structures, (ii) primary early postmortem silicification of almost intact cyanobacterial cells that leave a record of spectacularly well-preserved cellular structures, (iii) replacement by silica of primary aragonite that has already recrystallized and obliterated the cellular structures, (iv) occasional replacement of primary aragonite precipitated in the mucopolysaccharide sheaths and extracellular polymeric substances by Al-Mg-Fe silicates. These observations suggest that the extremely scarce earliest fossil record may, in part, be the result of (a) secondary replacement by silica of primary carbonate minerals (aragonite, calcite, siderite), which, due to recrystallization, had already annihilated the cellular morphology of the mineralized microbiota or (b) relatively late primary silicification of already highly degraded and no longer morphologically identifiable microbial remains.
Studies of modern cyanobacterial mats and biofilms show that they can precipitate minerals as a consequence of metabolic and degradational activities paired with ambient hydrochemical conditions. This study looked at modern microbial mats forming giant, tower‐like, groundwater‐fed, calcareous microbialites in the world's largest, highly alkaline lake; Van Gölü (Lake Van), East Turkey. Results show that microbial systems play a role not only in carbonate precipitation but also in the formation of siliceous mineral phases. Transmitted light microscopy, scanning electron microscopy and spectral observations revealed that within the extracellular polymeric substances excreted by the mats abundant minute aragonite grains precipitated first in vivo. These minute grains were quickly succeeded and/or supplemented in the dead biomass of the cyanobacterial mat by authigenic Al‐Mg‐Fe siliceous phases. SiO2 is available in large concentrations in the highly alkaline water of Lake Van. Divalent cations (Ca, Mg) are delivered to the microbialites mostly by groundwater springs. The precipitation of the fine‐grained siliceous phases is probably mediated by bacteria degrading the cyanobacterial biomass and complexing the excessive cations with their extracellular polymeric envelopes. The bacteria serve as nucleation centres for the subsequent precipitation of siliceous mineral phases. Generally, the biphasic (calcareous and siliceous) mineralization ‐ characterizing Lake Van microbialites ‐ is controlled by their interior highly dynamic hydrogeochemical situation. There the dramatically different alkaline lake water and the Ca‐Mg‐charged groundwater mix at various rates. The early diagenetic replacement of the in vivo aragonite by authigenic siliceous phases significantly increases the fossilization potential of the mat‐forming cyanobacteria. Lake Van and its giant microbialite tufa towers act as a model explaining early diagenetic mineral phases transformation observed in many modern and ancient carbonate marine deposits, particularly those influenced by diffusion of silica‐enriched and metal‐enriched pore waters from below the water‐sediment interface.
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Fossil record of earliest Earth’s life is scant and restricted to simple kerogenous filaments and spheres, which origin and taxonomic affiliation are still ambiguous. Here we report clusters of cell-like bodies found in massive and weakly laminated black cherts of the ∼3.4 Ga Kromberg Formation (Onverwacht Group, Barberton greenstone belt, South Africa), known earlier for benthic microbial mats and microfossils. Morphological traits and mineralization of the Kromberg microfossils match those known from modern and fossil cyanobacteria. Micro-Raman and SEM/EDS analyses showed that the cell-like bodies fossilized mostly due to early mineralization with Al-silicates enclosing dispersed carbonaceous (kerogenous) matter derived from their thermally altered cell remains. Although in widespread opinion the early Archean life is predominantly represented by benthic microbial mats, the random (non-laminated) distribution of the studied cyanobacteria-like microfossils in the Kromberg cherts is suggestive for probably benthic-planktonic life cycle of these microbiota. The paper also discusses how the morphological similarity of the Kromberg Fm cell-like microfossils to geologically much younger and extant coccoidal cyanobacteria may influence the debate concerning the time of origin of Earth’s oxic atmosphere.
Pleurocapsa concharum Hansgirg was isolated and maintained in culture to observe various growth stages under controlled conditions. Observations of growth in BG-11 medium showed no clear separation between the phases of binary fission to form macrocytes and multiple fissions to form baeocytes. So long as favourable conditions and sufficient nutrients remained, divisions continued to take place forming small baeocytes. In old cultures, colonies turn brownish and were covered by sheaths, later individual baeocytes grew and enlarged in situ and divided to form 2-4-8 celled colonies. Further, individual cells may also become enveloped to form Chroococcus-like stages. When old and perennating colonies were transferred to fresh medium they became blue-green in colour and formed new clusters of colonies. In most of the observations based on material collected from natural habitats and culture conditions, the organism could be identified as Myxosarcina because pseudofilamentous stages were not seen in nature and for a long time not even in culture. During seven years of observation, pseudoheterotrichous or pseudofilamentous stages were only observed twice during the revival of senescent cultures. However these stages could not be reproduced even after repetition in the same growth conditions. It is concluded that pseudofilamentous stages are extremely rare usually found in senescent cultures and may be related to attachment or nutrient availability.
The presence in the Inferniglio cave (Italy) of four Myxosarcina-like species is reported. According to the new classification system of cyanophytes (Komarek & Anagnostidis, 1986) the species are assignable to the genera Myxosarcina Printz, Pseudocapsa Ercegovic and Cyanosarcina Kovacik. Owing to the abundancy of the material, it has been possible to observe many development stages. The life cycle of the species in the Italian cave has been outlined on the basis of a previous described development model. Taxonomic comments on the species are made.
The Swaziland Supergroup in the Barberton Greenstone Belt (BGB) consists of a lower, predominantly volcanic sequence, the Onverwacht Group; a middle volcaniclastic and quartz-poor clastic succession, the Fig Tree Group; and an upper quartzose terrigenous unit, the Moodies Group. In classic sections in the Onverwacht anticline, the Onverwacht Group includes 8 to 10 km of komatiitic, basaltic, and dacitic volcanic rocks and thin, silicified sedimentary layers that have been subdivided, from base to top, into the Komati, Hooggenoeg, and Kromberg Formations, and a new unit, the Mendon Formation. The ages and stratigraphic relationships of the highly altered Sandspruit and Theespruit Formations in the anticline are not fully resolved, but the latter includes felsic volcanic components that are in part older than the Komati Formation and in part correlative with dacitic volcanic units at the top of the Hooggenoeg Formation. However, in the Steynsdorp anticline, rocks assigned to the Theespruit Formation lie stratigraphically below the Komati Formation and include the oldest dated stratigraphic units in the Swaziland Supergroup. In the central part of the belt, north of the Granville Grove fault and south of the Inyoka fault, komatiitic volcanic rocks of the Onverwacht Group are younger than those of the Komati Formation and are here assigned to a new unit, the Mendon Formation. Exposed portions of the formation appear to young northward across a series of fault-bounded outcrop belts. North of the Inyoka fault, the Onverwacht Group includes a thick succession of komatiitic and basaltic volcanic rocks and tuffs, layered ultramafic intrusions, and thin cherty units. These rocks are here grouped into a new lithostratigraphic unit, the Weltevreden Formation. Age data suggest that the Weltevreden Formation is equivalent to at least the upper part of the Mendon Formation. The overlying Fig Tree Group consists of interstratified terrigenous clastic units and dacitic to rhyodacitic volcaniclastic and volcanic rocks. South of the Inyoka fault, these strata appear to include two formation-level units: the Mapepe and Auber Villiers Formations. The Mapepe Formation includes as much as 700 m of shale, chertgrit sandstone, and chert-clast conglomerate interstratified with fine-grained felsic pyroclastic and volcaniclastic rocks. Chert, jasper, and barite make up a minor part of most sections. Deposition took place in alluvial, fan-delta, and shallow to perhaps moderately deep subaqueous environments. Dacitic tuffs have yielded single-crystal zircon ages of 3,252 ± 6, 3,243 ± 4, and 3,226 ± 4 Ma. The Auber Villiers Formation includes 1,500 to 2,000 m of dacitic tuff; coarse vol- caniclastic sandstone, conglomerate, and breccia; and terrigenous chert-clast conglomerate, chert-grit sandstone, and shale. This sequence and the locally overlying Moodies strata form the hanging-wall succession above a thrust fault, named the 24-hour Camp fault. The footwall sequence includes rocks of the Mendon and Mapepe Formations. Volcanic breccia in the lower part of the exposed section of the Auber Villiers Formation has yielded a maximum age of 3,256 ± 4 Ma. The northern facies of the Fig Tree Group, north of the Inyoka fault, includes nearly 1,500 m of strata comprising the Ulundi, Sheba, Belvue Road, and Schoongezicht Formations. The Ulundi Formation is a 20- to 50-m-thick unit of carbonaceous shale, ferruginous chert, and iron-rich sediments at the base of the Fig Tree Group. The overlying Sheba Formation includes between 500 and 1,000 m of predominantly fine- to mediumgrained lithic graywacke. The Belvue Road Formation consists mainly of shale and thin, fine-grained turbiditic sandstone. Toward the top, it includes an increasing proportion of dacitic volcaniclastic rocks. Along the northeast end of the Stolzburg syncline, sedimentary rocks of the Belvue Road Formation are succeeded by serpentinized komatiitic volcanic rocks that have been included within the Belvue Road by previous workers. Extensive shearing and brecciation of the komatiitic volcanic rocks and overlying black and banded cherts suggest that the contact between the Belvue Road Formation and this komatiitic unit is a fault. This komatiitic unit is interpreted to be the upper part of the Weltevreden Formation (Onverwacht Group), which, along with overlying units, has been thrust over rocks of the Belvue Road Formation. The ultramafic rock and chert are overlain by nearly 450 m of turbiditic, plagioclase-rich sandstone and mudstone of the Schoongezicht Formation. Juvenile dacite-clast conglomerate near the top of the Schoongezicht Formation has yielded maximum single-crystal zircon of 3,226 ± 4 Ma. The youngest rocks in the BGB are lithic, feldspathic, and quartzose sandstone, conglomerate, and siltstone of the Moodies Group. These strata reach about 3,500 m thick in the study area and include units correlative with the Clutha, Joe's Luck, and Baviaanskop Formations in the Eureka District. The wide development of conglomerate at the base of the Moodies and, in southern areas, of Moodies conglomerates resting with angular unconformity on rocks of the Onverwacht Group suggest that the base of the Moodies is an unconformity over much of the study area. Regionally, the Moodies Group and underlying Schoongezicht Formation are paraconformable but rest discordantly on older Fig Tree and Onverwacht units. This contact is thought to be a regional thrust fault that divides the northern sequences into footwall (Weltevreden, Ulundi, Sheba, and Belvue Road Formations) and hanging-wall (Weltevreden, Ulundi, and Schoongezicht Formations overlain by Moodies Group) sequences. The Moodies Group appears to include two and possibly more distinct facies. Rocks north of the Inyoka fault comprise sections commonly exceeding 2,000 m thick that include microcline and clasts of potassic plutonic rock. South of the Inyoka fault, Moodies sections are generally less than 1,000 m thick and lack microcline and granitic detritus. Within the southern facies, individual northeast-trending outcrop belts are characterized by distinctive conglomerate-clast compositions. These facies contrasts suggest derivation of Moodies sediments from several different sources and either deposition in separate parts of a large basin, with incomplete mixing of detritus from different sources, or deposition in several small basins. Although the stratigraphies of the Onverwacht, Fig Tree, and Moodies Groups north and south of the Inyoka fault are virtually identical, subtle but important petrologic differences suggest that they represent blocks that were separated until mid- to post-Moodies time. The present study emphasizes: (1) the diachronous nature of Onverwacht volcanic rocks across the study area, with a general younging trend from south to north; (2) the contrasting igneous facies between the classic formations of the Onverwacht Group in the south and the Weltevreden Formation in the north, with the Mendon Formation representing a transitional unit in the central part of the belt; (3) the stratigraphic complexity, widespread volcanic component, and predominantly shallow-water character of Fig Tree rocks in southern facies and the more uniform, almost exclusively detrital, turbiditic aspect of northern facies Fig Tree units; (4) the existence of several major Fig Tree dacitic volcanic units, including the Auber Villiers (circa 3,260 Ma), Mapepe (3,243-3,225 Ma), and Schoongezicht (circa 3,226 Ma) Formations; and (5) the distinction between northern microcline- and granite-clastbearing and southern, K-spar- and granite-clast-poor Moodies facies.
Three main types of carbonaceous chert occur in the Swaziland Supergroup, Barberton Greenstone Belt, South Africa: black-and-white banded chert, massive black chert, and laminated black chert. These cherts are composed of six main morphological types of carbonaceous matter: carbonaceous laminations, simple grains, composite grains, wisps, diffuse carbonaceous matter, and crystalline carbonaceous matter. The black bands in black-and-white banded cherts are generally composed of well-preserved fine carbonaceous laminations, representing the remains of microbial mats, interbedded with layers of simple and composite carbonaceous grains. Massive black cherts contain a large proportion of lithic grains as well as carbonaceous detritus, but lack matlike laminations. Laminated black cherts are also accumulations of detrital lithic and carbonaceous matter, but are commonly finer grained than massive black cherts and contain a high proportion of carbonaceous wisps. Comparison of the aspect ratios of carbonaceous grains among the various chert types suggests that the original sediments were silicified at different times relative to compaction. Black-and-white banded chert and to a lesser extent massive black chert contain round or lobate carbonaceous grains and were silicified before sediment compaction. Laminated cherts are dominated by wispy grains, indicating that compaction largely preceded silicification. The relationship between grain shape and total organic carbon (TOC) indicates that TOC in carbonaceous cherts is a function of both primary carbon content and the amount of prelithification sediment compaction. In general, laminated cherts show the greatest presilicification sediment compaction and the highest TOC contents. Carbon isotope values indicate that all of the carbonaceous matter probably had a biological origin. Most cherts in the Hooggenoeg, Kromberg, and lower cycles of the Mendon Formations contain carbonaceous matter deposited in shallow water as both loose detritus and microbial mats. During periods of explosive volcanism, volcaniclastic debris was locally mixed with accumulating carbonaceous matter. These shallowwater sediments were generally lithified soon after deposition, probably through the interaction of the uppermost sea-floor sediment layers and sea water. During deposition of basaltic sequences, detrital carbonaceous matter accumulated in deeper water along with fine volcanic ash. Cherts in the upper part of the Mendon Formation represent deep-water sediments that rarely contain mat accumulations. Carbonaceous matter was preserved mainly as fine detritus mixed with lithic grains deposited under low-energy conditions. Lithification and silicification occurred after sediment compaction, probably well below the sea-floor. The abundance of in situ bacterial mats and composite grains in shallow-water deposits and their paucity in deep-water carbonaceous cherts is consistent with the interpretation that some early Archean organisms were photosynthetic, with much primary production occurring in the photic zone.
BCL3 and Sheehy cite Bergey's manual of determinative bacteriology of which systematic bacteriology, first edition, is an expansion. With v.4 the set is complete. The volumes cover, roughly, v.1, the Gram-negatives except those in v.3 ($87.95); v.2, the Gram-positives less actinomycetes ($71.95); v.
The Strelley Pool Formation (SPF) is widely distributed in the East Pilbara Terrane (EPT) of the Pilbara Craton, Western Australia, and represents a Paleoarchean shallow-water to subaerial environment. It was deposited ~3.4 billion years ago and displays well-documented carbonate stromatolites. Diverse putative microfossils (SPF microfossils) were recently reported from several localities in the East Strelley, Panorama, Warralong, and Goldsworthy greenstone belts. Thus, the SPF provides unparalleled opportunities to gain insights into a shallow-water to subaerial ecosystem on the early Earth. Our new micro- to nanoscale ultrastructural and microchemical studies of the SPF microfossils show that large (20-70 μm) lenticular organic-walled flanged microfossils retain their structural integrity, morphology, and chain-like arrangements after acid (HF-HCl) extraction (palynology). Scanning and transmitted electron microscopy of extracted microfossils revealed that the central lenticular body is either alveolar or hollow, and the wall is continuous with the surrounding smooth to reticulated discoidal flange. These features demonstrate the evolution of large micro-organisms able to form an acid-resistant recalcitrant envelope or cell wall with complex morphology and to form colonial chains in the Paleoarchean era. This study provides evidence of the evolution of very early and remarkable biological innovations, well before the presumed late emergence of complex cells.
© 2015 John Wiley & Sons Ltd.
Modern microbial mats and microbialites are described from basaltic sea caves on the island of Kauai, HI. The mats grow on the ceilings and walls in the photic zone of several open caves where fresh water seeps out of the rock. Scanning (SEM) and transmission electron microscopy (TEM) showed that the active mats are dominated by filamentous and nonfilamentous cyanobacteria in the surface layers and heterotrophic bacteria in deeper layers. Energy dispersive X-ray analysis revealed that copious amounts of extracellular polymeric substances (EPS) are rich in Mg, Si, O, and Ca, likely concentrated from solution. Petrographic microscopy and electron microprobe analysis of the mineralized microbialites showed textures reminiscent of stromatolitic laminations, consisting mainly of alternating calcium carbonate (calcite and aragonite) and magnesium-rich silicate (kerolite). Thin coatings rich in magnesite, hydromagnesite and monohydrocalcite surround the microbialites on the rock surfaces and are likely inorganic in origin. Within the mats, minerals tend to form and concentrate within, or around, dense matrices of EPS. Microenvironments with geochemical conditions favorable for mineral crystallization likely develop in the mats as a result of the mucilaginous extracellular material and the development of bacterial microcolonies. In addition, copious amounts of extracellular polymers bind ions from solution and provide nucleation sites for mineral crystallization and growth. This combination of biological and inorganic processes can explain the occurrence of the secondary minerals in these caves, as well as the stromatolitic textures of the microbialites.
This chapter describes clays, microorganisms, and biomineralization. Microorganisms are ubiquitous on the Earth's surface. They are especially abundant in biofilms and microbial mats associated with ponds, hot springs, weathered feldspar, deep-sea floor vents, and mine drainage areas. In these environments, microorganisms can synthesize many kinds of clay minerals, both inside and outside their living cells. The study of biomineralization and the processes involved requires a multidisciplinary approach and the application of a range of analytical and instrumental techniques. At the same time, the results can provide valuable background information with respect to environmental protection, such as bioremediation of polluted sites, as well as an insight into the sustainable development of new, clean energy sources, mineral resources, and biomedical technologies.
A total of forty-three taxa were studied and described. The taxa belong to three orders, thirteen families and twenty two genera. Eighteen genera are new records for the country, as follows: Aphanothece, Aphanocapsa, Merismopedia, Gloeocapsa, Chroococcus, Entophysalis, Stanieria, Myxosarcina, Pleurocapsa, Pseudanabaena, Jaaginema, Geitlerinema, Porphyrosiphon, Spirulina, Leptolyngbya, Oscillatoria and Homoeothrix. Twelve species were fond in distinctly different habitats from those from which the original type species were collected and described: Aphanocapsa cf. muscicola, Gloeocapsa cf. atrata, G. cf. arenaria, G. cf. punctata, Chroococeus cf. helveticus, C. cf. minor, C. cf. polyedriformis, Myxosarcina cf. concinna, cf. Jaaginema pseudogeminatum, Phormidium cf. priestley, P. cf. puteale and Homoeothrix cf. gracilis. One new combination is proposed [Stanieria minima (Geitl.) Silva et Pienaar].
Morphology and reproduction of Chroococcidiopsis mysorensis sp. nov., isolated from enrichment cultures of paddy field soils of Naganahalli (India), have been described. The taxonomic status of the genus Chroococcidiopsis has been discussed suggesting it to be placed under the order Chroococcales.
Formative cell divisions utilizing precise rotations of cell division planes generate and spatially place asymmetric daughters to produce different cell layers. Therefore, by shaping tissues and organs, formative cell divisions dictate multicellular morphogenesis. In animal formative cell divisions, the orientation of the mitotic spindle and cell division planes relies on intrinsic and extrinsic cortical polarity cues. Plants lack known key players from animals, and cell division planes are determined prior to the mitotic spindle stage. Therefore, it appears that plants have evolved specialized mechanisms to execute formative cell divisions. Despite their profound influence on plant architecture, molecular players and cellular mechanisms regulating formative divisions in plants are not well understood. This is because formative cell divisions in plants have been difficult to track owing to their submerged positions and imprecise timings of occurrence. However, by identifying a spatiotemporally inducible cell division plane switch system applicable for advanced microscopy techniques, recent studies have begun to uncover molecular modules and mechanisms for formative cell divisions. The identified molecular modules comprise developmentally triggered transcriptional cascades feeding onto microtubule regulators that now allow dissection of the hierarchy of the events at better spatiotemporal resolutions. Here, we survey the current advances in understanding of formative cell divisions in plants in the context of embryogenesis, stem cell functionality and post-embryonic organ formation. © 2012 The Author 2012.reserved. For permissions, please email: [email protected]
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This chapter presents an overview of the geology of the Barberton Greenstone belt and vicinity. Rocks in the 3.55 to 3.22 Ga Barberton Granite Greenstone Terrain (BGGT), South Africa and Swaziland, represent one of the oldest, well-preserved pieces of continental crust on the Earth. Together with similar rocks of nearly identical ages in the Pil-bara Craton of Western Australia, rocks of the BGGT have provided most of the direct geologic evidence on the nature and evolution of the pre-3.0 Ga Earth, its crust, surface environment, ocean, atmosphere, and biota. The BGB includes volcanic, sedimentary, and shallow intrusive rocks ranging in age from >3547 to <3225 Ma. The rocks have traditionally been divided into three main lithostratigraphic units that includes from base to top. It is found that in the Komati Gorge section, silicified volcaniclastic turbidites and debris-flow deposits of H6 at the top of the Hooggenoeg Formation are overlain by 100–200 m of massive serpentinized ultramafic rock that mark the base of the type section of the Kromberg Formation.
The detection of Mg–silica associated with endolithic biofilms of cyanobacteria and bacterial communities living in lake shore sand tufa formations in Mono Lake (California) has been observed using scanning electron microscopy with backscattered electron imaging (SEM–BSE) and energy-dispersive X-ray spectroscopy (EDS). Mg accumulated in cell walls of living cyanobacteria is interpreted to promote Mg-silicification with structural preservation while non-cyanobacterial prokaryotes are not fossilized. However, extracellular polymeric substances around the living communities were prominent as nucleation sites for authigenic mineral precipitation: a Mg–silicate served to stabilize the biofilm remains. Further diagenesis of these textures resulted in occlusion of the cytoplasm pore spaces by carbonate precipitates. This precipitate damaged the microfossil structures but the Mg–silica fabric was still present. The paleontological significance of Mg–silica precipitates as a possible biosignature of the former presence of endolithic biofilms is addressed.
An ultrastructural examination of cell division in two baeocyte producing cyanobacteria,Pleurocapsa minor andDermocarpa violaceae, reveals two distinct patterns of binary (transverse) fission. Septate binary fission, inPleurocapsa minor, involves centripetal synthesis and deposition of the mucopolymer cell wall layer (L 2). The ingrowth of the cytoplasmic membrane and L 1 cell wall layer, along with the synthesis of the L 2 cell wall layer, results in the formation of a prominent septum. Partitioning of the cell occurs by the constriction of the outer cell wall layers (L 3 and L 4) through the septum. InDermocarpa violaceae, constrictive binary fission occurs by the simultaneous ingrowth or constriction of the cytoplasmic membrane and all cell wall layers (L1, L2, L3, L4). Septate and constrictive binary fission may proceed symmetrically (medially) or asymmetrically (nonmedially). Multiple fission occurs regularly inDermocarpa violaceae and provides for a rapid means of reproduction when compared to binary fission. Successive radial and tangential divisions of the protoplast result in formation of many small daughter cells (baeocytes). The process of multiple fission is similar to septate binary fission with reduced septa being formed. However, constriction of the outer cell wall layers, through the septa, proceeds concurrently with septum formation.
Transmission electron microscopic (TEM) analyses of freshwater biofilms and bacterial cells, grown in experimental culture, have shown that these microorganisms are commonly associated with fine-grained (Fe, Al)-silicates of variable composition. The inorganic phases develop in a predictable manner, beginning with the adsorption of cationic iron to anionic cellular surfaces, supersaturation of the proximal fluid with Fe3+, nucleation and precipitation of a precursor ferric hydroxide phase on the cell surface, followed by reaction with dissolved silica and aluminum and eventually the growth of an amorphous clay-like phase. Alternatively, colloidal species of (Fe, Al)-silicate composition may react directly with either the anionic cellular polymers or adsorbed iron, depending on their net charge. Over time, these hydrous precursors may dehydrate and convert to more stable crystalline phases. Because microbial biofilms are expansive and highly reactive surfaces at the sediment–water interface, coupled with their ability to bind soluble components and form solid inorganic phases, they should influence the chemical composition of the overlying aqueous microenvironment, and ultimately contribute to the makeup of river bottom sediment.
The influence of microbial surfaces in the formation of short-range ordered aluminosilicates of allophanic composition was studied in the presence of citric, tannic and fulvic acids (FA) and two metals (Cd and Zn). In all bacterial cases, the pH was raised from an initial value of 4.5 to close to neutrality (6 to 7), and the bacteria produced a mineral phase in which a higher proportion of silicate was incorporated, i.e., with 150 ppm of fulvic acid, molar Si:Al ratios in these precipitates were the highest of any system and oscillated between 0.12 and 0.41 in the bacterial systems, whereas they ranged from 0.02 and 0.12 in the abiotic controls. Bacterial surfaces enhanced the immobilization of the metallic cations, especially Al and Zn, and increased their residence time as bound metal. The significance of these results for soil environments is discussed.
Fossil evidence of the existence of life during the Archean Eon of Earth history (>2500 Ma) is summarized. Data are outlined for 48 Archean deposits reported to contain biogenic stromatolites and for 14 such units that contain a total of 40 morphotypes of described microfossils. Among the oldest of these putatively microfossiliferous units is a brecciated chert of the ∼3465 Ma Apex Basalt of Western Australia. The paleoenvironment, carbonaceous composition, mode of preservation, and morphology of the Apex microbe-like filaments, backed by new evidence of their cellular structure provided by two- and three-dimensional Raman imagery, support their biogenic interpretation. Such data, together with the presence of stromatolites, microfossils, and carbon isotopic evidence of biological activity in similarly aged deposits, indicate that the antiquity of life on Earth extends to at least ∼3500 Ma.
The geological record of carbonaceous matter from at least 3.5 Ga to the end of the Precambrian is fundamentally continuous in terms of carbonaceous matter structure, composition, environments of deposition/preservation, and abundance in host rocks. No abiotic processes are currently known to be capable of producing continuity in all four of these properties. Although this broad view of the geological record does not prove that life had arisen by 3.5 Ga, the end of the early Archean, it suggests a working hypothesis: most if not all carbonaceous matter present in rocks older than 3.0 Ga was produced by living organisms. This hypothesis must be tested by studies of specific early geological units designed to explore the form, distribution, and origin of enclosed carbonaceous matter.
Carbonaceous matter occurring in chert deposits of the 3.4–3.2 Ga old Barberton Greenstone Belt (BGB), South Africa, has experienced low grade regional metamorphism and variable degrees of local hydrothermal alteration. Here a detailed study is presented of in situ analysis of carbonaceous particles by LRS (laser Raman spectroscopy) and SIMS (secondary ion mass spectrometry), reporting degree of structural disorder, carbon isotope ratio and nitrogen-to-carbon ratio. This combination of in situ analytical tools is used to interpret the δ13C values of only the best preserved carbonaceous remains, enabling the rejection of non-indigenous (unmetamorphosed) material as well as the exclusion of strongly hydrothermally altered carbonaceous particles. Raman spectroscopy confirmed that all carbonaceous cherts studied here have experienced a regional sub- to lower-greenschist facies metamorphic event. Although this identifies these organics as indigenous to the cherts, it is inferred from petrographic observations that hydrothermal alteration has caused small scale migration and re-deposition of organics. This suggest that morphological interpretation of these carbonaceous particles, and in general of putative microfossils or microlaminae in hydrothermally altered early Archean cherts, should be made with caution. A chert in the Hooggenoeg Formation, which is older than and has been hydrothermally altered by a volcanic event 3445 Ma ago, contains strongly altered carbonaceous particles with a uniform N/C-ratio of 0.001 and a range of δ13C that is shifted from its original value. Cherts of the Kromberg Formation post-date this volcanic event, and contain carbonaceous particles with a N/C-ratio between 0.002 and 0.006. Both the Buck Reef Chert and the Footbridge Chert of the Kromberg Formation have retained fairly well-preserved δ13C values, with ranges from −34‰ to −24‰ and −40‰ to −32 ‰, respectively. Abiologic reactions associated with hydrothermal serpentinization of ultramafic crust (such as Fischer–Tropsch synthesis) were an unlikely source for carbonaceous material in these cherts. The carbonaceous matter in these cherts has all the characteristics of metamorphosed biologic material.
Single-celled organisms dividing by binary fission were thought not to age [1-4]. A 2005 study by Stewart et al. [5] reversed the dogma by demonstrating that Escherichia coli were susceptible to aging. A follow-up study by Wang et al. [6] countered those results by demonstrating that E. coli cells trapped in microfluidic devices are able to sustain robust growth without aging. The present study reanalyzed these conflicting data by applying a population genetic model for aging in bacteria [7]. Our reanalysis showed that in E. coli, as predicted by the model, (1) aging and rejuvenation occurred simultaneously in a population; (2) lineages receiving sequentially the maternal old pole converged to a stable attractor state; (3) lineages receiving sequentially the maternal new pole converged to an equivalent but separate attractor state; (4) cells at the old pole attractor had a longer doubling time than ones at the new pole attractor; and (5) the robust growth state identified by Wang et al. corresponds to our predicted attractor for lineages harboring the maternal old pole. Thus, the previous data, rather than opposing each other, together provide strong evidence for bacterial aging.