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Chapter 4.4 Evolutionary Processes in the Formation of Stromatolites
Abstract
This chapter discusses the evolutionary process in the formation of stromatolites. To understand the evolution of stromatolite-building organisms, it is necessary to understand the processes operating in the formation of microbial mats, the precursors of stromatolites. The chapter provides background information on biological and sedimentary processes, leading to stromatolite formation and preservation. This chapter illustrates the stromatolitic habit, which is notably polyphyletic. It involves communities of microorganisms interacting with the physical environment. It is likely that the morphological conservatism of blue-green algae optimally adapted to the stromatolite-building habit is highly evolved, in the sense that stability itself is under genetic control. Blue-green algae may have been subject to rapid rates of evolution of new highly fit genotypes; but limited morphology severely limits the phenotypic information that can be deduced by studies of fossil microorganisms. It is within the blue-green algal template that other organisms have contributed to the construction of stromatolites. In view of this polyphyletic origin, evolutionary information derivable directly from stromatolites and their microbiota is intrinsically limited.
... In these sites, the observed microbes have been considered to be the origin of the iron terraces, which could thus represent modern analogs of the ancient banded iron formations (BIF), and even Precambrian stromatolites . These acid mine drainage–related iron deposits meet all the criteria to be considered as modern stromatolites in the sense that they are accretionary, organosedimentary structures produced by sediment trapping, binding, and/or precipitation as a result of the growth and metabolic activity of mat-forming microorganisms (Walter, 1976; Awramik et al., 1976). However, there is an important difference between these two acid mine drainage stromatolite systems in the type of microbial communities involved in their formation. ...
... In addition to the calcareous travertines deposited by Ca-HCO 3 –rich groundwaters and hot springs, a comparison between the iron terraces of the Tintillo River and the calcareous stromatolites formed in marine (shore) environments seems pertinent. Interpretations of these typical sedimentary structures have evolved considerably, from the mainly organic (bacterial ) nature, which sedimentologists attributed to these formations in the 1970s (e.g., Awramik et al., 1976; Golubic, 1976; Walter, 1976 Walter, , 1994), to more recent abiotic theories based on current research using physical and numerical computer modeling (e.g., Grotzinger and Rothman, 1996; Grotzinger and Knoll, 1999; Batchelor et al., 2000 ). Grotzinger and colleagues have successfully modeled a growth pattern similar to those of stromatolites based upon diffusion-limited aggregation and sedimentation of either microbial mats or precipitated minerals. ...
Iron terraces that form in acidic mine drainage settings are unique and extreme geomicrobiological systems that can provide highly relevant information about the inter-action between microbes and their surround-ing aqueous environments. These singular systems can represent, additionally, poten-tial models for the study of ancient geologi-cal formations (e.g., banded iron formations, stromatolites) and/or for the cycling of iron on Mars. This work describes geochemical, mineralogical, morphological, and micro-biological evidence obtained in the highly acidic and Fe-rich Tintillo River (Riotinto mines, Huelva, SW Spain), which can be used to speculate about the origin and nature of the terraced iron formations (TIFs) that are being currently formed in acid mine drainage environments. The size (up to 36 m long and 1 m thick) and continuity (strong development over 3.5 km) of the iron ter-races offer a unique opportunity to study the different organic (mainly microbial) and inorganic processes involved in the construc-tion of these characteristic, travertine-like, sedimentary structures. Evidence presented in this study suggests that both types of pro-cesses appear to be controlling factors in the formation and internal arrangement of the TIFs, although no defi nitive evidence has been found to support the prevalence of any of these mechanisms with respect to another. The photosynthetic production of dissolved oxygen by eukaryotic microorganisms (green algae, euglenophytes, and diatoms) and the Fe-oxidizing metabolism of acidophilic pro-karyotes are critical factors for the formation of TIFs, whereas abiotic parameters, such as water composition, fl ow rate and velocity, or stream channel geometry, also appear to be essential variables.
... In recent decades, the study of microbialites and their contribution to the formation, mineralization, accumulation, and diagenesis of carbonate sediments, as well as their role in the increasing levels of oxygen in the Earth's oceans and atmosphere, has become a hot topic in carbonate sedimentology as it provides valuable information regarding paleobiodiversity and paleoenvironments of stromatolites (Dupraz et al., 2009;Riding, 2011;Reinhard et al., 2013;Soo et al., 2017;Lan et al., 2020;Gabriel et al., 2021;Hohl and Viehmann, 2021). Stromatolites, the most common microbialites, are layered biosedimentary structures formed in shallow marine environments through the trapping and binding of ambient sediments (agglutination) and/or the precipitation of minerals on or within microbial mats/biofilms (Awramik et al., 1976;Awramik and Sprinkle, 1999;Riding, 2000;Shi et al., 2008;Riding et al., 2014;Fralick and Riding, 2015;Qu et al., 2018;Baumgartner et al., 2020;Wang et al., 2021). These stromatolites were the predominant evidence of life in marine ecosystems throughout the Precambrian record (Walter, 1994;Hohl and Viehmann, 2021), reaching their peak abundance and morphological diversity in the Mesoproterozoic (Awramik and Sprinkle, 1999;Riding et al., 2014;Wang et al., 2021). ...
Stromatolites in the lower part of the Early Mesoproterozoic Fengjiawan Formation in the southern North China Craton (NCC) contain abundant microbial dolostones with well-preserved primary structures. This study comprehensively investigates the macromorphological, petrographical, mineralogical, and geochemical characteristics of these pure stromatolite-dominated microbialites for the first time. The results reveal diverse macromorphological structures distributed from the bottom to the top as conical (lower intertidal to upper subtidal zones), hemispherical domal (lower supratidal zone), spherical domal (upper to middle intertidal zone), and more abundant columnar (upper to middle subtidal zone) stromatolites. The mineral composition of these stromatolites indicates that they are pure dolostones, predominantly consisting of dolomite (avg. 94.24 wt%). These stromatolite-dominated microbialites were subjected to minimal detrital contamination and weak post-depositional alteration as documented by different geochemical multiproxy evaluations. This indicates that they have effectively retained the composition of the seawater in which they were initially formed. Geochemical indicators, such as Y/Ho, La/La*, Gd/Gd* and Ce/Ce* strongly suggest deposition of the stromatolites in an open shallow marine environment characterized by a passive continental margin setting. Different elemental ratios, such as Ce/Ce*, MoEF, UEF, MoEF/UEF, imply the stromatolites formation under a suboxic redox environment, where the photosynthesis of cyanobacteria likely played a significant role in the slight oxygenation of the seawater during their formation. These oxygenation levels decreased as water depth increased, with more nutrient supply promoting from deeper waters. The formation and growth of the stromatolite-dominated
microbialites were significantly influenced by various depositional conditions, including sea level fluctuations, biological processes, suboxic redox environment, low detritus contamination, and weak post-depositional alteration. These conditions all contributed to the excellent preservation of the Fengjiawan Formation stromatolites, providing valuable insights into deep time microbial carbonates.
... Kalkowsky introduced the term to describe layered carbonate structures in the Lower Triassic Buntsandstein of central Germany, and remarkably already proposed a biological origin of these fabrics. Following work showed that various biological processes are involved in stromatolite formation, including microbial mat-related mineral precipitation and/or trapping and binding of detrital sediments (Awramik et al. 1976;Reitner 2011;Riding 2011;Suarez-Gonzalez et al. 2019). ...
The so‐called Permian–Triassic mass extinction was followed by a prolonged period of ecological recovery that lasted until the Middle Triassic. Triassic stromatolites from the Germanic Basin seem to be an important part of the puzzle but have barely been investigated so far. Here, we analysed late Anisian (upper Middle Muschelkalk) stromatolites from across the Germanic Basin by combining petrographic approaches (optical microscopy, micro X‐ray fluorescence, Raman imaging) and geochemical analyses (sedimentary hydrocarbons, stable carbon and oxygen isotopes). Palaeontological and sedimentological evidence, such as Placunopsis bivalves, intraclasts and disrupted laminated fabrics, indicate that the stromatolites formed in subtidal, shallow marine settings. This interpretation is consistent with δ13Ccarb of about −2.1‰ to −0.4‰. Occurrences of calcite pseudomorphs after gypsum possibly suggest occasionally elevated salinities, which is well in line with the relative rarity of fossils in the host strata. Remarkably, the stromatolites are composed of microbes (perhaps cyanobacteria and sulphate‐reducing bacteria) and metazoans such as non‐spicular demosponges, Placunopsis bivalves and/or microconchids. Therefore, these ‘stromatolites’ should more correctly be referred to as microbe‐metazoan build‐ups. They are characterized by diverse lamination types, including planar, wavy, domal and conical ones. Microbial mats likely played an important role in forming the planar and wavy laminations. Domal and conical laminations commonly show clotted to peloidal features and mesh‐like fabrics, attributed to fossilized non‐spicular demosponges. Our observations not only point up that non‐spicular demosponges are easily overlooked and might be mistakenly interpreted as stromatolites, but also demonstrate that microbe‐metazoan build‐ups were widespread in the Germanic Basin during Early to Middle Triassic times. In the light of our findings, it appears plausible that the involved organisms benefited from elevated salinities. Another (not necessarily contradictory) possibility is that the mutualistic relationship between microbes and non‐spicular demosponges enabled these organisms to fill ecological niches cleared by the Permian–Triassic crisis. If that is to be the case, it means that such microbe‐metazoan associations maintained their advantage until the Middle Triassic.
... Laminae with abundant organic matter likely represent exopolymeric substances (EPS) secreted by microbial mat communities (see Suarez-Gonzalez and Reitner 2021). Those devoid of organic matter, in contrast, might be dominated by authigenic mineral precipitation and/or trapping and binding of detrital materials (Awramik et al. 1976;Reitner 2011;Suarez-Gonzalez et al. 2019). ...
The aftermath of the Permian – Triassic crisis is characterized by ubiquitous occurrences of microbial sediments around the world. For instance, Triassic deposits of the Germanic Basin have shown to provide a rich record of stromatolites as well as of microbe-metazoan build-ups with non-spicular demosponges. Despite their paleoecological significance, however, all of these microbialites have only rarely been studied. This study aims to fill this gap by examining and comparing microbialites from the Upper Buntsandstein (Olenekian, Early Triassic) and the lower Middle Muschelkalk (Anisian, Middle Triassic). By combining analytical petrography (optical microscopy, micro X-ray fluorescence, Raman spectroscopy) and geochemistry (δ13Ccarb, δ18Ocarb), we show that all studied microbialites formed in hypersaline lagoons to sabkha environments. Olenekian deposits in Jena and surroundings and Anisian strata at Werbach contain stromatolites. Anisian successions at Hardheim, in contrast, host microbe-metazoan build-ups. Thus, the key-difference is the absence or presence of non-spicular demosponges in microbialites. After the Permian – Triassic crisis, the widespread microbialites (e.g., stromatolites/microbe-metazoan build-ups) possibly resulted from suppressed ecological competition and occupied the vacant ecological niche. It seems plausible that microbes and non-spicular demosponges had a mutualistic relationship and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecologic association. Furthermore, both microbes and non-spicular demosponges may benefit from elevated salinities. Perhaps it was minor differences in salinities that controlled whether or not non-spicular demosponges could develop.
... Kalkowsky introduced the term to describe layered carbonate structures in the Lower Triassic Buntsandstein of central Germany, and remarkably already proposed a biological origin of these fabrics. Following work showed that various biological processes are involved in stromatolite formation, including microbial mat-related mineral precipitation and/or trapping and binding of detrital sediments (Awramik et al. 1976;Reitner 2011;Riding 2011;Suarez-Gonzalez et al. 2019). ...
The so-called Permian — Triassic mass extinction was followed by a prolonged period of ecological recovery that lasted until the Middle Triassic. Triassic stromatolites from the Germanic Basin seem to be an important part of the puzzle, but have barely been investigated so far. Here we analyzed late Anisian (upper Middle Muschelkalk) stromatolites from across the Germanic Basin by combining petrographic approaches (optical microscopy, micro X-ray fluorescence, Raman imaging) and geochemical analyses (sedimentary hydrocarbons, stable carbon and oxygen isotopes). Paleontological and sedimentological evidence, such as Placunopsis bivalves, intraclasts and disrupted laminated fabrics, indicate that the stromatolites formed in subtidal, shallow marine settings. This interpretation is consistent with δ13Ccarb of about -2.1 % to -0.4 %. Occurrences of calcite pseudomorphs after gypsum suggest slightly evaporitic environments, which is well in line with the relative rarity of fossils in the host strata. Remarkably, the stromatolites are composed of microbes (perhaps cyanobacteria and sulfate reducing bacteria) and metazoans such as non-spicular demosponges, Placunopsis bivalves, and/or Spirobis-like worm tubes. Therefore, these ″stromatolites″ should more correctly be referred to as microbe-metazoan build-ups. They are characterized by diverse lamination types, including planar, wavy, domal and conical ones. Microbial mats likely played an important role in forming the planar and wavy laminations. Domal and conical laminations commonly show clotted to peloidal features and mesh-like fabrics, attributed to fossilized non-spicular demosponges. Our observations not only point up that non-spicular demosponges are easily overlooked and might be mistakenly interpreted as stromatolites, but also demonstrate that microbe-metazoan build-ups were widespread in the Germanic Basin during Early to Middle Triassic times. In the light of our findings, it appears plausible that the involved organisms benefited from elevated salinities. Another (not necessarily contradictory) possibility is that the mutualistic relationship between microbes and non-spicular demosponges enabled these organisms to fill ecological niches cleared by the Permian — Triassic Crisis. If that is to be the case, it means that such microbe-metazoan associations maintained their advantage until the Middle Triassic.
... Stromatolites are a type of microbialite and are one of the most prevalent and most recognized constituents of the Precambrian carbonate platforms Reid et al. 2011;Suosaari et al. 2016bSuosaari et al. , 2019. Stromatolites are laminated, lithified organosedimentary structures that have formed as a result of the sediment trapping, binding, and precipitating activities of microbes in response to their local environment (Awramik et al. 1976). As products of interactions between microbes, minerals and the environment, stromatolites are a quintessential embodiment of linkages between the biosphere and the geosphere. ...
Extant stromatolites have been considered ecological similes to their ancient counterparts. We now know that these microbial assemblages are composed of a great diversity of microbes, which couple and intertwine their metabolic capabilities to create self-sustained microbial ecosystems. Presently, stromatolites thrive in a vast diversity of aquatic environments including freshwater, hypersaline, coastal lagoons, alkaline lakes, oligotrophic pools, abandoned pits, few marine systems, and brackish waters. In this chapter, we will summarize the research that has been done in two stromatolite-harboring sites in Mexico: the alkaline crater-lake Alchichica and the oligotrophic karstic coastal lagoon of Bacalar. Alchichica is located in the Transvolcanic belt in Central Mexico. It is a maar-alkaline crater lake (salinity 8.5 gl−1, pH 9.5) with water chemistry determined by high contents of carbonates, sodium, a high Mg/Ca ratio, and particularly low Ca2+ concentrations (~0.3 mM). Two main stromatolite-types, as defined by mineralogy, texture, and microbial composition, develop along its periphery from surface to over 30 m in depth. Alchichica is a modern environment that resembles Precambrian oceanic conditions. Stromatolites from Alchichica have been dated radiometrically in ~1.1–2.8 ka BP. Bacalar is a coastal lagoon located in the Yucatan Peninsula, which is a carbonate platform that emerged above sea level during the Oligocene. The Bacalar lagoon has high carbonate concentrations as a result of the influx of groundwater (salinity 1.2 gl−1, pH 7.6–8.3). It holds the largest freshwater stromatolite structures known, which have been radiometrically dated in ~6.8–9.2 ka BP. Their mineralogy, shape, and texture are similar along the lagoon’s coast, but microbial composition changes, possibly due to anthropogenic impact.
... Stromatolites are a type of microbialite and are one of the most prevalent and most recognized constituents of the Precambrian carbonate platforms (Grotzinger and Knoll 1999;Reid et al. 2011;Suosaari et al. 2016bSuosaari et al. , 2019. Stromatolites are laminated, lithified organosedimentary structures that have formed as a result of the sediment trapping, binding, and precipitating activities of microbes in response to their local environment (Awramik et al. 1976). As products of interactions between microbes, minerals and the environment, stromatolites are a quintessential embodiment of linkages between the biosphere and the geosphere. ...
For millennia, humanity has looked to stars and wondered, “Are we alone in the universe?” Although this question was initially the purview of philosophers, now, with leaps in scientific and technological advances, we have changed the nature of this question from existential to empirical. Today, the question “Are we alone?” serves as a crux to the field of astrobiology. To search for life elsewhere in the universe, we must first understand how life originates and evolves on Earth but also how biology leaves behind residual signatures of its existence. To address these questions, many astrobiology researchers have targeted stromatolite-forming communities as model ecosystems to explore how microbe–mineral interactions, under a range of environmental conditions, can lead to the formation of biosignatures. Stromatolites are depositional structures formed by the activities and interactions of microbes and have a fossil record dating back billions of years. Due to their long evolutionary history and abundance on the modern Earth, research on the biological, chemical and geological processes of stromatolite formation have provided important insights into the field of astrobiology, including the diversity and preservation of biosignatures. In this chapter, we examine the range of biosignatures found in stromatolites and how these markers improve our understanding of the past, present, and future of life in the context of astrobiology. We also discuss whether stromatolite research can play a role in the future exploration of habitable worlds in our own solar system and beyond.
... Stromatolites are a type of microbialite and are one of the most prevalent and most recognized constituents of the Precambrian carbonate platforms Reid et al. 2011;Suosaari et al. 2016bSuosaari et al. , 2019. Stromatolites are laminated, lithified organosedimentary structures that have formed as a result of the sediment trapping, binding, and precipitating activities of microbes in response to their local environment (Awramik et al. 1976). As products of interactions between microbes, minerals and the environment, stromatolites are a quintessential embodiment of linkages between the biosphere and the geosphere. ...
Deep hydrothermal systems result from the magmatic and tectonic activity of the ocean floor. This deep extreme biosphere represents a unique oasis of life driven by sulfur-based chemosynthesis instead of photosynthesis. The organisms inhabiting these systems are adapted to cope with harsh environmental conditions such as the absence of sunlight, high temperatures and hydrostatic pressures, and elevated concentrations of hydrogen sulfide, as well as high concentrations of heavy metals. Therefore, this biome is different from any other environment on modern Earth. As expected from such conditions, chemoautotrophic prokaryotes are the leading primary producers at the base of the food web considered as an analog to the oldest signs of life on Earth. Herein, we discuss prokaryotic diversity and community structure from the newly discovered hydrothermal systems in the Alarcón Rise (AR), the Pescadero Basin (PB), and the Pescadero Transform Fault (PTF) at the mouth of the Gulf of California, Mexico, using 16S rRNA gene amplicon Illumina sequencing. Despite the spatial proximity of the studied vent systems (<100 km), they differ considerably in their physical, chemical, geological settings, and biotic characteristics. Our results indicated that beta prokaryotic diversity is associated to the sampling source, suggesting a strong effect of environmental conditions in shaping microbial distribution. The most abundant phyla were Proteobacteria, Bacteroidetes, Actinobacteria, Chloroflexi, and Epsilonbacteraeota. Also, we found evidence on the oxidation of methane as a prevalent process in PB and PTF, since methylotrophic bacteria and Atribacteria were abundant, in contrast to AR basalt-hosted system. Bacteria associated with the sulfur cycle, in particular sulfur compounds reducing and sulfur compounds oxidizing bacteria predominated in all samples, confirming the importance of sulfur supporting vent communities. It is possible that vent systems played a significant role in the origins of life on Earth. Hence, they represent useful models when searching for life elsewhere in the universe.
... The Gunflint Iron Formation is known for its microfossiliferous black cherts. These cherts have 42 produced compelling evidence for very ancient (1.88 Ga [3]) microbial life [4][5][6][7][8][9][10][11][12]. Tyler and Barghoorn 43 [4] noted that as "far as we are aware, these [microbes] are the oldest structurally preserved 44 Preprints (www.preprints.org) ...
We report here a giant microfossil resembling the conidium of an ascomycete fungus (cf. Alternaria alternata). The specimen is preserved in stromatolitic black chert of the Gunflint Iron Formation (Paleoproterozoic Eon, Orosirian Period, ca. 1.9-2.0 Ga) of southern Ontario, Canada, and the rock that provided the thin section may have been collected by Elso Barghoorn as part of the original discovery of the Gunflint microbiota. The large size of the fossil sets it apart from other, tiny by comparison, Gunflint microfossils. The fossil is 200 microns in length and has cross walls. Individual cells are 30-46 microns in greatest dimension. The apical ‘spore’ is cap-shaped, and has partly separated from the rest of the structure. Cloulicaria gunflintensis gen. nov. sp. nov. may provide early evidence for eukaryotes (fungi) in the fossil record, and may also represent the earliest evidence for asexual reproduction in a eukaryote by means of mitospores.
... The biogeomorphological processes primarily focussed on are, bioconstruction and biostabilisation, with additional observations presented on bioerosion and bioturbation. The framework relies on both early studies (Davies, 1970a;Logan and Cebulski, 1970;Logan et al., 1974;Awramik et al., 1976;Brock, 1976;Golubic, 1976a;Golubic, 1976b;Monty, 1976;Playford and Cockbain, 1976;Read, 1976;Walter, 1976;Burne and Hunt, 1990) and recent advances in the understanding of the lithifying peritidal benthic microbial ecosystem (Visscher et al., 1998;Reid et al., 2003;Dupraz et al., 2009;Burne and Johnson, 2012;Jahnert and Collins, 2013;Summons et al., 2013;Edgcomb et al., 2014;Giusfredi, 2014;Pages et al., 2014b;Wong et al., 2015;Ruvindy et al., 2016;Suosaari et al., 2016a;Suosaari et al., 2016b;Suosaari et al., 2019;Wong et al., 2017;Babilonia et al., 2018;Wong et al., 2018;Charlesworth et al., 2019) as well as the coastal coquina ridge deposits Playford et al., 2013;Correia Baptista da Silva, 2014) and the underlying geological and geomorphological framework (Butcher et al., 1984;O'Leary et al., 2008;Eliot et al., 2011;Playford et al., 2013). Our efforts are focused on three areas of approximately 0.3 km 2 each, covering the upper subtidal through to the supratidal zones (locations NIL, RRBS and RRBN Fig.1, and Fig.2). ...
The World Heritage microbialites, coquinas, and microbialitic-sediments of Hamelin Pool, Shark Bay, Western Australia, form through complex community and environmental interactions resulting in surficial CO2 sequestration. Predicted climate-change impacts threaten the stability of this setting and the balance of biogeomorphological processes that generate them. In this setting, long-term surficial CO2 sequestration occurs through the combination of biomineralisation, organomineralisation and lithification, which locks atmospheric CO2 into CaCO3 rocks and sediments. Biogeomorphlogical processes that control the distribution and effectiveness of these mechanisms including biostabilisation, bioconstruction, bioerosion and bioturbation have not been addressed in this setting to date.
A 3-dimensional biogemorphological mapping unit framework is defined at three locations in Hamelin Pool within the shallow-subtidal to supratidal zones based on historical and recent multiscale datasets. This framework is used to review the complex distribution of geomorphological and biogeomorphological processes. The distribution of lithification within a biogeomorphological process context is examined, and current geological analogue models of microbialite and coquina development are expanded to include biogeomorphological processes.
The framework is used to understand the impacts of increased water elevation in response to intensification of the Leeuwin Current during the 2010–2012 Ningaloo Nino event. Potential climate change impacts are identified including reduced intertidal bioconstruction, and a shift from microbialitic structure generation to increased grain generation reducing stromatolite development. Review of the uncertainty of impacts of ocean acidification and temperature shifts on biomineralisation, organomineralisation and lithification processes, crucial to the maintenance of the outstanding universal values in this World Heritage setting, suggests urgent research is required.
... Stromatolites form when layers of bacterial mat grow sequentially on top of one another and lithify to form a three-dimensional rock framework. This framework forms by microbial trapping and binding of detrital sediment and/or mineral precipitation and is further strengthened by diagenesis and continued cementation through time (Awramik, 1984;Awramik, Margulis, & Barghoorn, 1976;Burne & Moore, 1987). These processes produce distinct build-ups that have relief above the seafloor (Playford et al., 2013). ...
In tribute to Dr. Robert N. Ginsburg (1925‐2017), we examine his 1991 seminal paper Controversies about Stromatolites: Vices and Virtues, which summarized current ideas about stromatolites including controversies of definition, whether ancient stromatolites should be interpreted as sedimentary structures, and mechanisms of carbonate fixation. The accepted model of stromatolite morphogenesis in 1991 was that biology controls microscale internal structure whereas environment controls macroscale morphology. Ginsburg, however, predicted that biology and environmental influences on stromatolite growth were closely coupled at macro, meso, and micro scales. Recent research in Hamelin Pool and the Bahamas has advanced our understanding of the inherent duality of stromatolites and associated controversies. These studies suggest that at the macroscale, when physical forces are strong, the environment is the main control on the morphology; however, when physical forces are weak, biological communities become the main drivers of morphology. Therefore, stromatolites can be considered both as fossils and as sedimentary structures dependent on the energy in the environment of deposition. At the mesoscale, as predicted by Trompette, stromatolite fabrics are influenced equally by environment and biology. As the degree of lamination is often unknown or heterogeneous, a generic genetic term such as ‘microbialite’ is considered the most appropriate terminology for structures of probable microbial origin. At the microscale, stromatolite microfabrics reflect environmentally driven cycling of microbial communities, reflecting both biology and environment. With respect to carbonate fixation, research on modern stromatolites provides a model for biofilm precipitation of micritic laminae lacking microfossils in ancient stromatolites. As pointed out by Ginsburg, the inherent duality of environmental and biological controls of morphogenesis at all scales is at the root of many long‐standing controversies. Recent investigations corroborate the foresight of Ginsburg nearly 30 years ago, further confirming well‐preserved stromatolites can provide insight into both biology and environmental factors in ancient ecosystems. This article is protected by copyright. All rights reserved.
... Stromatolites are layered organo-sedimentary structures that form from sediment trapping, binding and/or carbonate precipitation as a result of the growth and metabolic action of cyanobacteria (Awramik et al. 1976). These deposits develop in areas where CaCO 3 supersaturation is reached (Smith et al. 2011), such as at the sediment water interface in freshwater, marine, and evaporitic environments (Riding 2011). ...
Stromatolites are rare in modern ecosystems due to factors associated with seawater chemistry or biological competition that restrict their formation. Actively calcifying stromatolites, near the Kei Mouth in the Eastern Cape, South Africa, were discovered in the early 2000s. Similar deposits were later described along a 200 km stretch on the south coast of Port Elizabeth. This study aims to describe the environmental setting, the macro- and meso-structures, as well as the evolution of the deposits near Port Elizabeth compared to other similar formations. Results show that the general environmental setting is consistent amongst peritidal stromatolites, including those described in this study. In all instances stromatolite growth occurs on a wave-cut rocky platform in and around rock pools. Growth is maximal within the intertidal to supratidal zone, as a result of freshwater inflow via emerging mineral springs at the base of landward slopes, and the periodic intrusion of seawater via storm surges or wave splash. In comparison with other systems, the South African stromatolite formations exhibit an additional macro-structure (beachrock/conglomerate) and four previously undescribed meso-structures: wrinkled laminar, laminar flat, rhizoliths, and blistered types. The South African stromatolites are also larger and more concentrated than other peritidal stromatolites, which could be due to this area having more suitable growth conditions.
... The pH of the acidic water ranges from 2.0 to 3.5 with concentrations of TDS up to 20 g L − 1 , including up to 63,000 mg L − 1 SO 4 − 2 , 3000 mg L − 1 Al 3+ , 2300 mg L − 1 Fe + 2 , and up to 600 mg L − 1 Cl − (Brake et al., 2001). Such low pH and elevated TDS precludes all metazoans and most protozoans, and limits many prokaryotes from inhabiting this environment (e.g., Awramik et al., 1976;Golubic, 1976;Bond et al., 2000;Brake and Hasiotis, 2010, and references therein). Numerous acidic seeps have developed in the reclaimed waste pile, with AMD flowing across the surface through several constructed channels (Brake et al., 2004). ...
... The Trendall Reserve contains some of the world's best fossil evidence for ancient life on Earth. This evidence is provided by fossil stromatolites, laminated organo-sedimentary structures built by the activities of microorganisms, predominantly cyanobacteria, and possibly other microorganisms such as other bacteria, archea, and algae (Awramik et al., 1976). Stromatolites are widespread throughout Earth's geological record from the Neoarchean onwards, but Paleoarchean stromatolites such as those in the Trendall Reserve are very rare, and where they do occur they are commonly far less well preserved than much younger stromatolites. ...
Abstract
State Geoheritage Reserve R50149, also known as the Trendall Reserve, was established by the Government of Western
Australia to preserve geological features of exceptional national and international significance. This and five other reserves
cover some of the world’s most important early Archean stromatolite and microfossil sites in the Pilbara Craton. Since the
discovery of fossil stromatolites in the Trendall Reserve was reported in 1999, the North Pole area has been the focus of
many field excursions by scientists and documentary-makers from Australia and overseas.
Within the Trendall Reserve, the initial discovery site, now known as the ‘Trendall locality’, reveals a diverse assemblage
of stromatolites and sedimentary facies preserved in carbonate rocks of the 3426−3350 Ma Strelley Pool Formation.
Stromatolites are laminated organo-sedimentary structures built by microorganisms, predominantly cyanobacteria, although
for Archean stromatolites the role of cyanobacteria in relation to other bacteria and archea is unclear.
Detailed examination of Archean fossil stromatolites can provide key evidence on the origin of life as well as on ancient
depositional environments. Modern stromatolites occur in diverse forms and inhabit a range of environments including
shallow-water marine, lacustrine, and hydrothermal hot-spring settings. In the Proterozoic, stromatolites were much more
morphologically diverse and widespread and were established in many niches that today are occupied by other organisms.
However, fossil stromatolites are rarely preserved in early Archean rocks, and this makes the few outcrops that do contain
them especially important. Stromatolites at the Trendall Reserve are particularly significant because their complex
morphologies demonstrate that even some of the oldest known Archean stromatolites were not simple structures.
This Record reviews previous research on the Strelley Pool Formation of the Trendall Reserve, and discusses the significance
of the findings in relation to the regional geology of the Pilbara Craton. Local background information, a geological review,
and a detailed description of the Trendall locality, are also provided.
KEYWORDS: Trendall Reserve, Trendall locality, stromatolites, origin of life, carbonate rocks, North Pole, Pilbara Craton,
Archean, Strelley Pool Formation, field excursions
... The pH of the acidic water ranges from 2.0 to 3.5 with concentrations of TDS up to 20 g L − 1 , including up to 63,000 mg L − 1 SO 4 − 2 , 3000 mg L − 1 Al 3+ , 2300 mg L − 1 Fe + 2 , and up to 600 mg L − 1 Cl − (Brake et al., 2001). Such low pH and elevated TDS precludes all metazoans and most protozoans, and limits many prokaryotes from inhabiting this environment (e.g., Awramik et al., 1976;Golubic, 1976;Bond et al., 2000;Brake and Hasiotis, 2010, and references therein). Numerous acidic seeps have developed in the reclaimed waste pile, with AMD flowing across the surface through several constructed channels (Brake et al., 2004). ...
Lipid analysis of benthic and floating biofilms from an acid mine drainage (AMD) site in western Indiana revealed the dominance of photosynthetic organisms, mainly Euglena, as indicated by the detection of abundant phytadiene, phytol, phytanol, polyunsaturated n-alkenes, polyunsaturated fatty acids, short-chain (C25–32) wax esters (WE), ergosterol, and tocopherols. The WE were probably synthesized in mitochondria under anoxic conditions, whereas the sterols (ergosterol and ergosta-7,22-dien-3β-ol) were likely synthesized in the cytosol in the presence of molecular oxygen by the acidophilic, photosynthetic microeukaryotes Euglena. The dual aerobic and anaerobic biosynthetic pathways may be the biochemical relics of the anaerobic past of the Earth. The conserved compartmentalization of Euglena biosynthetic machinery may have allowed early eukaryotes to survive and diversify on early Earth, and despite their evolution, they preserved this physiology. Euglenids are capable of producing suites of short-chain WE that can be considered precursors for low-molecular weight (C12–16) n-alkanes, which can serve as proxies for the detection of such eukaryotic organisms in Precambrian strata. In addition, ergosterol and its derivative can also serve as biomarkers for such organisms.
... On planet Earth life usually manifests not as isolated individual organisms, but overwhelmingly appears as complex communities. Stromatolites are laminated organo-sedimentary structures [1][2][3][4] classically thought to arise from microbial trapping and binding of sediment or precipitation of dissolved minerals [5,6]. However, some investigators have posited more descriptive definitions leaving the biogenicity of these structures as an open question [7][8][9][10]. ...
Stromatolites offer a unique fossil record across 3.5 Ga of microbial community evolution within the context of an evolving Earth. Our interest is in developing quantitative metrics to follow the evolution of stromatolite morphological complexity. Adopting the canonical definition of complexity as the emergence of previously unseen properties in a dynamic phenomenon, we have previously proposed in these proceedings that laminations are the defining emergent property of stromatolites and we have employed a set of statistical information metrics to quantify laminae complexity in two spatial dimensions. We now demonstrate computer x-ray tomography of stromatolites and discuss the advantages of this 3D volume density distribution technique for characterizing stromatolite samples. CT imaging makes it possible to create a virtual stromatolite, enabling both research and educational efforts previously hampered by the costs of obtaining, preparing, and distributing precious Archean stromatolite fossils. We discuss recent advances in instrument miniaturization making it feasible to provide nondestructive 3D density and elemental abundance information on endolithic geobiological targets during future manned and unmanned missions to Mars.
... Based on the observations presented here and those of Brake et al. (2002), the iron-rich biolaminates that cover the channel bottom can be classified as stromatolites, since they meet the criteria established by Awramik et al. (1976) as being accretionary, laminated, organosedimentary structures produced by trapping and binding or precipitation of minerals as a result of the metabolic activities of microorganisms. ...
Adverse conditions in the acid mine drainage (AMD) system at the Green Valley mine, Indiana, limit diatom diversity to one species, Nitzschia tubicola. It is present in three distinct microbial consortia: Euglena mutabilis-dominated biofilm, diatomdominated biofilm, and diatom-exclusive biofilm. E. mutabilis dominates the most extensive biofilm, with lesser numbers of N. tubicola, other eukaryotes, and bacteria. Diatom-dominated biofilm occurs as isolated patches containing N. tubicola with minor fungal hyphae, filamentous algae, E. mutabilis, and bacteria. Diatom-exclusive biofilm is rare, composed entirely of N. tubicola.
Diatom distribution is influenced by seasonal and intraseasonal changes in water temperature and chemistry. Diatoms are absent in winter due to cool water temperatures. In summer, isolated patchy communities are present due to warmer water temperatures. In 2001, the diatom community expanded its distribution following a major rainfall that temporarily diluted the effluent, creating hospitable conditions for diatom growth. After several weeks when effluent returned to preexisting conditions, the diatom biofilm retreated to isolated patches, and E. mutabilis biofilm flourished.
Iron-rich stromatolites underlie the biofilms and consist of distinct laminae, recording spatial and temporal oscillations in physicochemical conditions and microbial activity. The stromatolites are composed of thin, wavy laminae with partially decayed E. mutabilis biofilm, representing microbial activity and iron precipitation under normal AMD conditions. Alternating with the wavy layers are thicker, porous, spongelike laminae composed of iron precipitated on and incorporated into radiating colonies of diatoms. These layers indicate episodic changes in water chemistry, allowing diatoms to temporarily dominate the system.
Microbialites are microbial sedimentary structures that constitute some of the oldest traces of life on Earth. By their deposition in a wide range of sedimentary environments and their presence throughout most of geological time, the sedimentological and geochemical signatures they preserve represent important paleoenvironmental archives for understanding Earth’s biological and geochemical co-evolution. Here we present a large microbialite collection containing more than 1370 curated specimens, covering all continents except Antarctica and spanning more than 3.5 Ga of Earth history, that is accessible to the international scientific community for examination and sampling at the Muséum National d’Histoire Naturelle (MNHN) in Paris, France. After cataloguing and evaluating the samples for their lithology, biogenicity, and inferred depositional environments, we characterized the collection for selected geochemical parameters, notably carbonate stable carbon and oxygen isotope ratios, as well as major, trace, and rare earth element compositions. Finally, we explore the different geochemical proxies analyzed with regards to their utility for reconstructing evolving Earth surface environments and/or microbial metabolisms via comparison of geochemical data from the MNHN Microbialite Collection to a compilation of similar proxy data for carbonates worldwide. We demonstrate that certain temporal trends previously recognized in carbonates worldwide (e.g., with respect to variations in C and O stable isotope compositions and redox sensitive trace element enrichments) are well reflected in this collection. Our findings highlight the utility of the MNHN Microbialite Collection and microbialites more generally for reconstructing the conditions associated with habitable environments in deep time and for tracing the response of microbial communities to the geochemical evolution of Earth’s surface.
Abstract Stromatolites are laminated biosedimentary structures of great importance for paleobiological, paleoecological, and paleoenvironmental analyses, mainly in Precambrian rocks. Their value is related to the glimpse of past life recorded in their lamination, fabric, and, eventually, due to the preservation of organic matter, including microfossils, and because their deposition is directly influenced by environmental conditions. Although stromatolites are widely described in microscopic scale, there is a lack of standardization of their nomenclature, precluding better paleoenvironmental and paleobiological interpretations. In this study, we propose a guide for the microscopic analysis of fossil stromatolites and, possibly, thrombolites, and provide a review of specialized literature and the bibliometric context of main terms. The goal is to contribute to the improvement of their application through systematization of microscopic data, in the face of novel paleoecological and paleobiological approaches and for astrobiological prospection for microbialites in therock record of Mars.
The explanation of the origin of microbialites and specifically stromatolitic structures is a problem of high relevance for decoding past sedimentary environments and deciphering the biogenicity of the oldest plausible remnants of life. We have investigated the morphogenesis of gypsum stromatolite-like structures currently growing in shallow ponds (puquíos) in the Salar de Llamara (Atacama Desert, Northern Chile). The crystal size, aspect ratio, and orientation distributions of gypsum crystals within the structures have been quantified and show indications for episodic nucleation and competitive growth of millimetric to centimetric selenite crystals into a radial, branched, and loosely cemented aggregate. The morphogenetical process is explained by the existence of a stable vertical salinity gradient in the ponds. Due to the non-linear dependency of gypsum solubility as a function of sodium chloride concentration, the salinity gradient produces undersaturated solutions, which dissolve gypsum crystals. This dissolution happens at a certain depth, narrowing the lower part of the structures, and producing their stromatolite-like morphology. We have tested this novel mechanism experimentally, simulating the effective dissolution of gypsum crystals in stratified ponds, thus providing a purely abiotic mechanism for these stromatolite-like structures.
The travertine deposits could provide a prospective geomicrobiological snapshot of the early Earth considering its the similarity with ancient stromatolites. Herein, we investigated the travertine carbonate deposits in a calcareous and sulfidic hot spring at Sipoholon, Northern Sumatra, Indonesia, with an emphasis on the formation of a laminated texture similar to one observed in the stromatolites. The travertine deposits were associated with microbial mats of different colors that change from yellow, pink, and green, lowering water temperature and H2S concentration along the water passages. A phylotype analysis of the Prokaryotic 16S rRNA gene revealed that chemolithoautotrophic sulfur and hydrogen oxidizing bacteria were abundant in the yellow sediment around vent sites. At the same time, photoheterotrophs, a linage of Anaerolineae, dominated the pink to green sediment at mid- to downstream. Laminated textures are developed in pink and green-colored sediments among these categories. Monitoring water chemical parameters and sediment texture for an Anaerolineae-dominated sample collected from a midstream site revealed that daily lamination was formed due to the daytime biofilm formation under the stable chemical composition of water conditions except for dissolved oxygen concentration. Daily laminations were also formed in other samples by microbial communities with lower Anaerolineae abundance. Further, the results showed that if mineral precipitation and microbial growth rates are appropriately balanced, cyanobacteria were not essential for the lamina development. This indicating that such a geochemical system could occur in early-time stromatolites even before the evolution of cyanobacteria. The microbial-mineral precipitation systems identified in this study provide significant insights into the formation of stromatolites, which can in turn shed light on the history of early Earth.
We report morphology and microstructure of the stromatolites of the Ganjingzi Formation in southern Liaoning. Sedimentologic and morphologic analyses indicate that the lower stromatolite mounds formed in a transgressive succession, while the stromatolite columns in the more complex upper biostrome changed vertically from dispersed growth to dense clumping. Biostratigraphic analysis shows that the stromatolites in the Ganjingzi Formation are similar to those from coeval strata in the Xuzhou-Huainan Region and in southern Jilin. Comparisons of the morphotype genera of stromatolites and the sedimentary setting between different areas, imply that sea-level was fluctuating in the east of the North China Craton (NCC) during the Ganjingzi interval and that the transgressions were beneficial to stromatolite growth, as indicated by the increased number of stromatolites in the study area.
Following the end-Permian crisis, microbialites were ubiquitous worldwide. For instance, Triassic deposits in the Germanic Basin provide a rich record of stromatolites as well as of microbe-metazoan build-ups with nonspicular demosponges. Despite their palaeoecological significance, however, all of these microbialites have only rarely been studied. This study aims to fill this gap by examining and comparing microbialites from the Upper Buntsandstein (Olenekian, Lower Triassic) and the lower Middle Muschelkalk (Anisian, Middle Triassic) in Germany. By combining analytical petrography (optical microscopy, micro X-ray fluorescence, and Raman spectroscopy) and geochemistry (δ13Ccarb, δ18Ocarb), we show that all the studied microbialites formed in slightly evaporitic environments. Olenekian deposits in the Jena area and Anisian strata at Werbach contain stromatolites. Anisian successions at Hardheim, in contrast, host microbe-metazoan build-ups. Thus, the key difference is the absence or presence of nonspicular demosponges in microbialites. It is plausible that microbes and nonspicular demosponges had a mutualistic relationship, and it is tempting to speculate that the investigated microbial-metazoan build-ups reflect an ancient evolutionary and ecological association. The widespread occurrence of microbialites (e.g., stromatolites/microbe-metazoan build-ups) after the catastrophe may have resulted from suppressed ecological competition and the presence of vacant ecological niches. The distribution of stromatolites and/or microbe-metazoan build-ups might have been controlled by subtle differences in salinity and water depth, the latter influencing hydrodynamic processes and nutrient supply down to the microscale. To obtain a more complete picture of the distribution of such build-ups in the earth’s history, more fossil records need to be (re)investigated. For the time being, environmental and taphonomic studies of modern nonspicular demosponges are urgently required.
The design and construction of sustainable and durable civil infrastructure provides modern societies higher qualities of life. Continued population growth and urbanization, however, is increasing global demand for building materials, like concrete and steel, whose production are not without environmental consequences. Reducing the environmental impacts of construction materials through the development of innovative, sustainable, and durable material technologies is critical if urban environments are ever to thrive in harmony with the natural world. Civil engineers can aspire to achieve equilibrium with the natural world by drawing inspiration from nature and implementing many of its design principles. This chapter provides a state-of-the-art review of the field of engineered living materials (ELMs) that are specifically designed for construction applications. ELM technologies based on microorganisms, fungal mycelium, and plants are reviewed in light of their biological functions and end-use applications. In addition, challenges that new ELM technologies designed for the built environment must overcome, including economic feasibility, uncertainty, scale-up, long-term organism viability, and biocontainment, are also reviewed and discussed herein.KeywordsEngineered living materials (ELMs)Self-healing materialsLiving building materials (LBMs)BiocementationLiving façadesMycotectureSoil stabilizationAlgae building technology
Stromatolites and microbialites contain a rich repository of environmental and biological information. Despite extensive research, questions remain regarding the biological, chemical, and physical processes that control stromatolite macro, meso, and microstructure. We report unusual deep water cuspate stromatolites from the Cryogenian Trezona Formation, South Australia, from a mixed siliciclastic–carbonate open marine ramp setting. Cuspate stromatolite horizons develop near the base of decameter-scale transgression–regression cycles and typically overlie decimeter-scale irregular erosion surfaces. The cuspate structure within the stromatolites form near vertical, stacked cusp structures in cross section. In plan view, the cusps form cm-scale sharp parallel ridges oriented perpendicular to the regional downslope direction and perpendicular to the elongation direction of stromatolites. Stromatolites colonized topographic highs of irregular erosion surfaces (often hardgrounds) and grew in carbonate supersaturated, iron-rich marine waters in low turbidity sediment-starved settings. Cuspate stromatolites are interpreted as forming in deep water environments during maximum transgression as condensed intervals. Their microbial metabolism may require low light and low oxygen. A deep water origin for the Trezona Formation cuspate stromatolites and other Precambrian cuspate stromatolites suggests a link between the cuspate morphology and physical/chemical (carbonate supersaturated, low light, and low oxygen) conditions of Precambrian deep water marine settings.
Iron isotope compositions of sedimentary pyrites (FeS2) are used to constrain the redox evolution of the Precambrian ocean and early Fe-based metabolisms such as Dissimilatory Iron Reduction (DIR). Sedimentary pyrites can record biotic and abiotic iron reduction, which have similar ranges of Fe isotopic fractionation, as well as post-depositional histories and metamorphic overprints that can modify Fe isotope compositions. However, some exceptionally well-preserved sedimentary records, such as the stromatolite-bearing Tumbiana Formation (ca. 2.7 Ga, Western Australia) have been proven to retain primary information on Early Neoarchean microbial ecosystems and associated metabolic pathways. Here, we present in situ Fe isotope measurements of micropyrites included in four stromatolites from the Tumbiana Formation in order to assess iron respiration metabolism using Fe isotope signatures. A set of 142 micropyrites has been analyzed in three lamina types, i.e. micritic, organic-rich and fenestral laminae, by Secondary Ion Mass Spectrometry (SIMS), using a Hyperion radio-frequency plasma source. The diversity of laminae is attributed to specific depositional environments, leading to the formation of Type 1 (micritic laminae) and Type 2 (organic-rich laminae) and early diagenetic effects (Type 3, fenestral laminae). Type 1 and 2 laminae preserved comparable δ⁵⁶Fe ranges, respectively from -1.76% to +4.15% and from -1.54% to +4.44%. Type 3 laminae recorded a similar range, although slightly more negative δ⁵⁶Fe values between -2.20% and +2.65%. Globally, our data show a large range of δ⁵⁶Fe values, from -2.20% to +4.44%, with a unimodal distribution that differs from the bimodal distribution previously reported in the Tumbiana stromatolites. Such a large range and unimodal distribution cannot be explained by a unique process (e.g., biotic/abiotic Fe reduction or pyrite formation only controlled by the precipitation rate). It rather could reflect a two-step iron cycling process in the sediment pore water including i) partial Fe oxidation forming Fe(OH)3 with positive δ⁵⁶Fe values followed by ii) partial, possibly microbially induced, Fe reduction leading to Fe²⁺ availability for pyrite formation by sulfate reducers carrying both a negative δ⁵⁶Fe and δ³⁴S signatures. In this model, the buildup and subsequent reduction through time of a residual Fe(OH)3 reservoir arising from the activity of methanotrophs, can explain the strongly positive δ⁵⁶FeFe(OH)3 values up to 4%. These results indicate that Archean microbial mats have been the site of the interaction of several closely linked biogeochemical cycles involving Fe, S and C.
Following the End Permian Mass Extinction (EPME) event, global stromatolite distributions rapidly increased. There are three documented examples of stromatolites occurring in intracontinental settings following the EPME; Greenland, Madagascar and west Australia’s Northern Perth Basin. We are reporting the west Australian stromatolite occurrence to be much further than previously thought, through detailed mapping and analysis of stromatolites in the Northampton region. A review of the stromatolites show alternating stromatolite morphologies, with a minimum of four phases. These alternating morphologies are attributed to environmental changes in energy and detrital input, as well as microbe community induced structural changes. Two primary morphologies are documented, large smooth domal and micro-digitate structures. Using sedimentological characteristics of the conglomerates below and the stromatolite unit itself, a revised palaeo-environmental model is proposed for the growth of the stromatolitic unit. The model places the system in an intracontinental marine setting with shallow basinal margins defined by the mid-Palaeozoic Tumblagooda Sandstone. Mass flow deposits come in from the margins, defined by cobble conglomerates, on which the stromatolites grow. The stromatolites have previously been considered to be Early Triassic in age. However, revised stratigraphic relationships suggest they may be older than previously thought. Raising the question, do the northern Perth Basin stromatolites truly correlate with a mass extinction event?
The Late Quaternary carbonate sediments and sedimentary rocks from the platform off western India were reviewed for their genesis and relationship with their ancient counter parts. Sub-marine cemented and vadose diagenetic limestones were recovered at different locations on the platform and, neomorphic limestones and caliche pisolites were recovered on the continental shelf south of the platform. Dolomites on the platform were primary and formed by microbial processes under hypersaline, sulphate-reducing conditions during the lowered sea levels. Aragonite ooids were formed from the mineralization of microbial filaments that enveloped their cortex portions. Phosphorites were found in organic-rich, aragonite muds on the continental slope adjacent to the platform and formed from the microbial mineralization of organic matter and replacement of carbonate by apatite during early diagenesis. Microbial processes thus played an important role in the formation of dolomites, ooids and phosphorites reported here and those in ancient deposits. Halimeda bioherms on the platform were grown luxuriantly from the nutrients brought by upwelling currents during the Late Pleistocene–Early Holocene sea level transgression and are similar to the Holocene–Recent deposits in the Indo-Pacific region. Lime muds were bio-detrital and formed primarily from the disintegration of Halimeda bioherms and carbonate skeletal on the platform and then exported to the slope. They resemble fine-grained limestones abundantly reported in ancient platforms. Thus, the different carbonate components on the platform are genetically related to their ancient ones and serve as Late Quaternary analogues for the ancient platform carbonates.
Stromatolites are organosedimentary build-ups that have formed as a result of the sediment trapping, binding and precipitating activities of microbes. Today, extant systems provide an ideal platform for understanding the structure, composition, and interactions between stromatolite-forming microbial communities and their respective environments. In this study, we compared the metagenomes of three prevalent stromatolite-forming microbial mat types in the Spaven Province of Hamelin Pool, Shark Bay located in Western Australia. These stromatolite-forming mat types included an intertidal pustular mat as well as a smooth and colloform mat types located in the subtidal zone. Additionally, the metagenomes of an adjacent, non-lithifying mat located in the upper intertidal zone were also sequenced for comparative purposes. Taxonomic and functional gene analyses revealed distinctive differences between the lithifying and non-lithifying mat types, which strongly correlated with water depth. Three distinct populations emerged including the upper intertidal non-lithifying mats, the intertidal pustular mats associated with unlaminated carbonate build-ups, and the subtidal colloform and smooth mat types associated with laminated structures. Functional analysis of metagenomes revealed that amongst stromatolite-forming mats there was an enrichment of photosynthesis pathways in the pustular stromatolite-forming mats. In the colloform and smooth stromatolite-forming mats, however, there was an increase in the abundance of genes associated with those heterotrophic metabolisms typically associated with carbonate mineralization, such as sulfate reduction. The comparative metagenomic analyses suggest that stromatolites of Hamelin Pool may form by two distinctive processes that are highly dependent on water depth. These results provide key insight into the potential adaptive strategies and synergistic interactions between microbes and their environments that may lead to stromatolite formation and accretion.
In the western part of the Anti Atlas, the Cambrian formations known since long (Nice, 1924; Bourcard, 1927; Bung & Netlner, 1933), have been the subject of a very detailed stratigraphic approach. The sedimentological work affecting the lower Cambrian of the Western Anti Atlas is rare. It concerns mainly: a lithostratigraphic description and a summary interpretation of the environmental sedimentary formations.
Indeed, recent sedimentological data allowed a new lithostratigraphic division of the Lower Cambrian succession in three lithostratigraphic series. These, are characterized, each one, by a particular microbialitic builds ups, resulted, probably, from Lower Cambrian marine ecosystem variations. The first step, the stromatolitic series, represents a long period in which a large area of the Anti-Atlasic platform was occupied by peritidal cyanobacterial mats.
The second step, represented by the Thrombolites series, marks a global sea level rise (Thrombolites proliferation) relayed by a massive silico-clastic supply inputs. And the third stage of evolution during which the medium sufficiently deepened to allow the installation of the distal facies platform. The development of the Archaeocyaths seems to be one of the precursors of dendritic form acquired by certain microbial communities.
Dating Neoproterozoic, the Archaeocyaths of Ifrane, whose morphological diversity and the microbial communities associated with them, are a focus for the industry of ornamental rocks in Ifrane in the Moroccan western Anti-Atlas.
In this area, the structural diversity of the Archaeocyaths is very pronounced, which facilitates the systematic division and their use in stratigraphy. The perforations or pores vary in number, size and arrangement, those whose walls are stretched in the subdivision pore channels or tubes which may be linear, bent, or inclined in the case of thicker walls.
Indeed, the main objective of this work is to highlight the “Archaeocyathans mounds” as a geological heritage through the study of varied silico-clastic and carbonate predominate components facies. Regrettably, the surexploitation of the Archaeocyaths in the ornamental industry and mining activities may lead to their death in the absence of regulatory laws of their exploitation. Clues are offered to make the Archaeocyaths a geological site, a geological destination, thing that necessitate the conduct of a number of approaches, the mobilization of various networks of actors but also the establishment or adaptation of infrastructure, home or mediation.
With the development of space technology, exobiology has been established as a scientific discipline. The term ‘exobiology’ was introduced by Lederberg (1960), who recognised the emerging capabilities of space exploration for giving new impetus to biology much as the development of astronomy has broadened our understanding of the physical world and the spectral analysis of the stars has proven the universality of the concepts of chemistry. Classical biological research has concentrated on the only example of life so far known, i.e. life on Earth. In contrast, exobiology extends the boundaries of biological investigations beyond the Earth, to other planets, comets, meteorites, and space at large. To date, the over-riding objective of exobiological research has been to attain a better understanding of the principles leading to the emergence of life from inanimate matter, its evolution, and its distribution on Earth and throughout the Universe (Klein 1986). To reach this goal, exobiological research has focused on the different steps of the evolutionary pathways through cosmic history that may be related to the origin, evolution and distribution of life.
A recent field-intensive program in Shark Bay, Western Australia provides new multi-scale perspectives on the world’s most extensive modern stromatolite system. Mapping revealed a unique geographic distribution of morphologically distinct stromatolite structures, many of them previously undocumented. These distinctive structures combined with characteristic shelf physiography define eight ‘Stromatolite Provinces’. Morphological and molecular studies of microbial mat composition resulted in a revised growth model where coccoid cyanobacteria predominate in mat communities forming lithified discrete stromatolite buildups. This contradicts traditional views that stromatolites with the best lamination in Hamelin Pool are formed by filamentous cyanobacterial mats. Finally, analysis of internal fabrics of stromatolites revealed pervasive precipitation of microcrystalline carbonate (i.e. micrite) in microbial mats forming framework and cement that may be analogous to the micritic microstructures typical of Precambrian stromatolites. These discoveries represent fundamental advances in our knowledge of the Shark Bay microbial system, laying a foundation for detailed studies of stromatolite morphogenesis that will advance our understanding of benthic ecosystems on the early Earth.
Stromatolitic microbial mats composed primarily of bacteria, cyanobacteria, eukaryotic algae are found in the cold, dimly-lit, perennially ice-covered antarctic lakes of southern Victoria Land, Antarctica (77°32-43’S, 161° 33’ -163°7’ E). The morphology of a particular mat results from a combination of biological, geochemical, and sedimentological processes, some of which may be unique to ice-covered lakes. Prostrate, lift-off, columnar, and pinnacle mats are trapping, binding and/or precipitating carbonates and various other minerals forming organosedimentary structures. The ice-covered lakes of Antarctica may serve as an important model for understanding the formation of stromatolites in cold environments. Studies of antarctic stromatolitic mats enhance our understanding of the range of environmental conditions capable of supporting stromatolite formation, particularly cold facies including those present during the Precambrian.
Causes of lamination, the most salient property of stromatolitic structures, are examined in terms of sedimentary kinetics and stasis using case histories of modern stromatolite-building biota from the Bahama Carbonate Platform, Great Sippewissett Salt Marsh of New England, and Hamelin Pool, Shark Bay, Australia. The findings are compared with fossil evidence preserved in silicified Mesoproterozoic stromatolites of the Gaoyuzhuang Formation, northern China. Multitrichomous cyanobacteria and their responses to sedimentation characterize the conditions of fluctuating sedimentation rates, whereas coccoid cyanobacteria colonize and stabilize sediments during periods of sedimentary stasis.
We report here the discovery of the first modern (Holocene) bluegreen algal stromatolites from Antarctica. Several types occur within these permanently ice-covered, dimly lit, nonturbulent, seasonally glacier-fed lakes, where the habitats vary chemically from fresh-to-saline and anoxic to oxygen-supersaturated. The dominant blue-green alga and principal stromatolite builder in all types is the oscillatoriaceous species, Phormidium frigidum, which can grow under the wide range of microhabitat conditions occurring in these Antarctic lakes and in the absence of competitive exclusion by eukaryotes, or significant browsing or burrowing by Metazoa. Precipitation of calcite and binding or trapping of sediments give the bluegreen algal mats internal laminae, preserving them as stromatolites. We suggest these frigid Antarctic lake stromatolites may be analogs of those that once inhabited deepwater Precambrian ecosystems.
A wide variety of calcareous nodules have been regarded as being probably formed by the activity of cyanophytes. Stromatolites too, are generally assumed to be mainly of cyanophyte origin. But it is only possible to be confident about these interpretations if the deposits contain direct internal evidence of the algae involved. In the Recent this can be provided by the presence of the living algae themselves, but in ancient material the evidence must be in the form of mineralized fossil remains or distinctive petrographic fabrics. In many cases it must be admitted that a cyanophyte origin for stromatolites and oncoids is inferred only from general similarities in form and structure between them and Recent cyanophyte mats which have trapped and bound particulate sediment. Specific evidence for the type of algae (or other microorganisms) involved is usually lacking. However, some fossil examples of stromatolites and oncoids contain convincing evidence of their cyanophyte origin due to the presence of mineralized algal remains within them. Many of the Precambrian silicified microfloras, which provide valuable information about the early history of life on Earth, occur within stromatolites. Examples include the microfloras from the Transvaal Dolomite, Gunflint Iron Formation, Belcher Group, Paradise Creek Formation, Bungle Bungle Dolomite, Beck Spring Dolomite, Bitter Springs Formation, and many others (Schopf 1977, Table 2).
Modern decimeter-scale columnar stromatolites from Lake Joyce, Antarctica, show a change in branching pattern during a period of lake level rise. Branching patterns correspond to a change in cyanobacterial community composition as preserved in authigenic calcite crystals. The transition in stromatolite morphology is preserved by mineralized layers that contain microfossils and cylindrical molds of cyanobacterial filaments. The molds are composed of two populations with different diameters. Large diameter molds (>2.8 μm) are abundant in calcite forming the oldest stromatolite layers, but are absent from younger layers. In contrast, <2.3 μm diameter molds are common in all stromatolites layers. Loss of large diameter molds corresponds to the transition from smooth-sided stromatolitic columns to branched and irregular columns. Mold diameters are similar to trichome diameters of the four most abundant living cyanobacteria morphotypes in Lake Joyce: Phormidium autumnale morphotypes have trichome diameters >3.5 μm, whereas Leptolyngbya antarctica, L. fragilis, and Pseudanabaena frigida morphotypes have diameters <2.3 μm. P. autumnale morphotypes were only common in mats at <12 m depth. Mats containing abundant P. autumnale morphotypes were smooth, whereas mats with few P. autumnale morphotypes contained small peaks and protruding bundles of filaments, suggesting that the absence of P. autumnale morphotypes allowed small-scale topography to develop on mats. Comparisons of living filaments and mold diameters suggest that P. autumnale morphotypes were present early in stromatolite growth, but disappeared from the community through time. We hypothesize that the mat-smoothing behavior of P. autumnale morphotypes inhibited nucleation of stromatolite branches. When P. autumnale morphotypes were excluded from the community, potentially reflecting a rise in lake level, short-wavelength roughness provided nuclei for stromatolite branches. This growth history provides a conceptual model for initiation of branched stromatolite growth resulting from a change in microbial community composition.
© 2015 John Wiley & Sons Ltd.
Spatial and temporal distribution and characteristics of three eukaryotic biofilms were monitored for an 18-month period in an acid mine drainage environment at the Green Valley coalmine in Indiana, USA. Each biofilm is dominated ([90 %) by a single eukaryotic microorganism based on enumeration: Euglena mutabilis, the diatom species Nitzschia tubicola, and a filamentous alga belonging to the genus Klebsormidium sp. The E. mutabilis- dominated biofilm occurs year round, covering up to 100 % of the channel bottom in spring and fall. The N. tubicola-dominated biofilm is less abundant, exists as small patches in spring and fall, expands from these patches to cover up to 50 % of the channel bottom in June, and is absent in winter. The Klebsormidium-dominated biofilm is restricted to small patches covering \5 % of the channel bottom from spring through fall and is absent in winter. Also present are floating microbial scum layers. The eukaryotic biofilms and scum layers contribute to the attenuation of precipitates and to the formation of orga- nosedimentary structures, or stromatolites, by trapping and binding chemical precipitates via aerotaxis and phototaxis and by serving as a medium for passive accumulation of precipitates. Each stromatolite layer represents the morphological characteristics of each eukaryotic biofilm that served as the architect of the layer and the time of year the biofilm populated the channel. Processes involved in stromatolite formation also attenuate chemical sediments by binding them to the channel bottom and prior stromatolite surface rather than allowing them to be carried to the adjacent drainage system where they may become bioavailable to other forms of life.
Abstract The ∼5.3-6.0 million-year-old evaporitic gypsum deposits of Cyprus and Crete contain a variety of stromatolites that formed during the Messinian salinity crisis. We recognize four stromatolite morphotypes, including domical, conical, columnar, and flat-laminated structures. Observations of morphological and textural variations among the different morphotypes reveal significant diversity and complexity in the nature of interactions between microorganisms, gypsum deposition, and gypsum crystal growth. Nonbiological processes (detrital gypsum deposition, in situ crust precipitation, syntaxial crystal growth, subsurface crystal growth, and recrystallization) interacted with inferred microbial processes (including localized growth of biofilms, trapping and binding of grains in mats, nucleation of gypsum on cells) to produce distinct morphological-textural assemblages. Evidence for biological origins is clear in some stromatolite morphotypes and can come from the presence of microfossils, the spatial distribution of organic matter, and stromatolite morphology. In one stromatolite morphotype, the presence of the stromatolite, or the biota associated with it, may have determined the morphology of gypsum crystals. In some stromatolite morphotypes, definitive evidence of a microbial influence is not as clear. There are broad similarities between the Messinian gypsum stromatolites and carbonate stromatolites elsewhere in the geologic record, such as the formation of precipitated and granular layers; the development of domed, columnar, and conical morphotypes; the potential for microbes to influence mineral precipitation; and the recrystallization of deposits during burial. However, in detail the array of microbial-sedimentary-diagenetic process interactions is quite distinct in gypsiferous systems due to differences in the way gypsum typically forms and evolves in the paleoenvironment compared to carbonate. Unique aspects of the taphonomy of gypsum compared to carbonate chemical sediments, generally speaking, include the following: the potential for growth of individual crystals to determine the shape of a stromatolite (and possibly vice versa), a more diverse set of outcomes relating to preservation versus destruction of textures through crystal growth and recrystallization, and a greater likelihood of preserving microfossils through encapsulation in large crystals. These insights gained from the study of terrestrial gypsum sedimentary rocks provide valuable guidance for the search for clues to past life in sulfate chemical sediments on Mars. Key Words: Stromatolites-Gypsum-Evaporites-Mars-Biosignature. Astrobiology 13, 870-886.
A number of inorganic elements are now known to be essential to organisms. Chemical evolutionary processes involving carbon, hydrogen, nitrogen and oxygen have been studied intensively and extensively, but the other essential elements have been rather neglected in the studies of chemical and biological evolution. This article attempts to assess the significance of inorganic chemistry in chemical and biological evolutionary processes on the earth. Emphasis is placed on the catalytic effects of inorganic elements and compounds, and also on possible studies on the earliest sediments, especially banded iron formation and strata-bound copper from the inorganic point of view in the hope of shedding some light on the evolution of the environment and the biological effects on it.
The foregoing two lecturers covered the atmosphere, hydrosphere and lithosphere (rocks) during the earlier stages of the earth’s history. This lecture, accordingly, attempts to discuss the interactions between the environment (atmosphere/hydrosphere/lithosphere) and the biosphere from the inorganic chemistry in the study of chemical and biological evolution.
Abundant stromatolites have been found from the Neoproterozoic Jiudingshan and Niyuan formations in northern Jiangsu Province
and northern Anhui Province, South China. The stromatolites are mostly stratiform, rarely small domed and conical (Conophyton-like), with more or less clearly laminated structures. Well-preserved, silicified microbial mats containing mat-building
microfossils have been found in small domed, conical, and stratiform stromatolites of the Neoproterozoic Jiudingshan Formation
and Niyuan Formation. Main mat-producing species are Gloeodiniopsis suxianensis among the coccoids, and Siphonophycus inornatum and Siphonophycus sp. among the filamentous cyanophytes, although Myxococcoides sp., Leiosphaeridia sp., and Eoentophysalis robusta are often included in the community. Present study indicates that the morphology of a microbial mat, particularly first microbial
mat, plays an important role in the morphogenesis of stromatolites, and the community dominated by Gloeodiniopsis suxianensis, which is largely confined to stratiform stromatolites, may represent an intertide setting.
Biological activity of Euglena
mutabilis , an acidophilic, photosynthetic protozoan,
contributes to the formation of Fe-rich stromatolites in acid mine
drainage systems. E.
mutabilis is the dominant microbe in bright green benthic
mats (biofilm), coating drainage channels at abandoned coal mine sites
in Indiana. It builds biolaminates through phototactic and aerotactic
behavior, similar to prokaryotes, by moving through precipitates that
periodically cover the mats. E.
mutabilis also contributes to formation of Fe-rich
stromatolites by (1) intracellularly storing Fe compounds released after
death, contributing to the solid material of stromatolites and acting as
nucleation sites for precipitation of authigenic Fe minerals, and (2)
generating O2 via photosynthesis that further facilitates
precipitation of reduced Fe, any excess O2 not consumed by Fe
precipitation being released to the atmosphere. Recognition of E.
mutabilis dominated biofilm in acidic systems raises a
provocative hypothesis relating processes involved in formation of
Fe-rich stromatolites by E.
mutabilis to those responsible for development of
Precambrian stromatolitic Fe formations and oxygenation of the early
atmosphere.
Lake Vanda is a cold nonturbulent, perennially ice‐covered lake in the valleys of southern Victoria Land, Antarctica. Observations made and samples collected under the 3.5 m ice in 1980 by SCUBA divers reveal that an extensive benthic microbial mat dominated by the filamentous blue‐green algae (cyanobacteria) Phormidium frigidum and Lyngbya martensiana is growing there. As is the case in other Antarctic lakes investigated by us thus far, the mat in Lake Vanda traps and binds sediment and precipitates calcite and is undisturbed by grazers and burrowers. Therefore, stromatolitic laminae are being generated. Unlike the other Antarctic lakes investigated in this region, Lake Vanda has (a) an ice cover and water that transmits significantly more light; (b) an ice cover that is permeable to gases and aeolian sediment; (c) no zone of lift‐off mat where photosynthetically generated oxygen would render the mat buoyant and cause it to separate from the substrate and float away; and (d) mat that has a distinctive pinnacle macrostructure. Although the laminae being laid down by the Lake Vanda mat do not retain the cone and ridge morphology of the living mat, the pinnacle macrostructure of the mat is similar to the Precambrian Conophyton stromatolites as well as microbial structures forming in Yellowstone hot springs, freshwater marshes in the Bahamas, and hypersaline intertidal mats in Baja California, Mexico, and Shark Bay, Australia. This suggests (a) Conophyton‐like structures similar to those abundant during the Precambrian can form under widely varying environmental conditions and (b) high latitudes should not be overlooked as sites of formation of ancient stromatolites.
The search for evidence of life on Mars will require highly capable rovers. The strategy for the search for life divides into four major objectives: understanding the history of the Martian climate, finding evidence of chemical evolution, searching for evidence of past life, and searching for evidence of living organisms. To achieve these objectives, rovers will need to traverse distances of the order of 10 km to study a variety of relevant features (e.g., fluvial channels, shorelines, eskers, hydrothermal deposits) in sufficient detail to diagnose their formation processes. Requirements for rovers include color imaging at a range of spatial scales (wide angle for geologic context, high resolution for resolving millimeter- to centimeter-scale textures, submillimeter resolution of selected areas); in situ capabilities to perform chemical, mineralogical, and organic analysis; and the ability to collect samples of rocks, soils, and subsurface samples for return to Earth for detailed laboratory analysis. Simulations of rover missions to Mars and the Moon were performed for 3 days each in February 1995 using the Marsokhod rover deployed on Kilauea volcano, Hawaii, and operated via satellite communications from Moffett Field, California. During the simulations, science teams analyzed images taken by the Marsokhod and deduced the geologic setting and history of the field site. Imaging for the Mars mission simulation used mast-mounted monochrome stereo cameras with 30° field of view, 1.4 mrad/pixel resolution, 1.7 m height, 1 m horizontal spacing, and fixed pointing. A camera mounted on a manipulator arm was capable of resolving 1 mm features. During 18 hours of operation, the rover traversed 800 m of terrain, made observations at eight science targets, and obtained 52 stereo images and 18 arm camera images. For the lunar mission simulation, stereo cameras were mounted on a mast with a pointable platform at 1.5 m height and 10 cm horizontal spacing. Stereo color video was transmitted to mission operations. Command cycles with 1.5 s time delay allowed live teleoperation. In 15 hours of operation the rover traversed 1.2 km, and nine science targets were studied in detail. These experiments show that rovers can be used to successfully perform field geology on other planets.
The study of the algal mats in salinas used for salt production on Hai-nan Island (China) revealed the presence of five morphological types, which are dominated by different blue-green algae: (1) Orange high pinnacle mats dominated by Phormidium hypersalinum and P. hypolimneticum; (2) Pale green, low pinnacle mats dominated by P. hypolimneticum, P. hypersalinum and Aphanothece sp.; (3) Pale green spongy mats dominated by the unicellular blue-green algae Aphanothece sp., Chroococcus minutus, Aphanocapsa concharum; (4) Yellow green gelatinous mats dominated by P. hypolimneticum and Microcoleus tenerrimus; (5) Blue-green flat mats with finger-like projections dominated by M. tenerrimus and M. chthonoplastes. The distribution of these different mats is correlated to the main ecological parameters monitored: salinity, temperature and water depth. The role of the constituent blue-green algae and of the ecological parameters in the control of the composition and morphology of algal mats, as well as the implications of these findings for the interpretation of Precambrian stromatolites are discussed.
Small, partly silicified, laterally linked stromatolites exhibiting a high degree of inheritance in the shape of successively stacked laminae from the lower Bambuí Group (late Proterozoic), near Unaí, Minas Gerais, south-central Brazil, are here described and classified as Stratifera undata Komar 1966. Rare, moderately preserved microfossils and stratiform concentrations of poorly preserved probable microfossils consist essentially of coccoidal forms, there being no convincing evidence of filamentous forms. If, in fact, coccoidal micro-organisms were the overwhelmingly dominant mat-formers, this represents an unusual situation among silicified microflorules from morphologically distinct Precambrian stromatolites yet is quite similar to that observed in another form of Stratifera (S. biwabikensis from the Gunflint Iron-Formation), which, like the Unaí stromatolites, also formed under permanently submerged conditions. S. undata is now known in Brazil from similar upper Proterozoic settings in two very widely separated localities, which suggests its potential use in regional stratigraphic correlation. As a result of this study, we recommend that greater attention be paid to the ‘simpler’ stromatolite morphologies, especially when micro fossiliferous, since such forms commonly comprise the basal portions of biostratigraphically significant stromatolites and may share a common microstructure with these same, more complex forms. Inasmuch as microstructure is generally acknowledged as the stromatolite property most closely controlled by biological factors, study of silicified microflorules within simpler forms may permit inferences regarding not only the microbial communities responsible for more complex stromatolites having the same microstructure but also possible biological reasons for the succession of distinct stromatolite assemblages observed in upper Precambrian rocks.
Periods of widespread abundance of calcified cyanobacteria in marine environments are here termed cyanobacterial calcification episodes (CCEs). Intense such episodes occurred in the Cambrian-early Ordovician, Late Devonian and Mid-Late Triassic. Mild episodes occurred during the Early Carboniferous and Jurassic-Early Cretaceous. Possible episodes during parts of the Silurian and Permian await confirmation. Cyanobacterial calcification is not obligate but is dependent upon environmental factors favouring carbonate precipitation. Cyanobacterial calcification episodes are therefore interpreted as reflecting periods of increased marine carbonate precipitation. This is supported by the general abundance of marine carbonate cements and oolites during cyanobacterial calcification episodes.
Cyanobacterial calcification episodes correspond with Phanerozoic periods for which elevated global temperatures have been inferred. They do not appear to correlate closely with inferred changes in aragonite/calcite facilitation (the Sandberg curve) and P CO2 through time. It is concluded that variations in the precipitation rates of abiotic and quasi-abiotic marine carbonates through time have been controlled primarily by temperature. The mineralogy of these precipitates appears instead to have been mainly influenced by P CO2 and the Mg/Ca ratio, also linked to sea-level through plate tectonic processes. Thus, the controls on rate and mineralogy of precipitation were independent of one another.
Our quest to find the earliest forms of microbial life and understand
their role in the mediation of surficial processes has focused primarily on
bacteria, since they represent the most primitive forms of life and possess
the ability to contribute to the precipitation and dissolution of minerals
(e.g., Fortin et al., 1997; Little et al., 1997; Nealson and Stahl, 1997) and
the formation of biolaminated structures such as stromatolites (e.g., Awramik
et al., 1976; Golubic, 1976). Stromatolites are the mineralized counterparts
of microbial mats and represent some of the most tangible sources
of morphological, biological, and chemical evidence for life on early
Earth (e.g., Grotzinger and Knoll, 1999; Hofmann, 2000; Schopf et al.,
2007). Stromatolite microstructures preserve paleobiological and paleoenvironmental
information that can provide insight on the early evolution
of the biosphere, atmosphere, and geosphere. Today, as in the distant past,
bacteria are found in a range of environments from oxic to anoxic, thermal
to arctic, and acid to alkaline, and they are important in biofilms and
in rare, modern stromatolites (Cavicchioli, 2002; Gupta et al., 2004; Edwards
et al., 2005). Our understanding of bacteria is, therefore, important
in our search for similar life forms in extraterrestrial locations. Our focus
on the role of bacteria, however, has created a gap in our understanding
of the importance of eukaryotic microorganisms in some of these same
environments.
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