Biogeochemical cycling and microbial diversity in the thrombolitic microbialites of Highborne Cay,Bahamas

Thrombolites are unlaminated carbonate build‐ups that are formed via the metabolic activities of complex microbial mat communities. The thrombolitic mats of Highborne Cay, Bahamas develop in close proximity (1–2 m) to accreting laminated stromatolites, providing an ideal opportunity for biogeochemical and molecular comparisons of these two distinctive microbialite ecosystems. In this study, we provide the first comprehensive characterization of the biogeochemical activities and microbial diversity of the Highborne Cay thrombolitic mats. Morphological and molecular analyses reveal two dominant mat types associated with the thrombolite deposits, both of which are dominated by bacteria from the taxa Cyanobacteria and Alphaproteobacteria. Diel cycling of dissolved oxygen (DO) and dissolved inorganic carbon (DIC) were measured in all thrombolitic mat types. DO production varied between thrombolitic types and one morphotype, referred to in this study as ‘button mats’, produced the highest levels among all mat types, including the adjacent stromatolites. Characterization of thrombolite bacterial communities revealed a high bacterial diversity, roughly equivalent to that of the nearby stromatolites, and a low eukaryotic diversity. Extensive phylogenetic overlap between thrombolitic and stromatolitic microbial communities was observed, although thrombolite‐specific cyanobacterial populations were detected. In particular, the button mats were dominated by a calcified, filamentous cyanobacterium identified via morphology and 16S rRNA gene sequencing as Dichothrix sp. The distinctive microbial communities and chemical cycling patterns within the thrombolitic mats provide novel insight into the biogeochemical processes related to the lithifying mats in this system, and provide data relevant to understanding microbially induced carbonate biomineralization.     

Evidence of oxygenic phototrophy in ancient phosphatic stromatolites from the Paleoproterozoic Vindhyan and Aravalli Supergroups,India

Fossil microbiotas are rare in the early rock record, limiting the type of ecological information extractable from ancient microbialites. In the absence of body fossils, emphasis may instead be given to microbially derived features, such as microbialite growth patterns, microbial mat morphologies, and the presence of fossilized gas bubbles in lithified mats. The metabolic affinity of micro‐organisms associated with phosphatization may reveal important clues to the nature and accretion of apatite‐rich microbialites. Stromatolites from the 1.6 Ga Chitrakoot Formation (Semri Group, Vindhyan Supergroup) in central India contain abundant fossilized bubbles interspersed within fine‐grained in situ‐precipitated apatite mats with average δ¹³C_org indicative of carbon fixation by the Calvin cycle. In addition, the mats hold a synsedimentary fossil biota characteristic of cyanobacterial and rhodophyte morphotypes. Phosphatic oncoid cone‐like stromatolites from the Paleoproterozoic Aravalli Supergroup (Jhamarkotra Formation) comprise abundant mineralized bubbles enmeshed within tufted filamentous mat fabrics. Construction of these tufts is considered to be the result of filamentous bacteria gliding within microbial mats, and as fossilized bubbles within pristine mat laminae can be used as a proxy for oxygenic phototrophy, this provides a strong indication for cyanobacterial activity in the Aravalli mounds. We suggest that the activity of oxygenic phototrophs may have been significant for the formation of apatite in both Vindhyan and Aravalli stromatolites, mainly by concentrating phosphate and creating steep diurnal redox gradients within mat pore spaces, promoting apatite precipitation. The presence in the Indian stromatolites of alternating apatite‐carbonate lamina may result from local variations in pH and oxygen levels caused by photosynthesis–respiration in the mats. Altogether, this study presents new insights into the ecology of ancient phosphatic stromatolites and warrants further exploration into the role of oxygen‐producing biotas in the formation of Paleoproterozoic shallow‐basin phosphorites.     

Microbial diversity in modern marine stromatolites, Highborne Cay, Bahamas

Living marine stromatolites at Highborne Cay, Bahamas, are formed by microbial mat communities that facilitate precipitation of calcium carbonate and bind and trap small carbonate sand grains. This process results in a laminated structure similar to the layering observed in ancient stromatolites. In the modern marine system at Highborne Cay, lamination, lithification and stromatolite formation are associated with cycling between three types of microbial communities at the stromatolite surface (Types 1, 2 and 3, which range from a leathery microbial mat to microbially fused sediment). Examination of 923 universal small-subunit rRNA gene sequences from these communities reveals that taxonomic richness increases during transition from Type 1 to Type 3 communities, supporting a previous model that proposed that the three communities represent different stages of mat development. The phylogenetic composition also changes significantly between these community types and these community changes occur in concert with variation in biogeochemical rates. The dominant bacterial groups detected in the stromatolites include Alphaproteobacteria , Planctomycetes , Cyanobacteria and Bacteroidetes . In addition, the stromatolite communities were found to contain novel cyanobacteria that may be uniquely associated with modern marine stromatolites. The implications of these findings are discussed in the context of current models for stromatolite formation.     

Bacterially mediated precipitation in marine stromatolites

Stromatolites are laminated, lithified (CaCO₃) sedimentary deposits formed by precipitation and/or sediment accretion by cyanobacterial–bacterial mat communities. Stromatolites have been associated with these communities as far back as the Precambrian era some 2+ billion years ago. The means by which microbial communities mediate the precipitation processes have remained unclear, and are the subject of considerable debate and speculation. Two alternative explanations for microbially mediated precipitation include: (i) cyanobacterial photosynthesis increases pH in a system supersaturated in respect of CaCO₃, resulting in CaCO₃ precipitation and then laminated lithification, and (ii) decomposition of cyanobacterial extracellular organic matter (e.g. sheaths, mucilage and organic acids) by microheterotrophs leads to release of organic-bound Ca²⁺ ions and CaCO₃ precipitation. We evaluated these explanations by examining metabolically active, lithifying stromatolitic mat communities from Highborne Cay, Bahamas, using microautoradiography. Microautoradiographic detection of ¹⁴CO₂ fixation and ³H organic matter ( d -glucose and an amino acid mixture) utilization by photosynthetically active cyanobacteria and microheterotrophs, combined with community-level uptake experiments, indicate that bacteria, rather than cyanobacteria are the dominant sites of CaCO₃ deposition. In the oligotrophic waters in which stromatolites exist, microheterotrophs are reliant on the photosynthetic community as a main source of organic matter. Therefore, autotrophic production indirectly controls microbially mediated precipitation and stromatolite formation in these shallow marine environments.     

Biotic and Abiotic Imprints on Mg-Rich Stromatolites: Lessons from Lake Salda,SW Turkey

Abstract

Modern hydrated Mg rich stromatolites are actively growing along the shallow shorelines of Lake Salda (SW Turkey). An integrated approach involving isotopic, mineralogical, microscopic, and organic/geochemical techniques along with culture-independent molecular methods were applied to various lake samples to assess the role of microbial processes on stromatolite formation. This study further explores the biosignature preservation potential of fossil stromatolites by comparing with textures, lipid profiles and isotopic composition of the modern stromatolites. Similar lipid profile and δ¹³C isotope values in active and fossil stromatolites argue that CO₂ cycling delicately balanced between photosynthetic and heterotrophic (aerobic) activity as in the active ones may have regulated stromatolite formation in the lake. A decrease in the exopolymeric substances (EPS) profile of the mat and concurrent hydromagnesite precipitation imply a critical role for EPS in the formation of stromatolite. Consistently, a discrete, discontinuous lamination and clotted micropeloidal textures with cyanobacterial remnants in the fossil stromatolites likely refer to partial degradation of EPS, creating local nucleation sites and allowing precipitation of hydrated Mg minerals and provide a link to the active microbial mat in the modern stromatolites. Our results for the first time provide strong evidence for close coupling of cyanobacterial photosynthesis and aerobic heterotrophic respiration on hydromagnesite textures involved in the stromatolite formation of Lake Salda. The creation of photosynthesis induced high-pH conditions combined with a change in the amount and properties of the EPS and the repetition of these processes over time seems to be a possible pathway for stromatolite growth in the lake. Understanding these microbial symbioses and their mineralized records may provide new insights on the formation mechanism of Mg-rich carbonates not only for terrestrial geological records but also for planetary bodies like Mars, where hydrated Mg-carbonate deposits have been identified in possible paleolake deposits at Jezero crater, the landing site of the NASA Mars 2020 rover.     

Facies selectivity of benthic invertebrates in a Permian/Triassic boundary microbialite succession: Implications for the “microbialite refuge” hypothesis

Thrombolite and stromatolite habitats are becoming increasingly recognized as important refuges for invertebrates during Phanerozoic Oceanic Anoxic Events (OAEs); it is posited that oxygenic photosynthesis by cyanobacteria in these microbialites provided a refuge from anoxic conditions (i.e., the “microbialite refuge” hypothesis). Here, we test this hypothesis by investigating the distribution of ~34, 500 benthic invertebrate fossils found in ~100 samples from a microbialite succession that developed following the latest Permian mass extinction event on the Great Bank of Guizhou (South China), representing microbial (stromatolites and thrombolites) and non‐microbial facies. The stromatolites were the least taxonomically diverse facies, and the thrombolites also recorded significantly lower diversities when compared to the non‐microbial facies. Based on the distribution and ornamentation of the bioclasts within the thrombolites and stromatolites, the bioclasts are inferred to have been transported and concentrated in the non‐microbial fabrics, that is, cavities around the microbial framework. Therefore, many of the identified metazoans from the post‐extinction microbialites are not observed to have been living within a microbial mat. Furthermore, the lifestyle of many of the taxa identified from the microbialites was not suited for, or even amenable to, life within a benthic microbial mat. The high diversity of oxygen‐dependent metazoans in the non‐microbial facies on the Great Bank of Guizhou, and inferences from geochemical records, suggests that the microbialites and benthic communities developed in oxygenated environments, which disproves that the microbes were the source of the oxygenation. Instead, we posit that microbialite successions represent a taphonomic window for exceptional preservation of the biota, similar to a Konzentrat‐Lagerstätte, which has allowed for diverse fossil assemblages to be preserved during intervals of poor preservation.     

Effect of high atmospheric CO2 concentration on δ13 of algae

Precambrian reduced carbon is more depleted in¹³C than what would be expected from the carbon isotopic composition of modern marine algae and algal mats. Since the photosynthetic carbon fixation by algae is the most likely source of the reduced carbon, the depletion has been considered an anomaly.We examined factors that might have contributed to the carbon isotope fractionation from inorganic sources through algae to organic matter in a sedimentary rock, and related laboratory obtainable data to those from Precambrian rocks. Laboratory culture experiments were then performed with nine strains of algae at various concentrations of carbon dioxide, and the result was interpreted according to the relationship.It indicated that the depletion could be understood in terms of a combined effect of fractionation factors, most depletion occurring at the fractionation during the photosynthetic carbon fixation. It also suggested that all but one algal strain incorporated bicarbonate as the source of carbon for its growth. The exception was a thermophilic, acidophilic alga, which must have used carbon dioxide as the carbon source.The present study suggests that Precambrian atmosphere was enriched in carbon dioxide roughly two orders of magnitude more than its present atmospheric level.     

Carbon Dioxide Fixation by Metallosphaera yellowstonensis and Acidothermophilic Iron-Oxidizing Microbial Communities from Yellowstone National Park

The fixation of inorganic carbon has been documented in all three domains of life and results in the biosynthesis of diverse organic compounds that support heterotrophic organisms. The primary aim of this study was to assess carbon dioxide fixation in high-temperature Fe(III)-oxide mat communities and in pure cultures of a dominant Fe(II)-oxidizing organism (Metallosphaera yellowstonensis strain MK1) originally isolated from these environments. Protein-encoding genes of the complete 3-hydroxypropionate/4-hydroxybutyrate (3-HP/4-HB) carbon dioxide fixation pathway were identified in M. yellowstonensis strain MK1. Highly similar M. yellowstonensis genes for this pathway were identified in metagenomes of replicate Fe(III)-oxide mats, as were genes for the reductive tricarboxylic acid cycle from Hydrogenobaculum spp. (Aquificales). Stable-isotope (¹³CO₂) labeling demonstrated CO₂ fixation by M. yellowstonensis strain MK1 and in ex situ assays containing live Fe(III)-oxide microbial mats. The results showed that strain MK1 fixes CO₂ with a fractionation factor of ∼2.5‰. Analysis of the ¹³C composition of dissolved inorganic C (DIC), dissolved organic C (DOC), landscape C, and microbial mat C showed that mat C is from both DIC and non-DIC sources. An isotopic mixing model showed that biomass C contains a minimum of 42% C of DIC origin, depending on the fraction of landscape C that is present. The significance of DIC as a major carbon source for Fe(III)-oxide mat communities provides a foundation for examining microbial interactions that are dependent on the activity of autotrophic organisms (i.e., Hydrogenobaculum and Metallosphaera spp.) in simplified natural communities.     

Living phosphatic stromatolites in a low‐phosphorus environment: Implications for the use of phosphorus as a proxy for phosphate levels in paleo‐systems

In the geological record, fossil phosphatic stromatolites date back to the Great Oxidation Event in the Paleoproterozoic, but living phosphatic stromatolites have not been described previously. Here, we report on cyanobacterial stromatolites in a supratidal freshwater environment at Cape Recife, South African southern coast, precipitating Ca carbonate alternating with episodes of Ca phosphate deposition. In their structure and composition, the living stromatolites from Cape Recife closely resemble their fossilized analogues, showing phosphatic zonation, microbial casts, tunnel structures and phosphatic crusts of biogenic origin. The microbial communities appear to be also similar to those proposed to have formed fossil phosphatic stromatolites. Phosphatic domains in the material from Cape Recife are spatially and texturally associated with carbonate precipitates, but form distinct entities separated by sharp boundaries. Electron Probe Micro‐Analysis shows that Ca/P ratios and the overall chemical compositions of phosphatic precipitates are in the range of octacalcium phosphate, amorphous tricalcium phosphate and apatite. The coincidence in time of the emergence of phosphatic stromatolites in the fossil record with a major episode of atmospheric oxidation led to the assumption that at times of increased oxygen release the underlying increased biological production may have been linked to elevated phosphorus availability. The stromatolites at Cape Recife, however, form in an environment where ambient phosphorus concentrations do not exceed 0.28 μM, one to two orders of magnitude below the previously predicted minimum threshold of >5 μM for biogenic phosphate precipitation in paleo‐systems. Accordingly, we contest the previously proposed suitability of phosphatic stromatolites as a proxy for high ambient phosphate concentrations in supratidal to shallow ocean settings in earth history.     

Carbon isotope discrepancy between precambrian stromatolites and their modern analogs: Inferences from hypersaline microbial mats of the sinai coast

The isotopic composition of organic carbon from extant stromatolite-type microbial ecosystems is commonly slanted toward heavy ¹³ C values as compared to respective compositions of average organic matter (including that from Precambrian stromatolites). This seems the more enigmatic as the bulk of primary producers from benthic microbial communities are known to fix carbon via the C3 pathway normally entailing the sizable fractionations of the RuBP carboxylase reaction.There is reason to believe that the small fractionations displayed by aquatic microorganisms result from the limitations of a diffusion-controlled assimilatory pathway in which the isotope effect of the enzymatic reaction is largely suppressed. Apart from the diffusion-control exercised by the aqueous environment, transport of CO₂ to the photosynthetically active sites will be further impeded by the protective slime (polysaccharide) coatings commonly covering microbial mats in which gas diffusivities are extremely low. Ineffective discrimination against¹³C becomes, however, most pronounced in hypersaline environments where substantially reduced CO₂ solubilities tend to push carbon into the role of a limiting nutrient (brine habitats constitute preferential sanctuaries of mat-forming microbenthos since the emergence of Metazoan grazers 0.7 Ga ago). As the same microbial communities had been free to colonize normal marine environments during the Precambrian, the CO₂ concentration effect was irrelevant to the carbon-fixing pathway of these ancient forms. Therefore, it might not surprise that organic matter from Precambrian stromatolites displays the large fractionations commonly associated with C3 photosynthesis. Increased mixing ratios of CO₂ in the Precambrian atmosphere may have additionally contributed to the elimination of the diffusion barrier in the carbon-fixing pathways of ancient mat-forming microbiota.     

Autotrophy of green non-sulphur bacteria in hot spring microbial mats: biological explanations for isotopically heavy organic carbon in the geological record

Inferences about the evidence of life recorded in organic compounds within the Earth's ancient rocks have depended on ¹³C contents low enough to be characteristic of biological debris produced by the well-known CO₂ fixation pathway, the Calvin cycle. 'Atypically' high values have been attributed to isotopic alteration of sedimentary organic carbon by thermal metamorphism. We examined the possibility that organic carbon characterized by a relatively high ¹³C content could have arisen biologically from recently discovered autotrophic pathways. We focused on the green non-sulphur bacterium Chloroflexus aurantiacus that uses the 3-hydroxypropionate pathway for inorganic carbon fixation and is geologically significant as it forms modern mat communities analogous to stromatolites. Organic matter in mats constructed by Chloroflexus spp. alone had relatively high ¹³C contents (−14.9‰) and lipids diagnostic of Chloroflexus that were also isotopically heavy (−8.9‰ to −18.5‰). Organic matter in mats constructed by Chloroflexus in conjunction with cyanobacteria had a more typical Calvin cycle signature (−23.5‰). However, lipids diagnostic of Chloroflexus were isotopically enriched (−15.1‰ to −24.1‰) relative to lipids typical of cyanobacteria (−33.9‰ to −36.3‰). This suggests that, in mats formed by both cyanobacteria and Chloroflexus , autotrophy must have a greater effect on Chloroflexus carbon metabolism than the photoheterotrophic consumption of cyanobacterial photosynthate. Chloroflexus cell components were also selectively preserved. Hence, Chloroflexus autotrophy and selective preservation of its products constitute one purely biological mechanism by which isotopically heavy organic carbon could have been introduced into important Precambrian geological features.     

Modern conophyton‐like microbial mats discovered in Lake Vanda,Antarctica

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.     

Growth of elaborate microbial pinnacles in Lake Vanda,Antarctica

Microbial pinnacles in ice‐covered Lake Vanda, McMurdo Dry Valleys, Antarctica, extend from the base of the ice to more than 50 m water depth. The distribution of microbial communities, their photosynthetic potential, and pinnacle morphology affects the local accumulation of biomass, which in turn shapes pinnacle morphology. This feedback, plus environmental stability, promotes the growth of elaborate microbial structures. In Lake Vanda, all mats sampled from greater than 10 m water depth contained pinnacles with a gradation in size from <1‐mm‐tall tufts to pinnacles that were centimeters tall. Small pinnacles were cuspate, whereas larger ones had variable morphology. The largest pinnacles were up to ~30 cm tall and had cylindrical bases and cuspate tops. Pinnacle biomass was dominated by cyanobacteria from the morphological and genomic groups Leptolyngbya, Phormidium, and Tychonema. The photosynthetic potential of these cyanobacterial communities was high to depths of several millimeters into the mat based on PAM fluorometry, and sufficient light for photosynthesis penetrated ~5 mm into pinnacles. The distribution of photosynthetic potential and its correlation to pinnacle morphology suggests a working model for pinnacle growth. First, small tufts initiate from random irregularities in prostrate mat. Some tufts grow into pinnacles over the course of ~3 years. As pinnacles increase in size and age, their interiors become colonized by a more diverse community of cyanobacteria with high photosynthetic potential. Biomass accumulation within this subsurface community causes pinnacles to swell, expanding laminae thickness and creating distinctive cylindrical bases and cuspate tops. This change in shape suggests that pinnacle morphology emerges from a specific distribution of biomass accumulation that depends on multiple microbial communities fixing carbon in different parts of pinnacles. Similarly, complex patterns of biomass accumulation may be reflected in the morphology of elaborate ancient stromatolites.     

Organic geochemical studies of modern microbial mats from Shark Bay: Part I: Influence of depth and salinity on lipid biomarkers and their isotopic signatures

The present study investigated the influence of abiotic conditions on microbial mat communities from Shark Bay, a World Heritage area well known for a diverse range of extant mats presenting structural similarities with ancient stromatolites. The distributions and stable carbon isotopic values of lipid biomarkers [aliphatic hydrocarbons and polar lipid fatty acids (PLFAs)] and bulk carbon and nitrogen isotope values of biomass were analysed in four different types of mats along a tidal flat gradient to characterize the microbial communities and systematically investigate the relationship of the above parameters with water depth. Cyanobacteria were dominant in all mats, as demonstrated by the presence of diagnostic hydrocarbons (e.g. n‐C₁₇ and n‐C_17:1). Several subtle but important differences in lipid composition across the littoral gradient were, however, evident. For instance, the shallower mats contained a higher diatom contribution, concordant with previous mat studies from other locations (e.g. Antarctica). Conversely, the organic matter (OM) of the deeper mats showed evidence for a higher seagrass contribution [high C/N, ¹³C‐depleted long‐chain n‐alkanes]. The morphological structure of the mats may have influenced CO₂ diffusion leading to more ¹³C‐enriched lipids in the shallow mats. Alternatively, changes in CO₂ fixation pathways, such as increase in the acetyl COA‐pathway by sulphate‐reducing bacteria, could have also caused the observed shifts in δ¹³C values of the mats. In addition, three smooth mats from different Shark Bay sites were analysed to investigate potential functional relationship of the microbial communities with differing salinity levels. The C_25:1 HBI was identified in the high salinity mat only and a lower abundance of PLFAs associated with diatoms was observed in the less saline mats, suggesting a higher abundance of diatoms at the most saline site. Furthermore, it appeared that the most and least saline mats were dominated by autotrophic biomass using different CO₂ fixation pathways.     

Carbon and nitrogen dynamics in a maritime Antarctic stream

• 1 The carbon and nitrogen dynamics in a maritime Antarctic lake outflow stream were investigated. The stream and the algal communities could be split into two zones: a semi-aquatic margin consisting of a perennial cyanobacteria/diatom mat and a flowing channel with a similar perennial mat that was overgrown by annual filamentous chlorophytes during the course of the summer.
• 2 Neither algal community was limited by nutrient availability. Major nutrients were always available in the stream water. There were slight differences in the atomic ratios of the mats, the N:P ratios in the channel mat being lower than those in the marginal mat. However, both these and the total dissolved N:P ratio in the stream water were all close to those that indicate a balanced supply.
• 3 There was no net carbon or nitrogen accumulation by the marginal mat suggesting that uptake processes were balanced by loss processes.
• 4 Maximum rates of carbon fixation (0.1–0.5mgCg^?1 dry weight h^?1) were similar to those of other perennial Antarctic algal mats. Productivity appeared to be limited by physical factors, but the effects of irradiance and temperature could not be separated.
• 5 There were no heterocystous cyanobacteria in the mat communities and rates of atmospheric nitrogen fixation were very low (0–10ngNmg^?1 mat Nh^?1). Fixation accounted for only 0.3% of the nitrogen accumulation of the channel mats, but was higher in the marginal mat where uptake of other sources of nitrogen was also low.
• 6 Nitrogen accumulation by the channel mat averaged 0.34gNm^?2 day^?1. Only 0.05gNm^?2 day^?1 was accounted for by the uptake of dissolved inorganic nitrogen (nitrate plus ammonium). The major (80%) source of nitrogen appeared to be dissolved organic nitrogen. Recycling of nitrogen within the stream ecosystem may also be important.

     

Microbial mats and the early evolution of life

Microbial mats have descended from perhaps the oldest and most widespread biological communities known. Mats harbor microbes that are crucial for studies of bacterial phylogeny and physiology. They illustrate how several oxygen-sensitive biochemical processes have adapted to oxygen, and they show how life adapted to dry land long before the rise of plants. The search for the earliest grazing protists and metazoa in stromatolites is aided by observations of mats: in them, organic compounds characteristic of ancient photosynthetic protists can be identified. Recent mat studies suggest that the 13C/12C increase observed over geological time in stromatolitic organic matter was driven at least in part by a long-term decline in atmospheric carbon dioxide levels.     

Temperature effects on carbon and nitrogen metabolism in some Maritime Antarctic freshwater phototrophic communities

Biofilms growing on ice and benthic mats are among the most conspicuous biological communities in Antarctic landscapes and harbour a high diversity of organisms. These communities are consortia that make important contributions to carbon and nitrogen input in non-marine Antarctic ecosystems. Here, we study the effect of increasing temperatures on the carbon and nitrogen metabolism of two benthic communities on Byers Peninsula (Livingston Island, Maritime Antarctica): a biofilm dominated by green algae growing on seasonal ice, and a land-based microbial mat composed mainly of cyanobacteria. Inorganic carbon photoassimilation, urea and nitrate uptake and N₂-fixation (acetylene reduction activity) rates were determined in situ in parallel at five different temperatures (0, 5, 10, 15, 25°C) using thermostatic baths. The results for the cyanobacterial mat showed that photosynthesis and N₂-fixation responded positively to increased temperatures, but urea and NO₃⁻ uptake rates did not show a significant variation related to temperature. This microbial mat exhibits relatively low activity at 0°C whereas at higher temperatures (up to 15°C), N₂-fixation rate increased significantly. Similarly, the maximum photosynthetic activity increased in parallel with temperature and showed no saturation up to 25°C. In contrast, the ice biofilm displayed higher photosynthetic activity at 0°C than at the other temperatures assayed, and it showed elevated photoinhibition at warmer temperatures.     

Comparative Characterization of the Microbial Diversities of an Artificial Microbialite Model and a Natural Stromatolite

Microbialites are organosedimentary structures that result from the trapping, binding, and lithification of sediments by microbial mat communities. In this study we developed a model artificial microbialite system derived from natural stromatolites, a type of microbialite, collected from Exuma Sound, Bahamas. We demonstrated that the morphology of the artificial microbialite was consistent with that of the natural system in that there was a multilayer community with a pronounced biofilm on the surface, a concentrated layer of filamentous cyanobacteria in the top 5 mm, and a lithified layer of fused oolitic sand grains in the subsurface. The fused grain layer was comprised predominantly of the calcium carbonate polymorph aragonite, which corresponded to the composition of the Bahamian stromatolites. The microbial diversity of the artificial microbialites and that of natural stromatolites were also compared using automated ribosomal intergenic spacer analysis (ARISA) and 16S rRNA gene sequencing. The ARISA profiling indicated that the Shannon indices of the two communities were comparable and that the overall diversity was not significantly lower in the artificial microbialite model. Bacterial clone libraries generated from each of the three artificial microbialite layers and natural stromatolites indicated that the cyanobacterial and crust layers most closely resembled the ecotypes detected in the natural stromatolites and were dominated by Proteobacteria and Cyanobacteria. We propose that such model artificial microbialites can serve as experimental analogues for natural stromatolites.     

Biogeochemistry of microbial mats under Precambrian environmental conditions: a modelling study

Microbial mats have arguably been the most important ecosystem on Earth over its 3.5 Gyr inhabitation. Mats have persisted as consortia for billions of years and occupy some of Earth's most hostile environments. With rare exceptions (e.g. microbial mats developed on geothermal springs at Yellowstone National Park, USA), today's mats do not exist under conditions analogous to Precambrian habitats with substantially lower oxygen and sulphate concentrations. This study uses a numerical model of a microbial mat to investigate how mat composition in the past might have differed from modern mats. We present a numerical model of mat biogeochemistry that simulates the growth of cyanobacteria (CYA), colourless sulphur bacteria (CSB), and purple sulphur bacteria (PSB), with sulphate‐reducing bacteria (SRB) and heterotrophic bacteria represented by parameterized sulphate reduction rates and heterotrophic consumption rates, respectively. Variations in the availability of light, oxygen, sulphide, and sulphate at the upper boundary of the mat are the driving forces in the model. Mats with remarkably similar biomass and chemical profiles develop in models under oxygen boundary conditions ranging from 2.5 × 10^?13 to 0.25 mm and sulphate boundary concentrations ranging from 0.29 to 29 mm , designed to simulate various environments from Archean to modern. The modelled mats show little sensitivity to oxygen boundary conditions because, independent of the overlying oxygen concentrations, cyanobacterial photosynthesis creates similar O₂ concentrations of 0.45–0.65 mm in the upper reaches of the mat during the photoperiod. Varying sulphate boundary conditions have more effect on the biological composition of the mat. Sulphide generated from sulphate reduction controls the magnitude and distribution of the PSB population, and plays a part in the distribution of CSB. CSB are the most sensitive species to environmental change, varying with oxygen and sulphide.