Diversity of phototrophic bacteria in microbial mats from Arctic hot springs (Greenland)

We investigated the genotypic diversity of oxygenic and anoxygenic phototrophic microorganisms in microbial mat samples collected from three hot spring localities on the east coast of Greenland. These hot springs harbour unique Arctic microbial ecosystems that have never been studied in detail before. Specific oligonucleotide primers for cyanobacteria, purple sulfur bacteria, green sulfur bacteria and Choroflexus/Roseiflexus-like green non-sulfur bacteria were used for the selective amplification of 16S rRNA gene fragments. Amplification products were separated by denaturing gradient gel electrophoresis (DGGE) and sequenced. In addition, several cyanobacteria were isolated from the mat samples, and classified morphologically and by 16S rRNA-based methods. The cyanobacterial 16S rRNA sequences obtained from DGGE represented a diverse, polyphyletic collection of cyanobacteria. The microbial mat communities were dominated by heterocystous and non-heterocystous filamentous cyanobacteria. Our results indicate that the cyanobacterial community composition in the samples were different for each sampling site. Different layers of the same heterogeneous mat often contained distinct and different communities of cyanobacteria. We observed a relationship between the cyanobacterial community composition and the in situ temperatures of different mat parts. The Greenland mats exhibited a low diversity of anoxygenic phototrophs as compared with other hot spring mats which is possibly related to the photochemical conditions within the mats resulting from the Arctic light regime.     

Cyanobacterial diversity and activity in modern conical microbialites

Modern conical microbialites are similar to some ancient conical stromatolites, but growth, behavior and diversity of cyanobacteria in modern conical microbialites remain poorly characterized. Here, we analyze the diversity of cyanobacterial 16S rRNA gene sequences in conical microbialites from 14 ponds fed by four thermal sources in Yellowstone National Park and compare cyanobacterial activity in the tips of cones and in the surrounding topographic lows (mats), respectively, by high‐resolution mapping of labeled carbon. Cones and adjacent mats contain similar 16S rRNA gene sequences from genetically distinct clusters of filamentous, non‐heterocystous cyanobacteria from Subsection III and unicellular cyanobacteria from Subsection I. These sequences vary among different ponds and between two sampling years, suggesting that coniform mats through time and space contain a number of cyanobacteria capable of vertical aggregation, filamentous cyanobacteria incapable of initiating cone formation and unicellular cyanobacteria. Unicellular cyanobacteria are more diverse in topographic lows, where some of these organisms respond to nutrient pulses more rapidly than thin filamentous cyanobacteria. The densest active cyanobacteria are found below the upper 50 μm of the cone tip, whereas cyanobacterial cells in mats are less dense, and are more commonly degraded or encrusted by silica. These spatial differences in cellular activity and density within macroscopic coniform mats imply a strong role for diffusion limitation in the development and the persistence of the conical shape. Similar mechanisms may have controlled the growth, morphology and persistence of small coniform stromatolites in shallow, quiet environments throughout geologic history.     

Diversity within cyanobacterial mat communities in variable salinity meltwater ponds of McMurdo Ice Shelf, Antarctica

This study investigated the diversity of cyanobacterial mat communities of three meltwater ponds--Fresh, Orange and Salt Ponds, south of Bratina Island, McMurdo Ice Shelf, Antarctica. A combined morphological and genetic approach using clone libraries was used to investigate the influence of salinity on cyanobacterial diversity within these ecosystems without prior cultivation or isolation of cyanobacteria. We were able to identify 22 phylotypes belonging to Phormidium sp., Oscillatoria sp. and Lyngbya sp. In addition, we identified Antarctic Nostoc sp., Nodularia sp. and Anabaena sp. from the clone libraries. Fresh (17 phylotypes) and Orange (nine phylotypes) Ponds showed a similar diversity in contrast to that of the hypersaline Salt Pond (five phylotypes), where the diversity within cyanobacterial mats was reduced. Using the comparison of identified phylotypes with existing Antarctic sequence data, it was possible to gain further insight into the different levels of distribution of phylotypes identified in the investigated cyanobacterial mat communities of McMurdo Ice Shelf.     

The biosynthesis of cyanobacterial sunscreen scytonemin in intertidal microbial mat communities

We have examined the biosynthesis and accumulation of cyanobacterial sunscreening pigment scytonemin within intertidal microbial mat communities using a combination of chemical, molecular, and phylogenetic approaches. Both laminated (layered) and nonlaminated mats contained scytonemin, with morphologically distinct mats having different cyanobacterial community compositions. Within laminated microbial mats, regions with and without scytonemin had different dominant oxygenic phototrophs, with scytonemin-producing areas consisting primarily of Lyngbya aestuarii and scytonemin-deficient areas dominated by a eukaryotic alga. The nonlaminated mat was populated by a diverse group of cyanobacteria and did not contain algae. The amplification and phylogenetic assignment of scytonemin biosynthetic gene scyC from laminated mat samples confirmed that the dominant cyanobacterium in these areas, L. aestuarii, is likely responsible for sunscreen production. This study is the first to utilize an understanding of the molecular basis of scytonemin assembly to explore its synthesis and function within natural microbial communities.     

Epilithic cyanobacterial communities of a marine tropical beach rock (Heron Island, Great Barrier Reef): diversity and diazotrophy

The diversity and nitrogenase activity of epilithic marine microbes in a Holocene beach rock (Heron Island, Great Barrier Reef, Australia) with a proposed biological calcification "microbialite" origin were examined. Partial 16S rRNA gene sequences from the dominant mat (a coherent and layered pink-pigmented community spread over the beach rock) and biofilms (nonstratified, differently pigmented microbial communities of small shallow depressions) were retrieved using denaturing gradient gel electrophoresis (DGGE), and a clone library was retrieved from the dominant mat. The 16S rRNA gene sequences and morphological analyses revealed heterogeneity in the cyanobacterial distribution patterns. The nonheterocystous filamentous genus Blennothrix sp., phylogenetically related to Lyngbya, dominated the mat together with unidentified nonheterocystous filaments of members of the Pseudanabaenaceae and the unicellular genus Chroococcidiopsis. The dominance and three-dimensional intertwined distribution of these organisms were confirmed by nonintrusive scanning microscopy. In contrast, the less pronounced biofilms were dominated by the heterocystous cyanobacterial genus Calothrix, two unicellular Entophysalis morphotypes, Lyngbya spp., and members of the Pseudanabaenaceae family. Cytophaga-Flavobacterium-Bacteroides and Alphaproteobacteria phylotypes were also retrieved from the beach rock. The microbial diversity of the dominant mat was accompanied by high nocturnal nitrogenase activities (as determined by in situ acetylene reduction assays). A new DGGE nifH gene optimization approach for cyanobacterial nitrogen fixers showed that the sequences retrieved from the dominant mat were related to nonheterocystous uncultured cyanobacterial phylotypes, only distantly related to sequences of nitrogen-fixing cultured cyanobacteria. These data stress the occurrence and importance of nonheterocystous epilithic cyanobacteria, and it is hypothesized that such epilithic cyanobacteria are the principal nitrogen fixers of the Heron Island beach rock.     

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.     

Grain trapping by filamentous cyanobacterial and algal mats: implications for stromatolite microfabrics through time

Archean and Proterozoic stromatolites are sparry or fine‐grained and finely laminated; coarse‐grained stromatolites, such as many found in modern marine systems, do not appear until quite late in the fossil record. The cause of this textural change and its relevance to understanding the evolutionary history of stromatolites is unclear. Cyanobacteria are typically considered the dominant stromatolite builders through time, but studies demonstrating the trapping and binding abilities of cyanobacterial mats are limited. With this in mind, we conducted experiments to test the grain trapping and binding capabilities of filamentous cyanobacterial mats and trapping in larger filamentous algal mats in order to better understand grain size trends in stromatolites. Mats were cut into squares, inclined in saltwater tanks at angles from 0 to 75° (approximating the angle of lamina in typical stromatolites), and grains of various sizes (fine sand, coarse sand, and fine pebbles) were delivered to their surface. Trapping of grains by the cyanobacterial mats depended strongly on (i) how far filaments protruded from the sediment surface, (ii) grain size, and (iii) the mat's incline angle. The cyanobacterial mats were much more effective at trapping fine grains beyond the abiotic slide angle than larger grains. In addition, the cyanobacterial mats actively bound grains of all sizes over time. In contrast, the much larger algal mats trapped medium and coarse grains at all angles. Our experiments suggest that (i) the presence of detrital grains beyond the abiotic slide angle can be considered a biosignature in ancient stromatolites where biogenicity is in question, and, (ii) where coarse grains are present within stromatolite laminae at angles beyond the abiotic angle of slide (e.g., most modern marine stromatolites), typical cyanobacterial‐type mats are probably not solely responsible for the construction, giving insight into the evolution of stromatolite microfabrics through time.     

LIPOPHILIC PIGMENTS FROM CYANOBACTERIAL (BLUE-GREEN ALGAL) AND DIATOM MATS IN HAMELIN POOL,SHARK BAY,WESTERN AUSTRALIA1

Lipophilic pigments were examined in microbial mat communities dominated by cyanobacteria in the intertidal zone and by diatoms in the subtidal and sublittoral zones of Hamelin Pool, Shark Bay, Western Australia. These microbial mats have evolutionary significance because of their similarity to lithified stromatolites from the Proterozoic and Early Paleozoic eras. Fucoxanthin, diatoxanthin, diadinoxanthin, β-carotene, and chlorophylls a and c characterized the diatom mats, whereas cyanobacterial mats contained myxoxanthophyll zeaxanthin, echinenone, β-carotene, chlorophyll a and, in some cases, sheath pigment. The presence of bacteriochlorophyll a with in the mats suggest a close association of photosynthetic bacteria with diatoms and cyanobacteria. The high carotenoids: chlorophyll a ratios (0.84–2.44 wt/wt) in the diatom mats suggest that carotenoids served a photoprotective function in this high light environment. By contrast, cyanobacterial sheath pigment may have largely supplanted the photoprotective role of carotenoids in the intertidal mats.     

Enrichment culture and microscopy conceal diverse thermophilic Synechococcus populations in a single hot spring microbial mat habitat

Recent molecular studies have shown a great disparity between naturally occurring and cultivated microorganisms. We investigated the basis for disparity by studying thermophilic unicellular cyanobacteria whose morphologic simplicity suggested that a single cosmopolitan species exists in hot spring microbial mats worldwide. We found that partial 16S rRNA sequences for all thermophilic Synechococcus culture collection strains from diverse habitats are identical. Through oligonucleotide probe analysis and cultivation, we provide evidence that this species is strongly selected for in laboratory culture to the exclusion of many more-predominant cyanobacterial species coexisting in the Octopus Spring mat in Yellowstone National Park. The phylogenetic diversity among Octopus Spring cyanobacteria is of similar magnitude to that exhibited by all cyanobacteria so far investigated. We obtained axenic isolates of two predominant cyanobacterial species by diluting inocula prior to enrichment. One isolate has a 16S rRNA sequence we have not yet detected by cloning. The other has a 16S rRNA sequence identical to a new cloned sequence we report herein. This is the first cultivated species whose 16S rRNA sequence has been detected in this mat system by cloning. We infer that biodiversity within this community is linked to guild structure.     

Enrichment culture and microscopy conceal diverse thermophilic Synechococcus populations in a single hot spring microbial mat habitat.

Recent molecular studies have shown a great disparity between naturally occurring and cultivated microorganisms. We investigated the basis for disparity by studying thermophilic unicellular cyanobacteria whose morphologic simplicity suggested that a single cosmopolitan species exists in hot spring microbial mats worldwide. We found that partial 16S rRNA sequences for all thermophilic Synechococcus culture collection strains from diverse habitats are identical. Through oligonucleotide probe analysis and cultivation, we provide evidence that this species is strongly selected for in laboratory culture to the exclusion of many more-predominant cyanobacterial species coexisting in the Octopus Spring mat in Yellowstone National Park. The phylogenetic diversity among Octopus Spring cyanobacteria is of similar magnitude to that exhibited by all cyanobacteria so far investigated. We obtained axenic isolates of two predominant cyanobacterial species by diluting inocula prior to enrichment. One isolate has a 16S rRNA sequence we have not yet detected by cloning. The other has a 16S rRNA sequence identical to a new cloned sequence we report herein. This is the first cultivated species whose 16S rRNA sequence has been detected in this mat system by cloning. We infer that biodiversity within this community is linked to guild structure.     

Epilithic Cyanobacterial Communities of a Marine Tropical Beach Rock (Heron Island, Great Barrier Reef): Diversity and Diazotrophy

The diversity and nitrogenase activity of epilithic marine microbes in a Holocene beach rock (Heron Island, Great Barrier Reef, Australia) with a proposed biological calcification “microbialite” origin were examined. Partial 16S rRNA gene sequences from the dominant mat (a coherent and layered pink-pigmented community spread over the beach rock) and biofilms (nonstratified, differently pigmented microbial communities of small shallow depressions) were retrieved using denaturing gradient gel electrophoresis (DGGE), and a clone library was retrieved from the dominant mat. The 16S rRNA gene sequences and morphological analyses revealed heterogeneity in the cyanobacterial distribution patterns. The nonheterocystous filamentous genus Blennothrix sp., phylogenetically related to Lyngbya, dominated the mat together with unidentified nonheterocystous filaments of members of the Pseudanabaenaceae and the unicellular genus Chroococcidiopsis. The dominance and three-dimensional intertwined distribution of these organisms were confirmed by nonintrusive scanning microscopy. In contrast, the less pronounced biofilms were dominated by the heterocystous cyanobacterial genus Calothrix, two unicellular Entophysalis morphotypes, Lyngbya spp., and members of the Pseudanabaenaceae family. Cytophaga-Flavobacterium-Bacteroides and Alphaproteobacteria phylotypes were also retrieved from the beach rock. The microbial diversity of the dominant mat was accompanied by high nocturnal nitrogenase activities (as determined by in situ acetylene reduction assays). A new DGGE nifH gene optimization approach for cyanobacterial nitrogen fixers showed that the sequences retrieved from the dominant mat were related to nonheterocystous uncultured cyanobacterial phylotypes, only distantly related to sequences of nitrogen-fixing cultured cyanobacteria. These data stress the occurrence and importance of nonheterocystous epilithic cyanobacteria, and it is hypothesized that such epilithic cyanobacteria are the principal nitrogen fixers of the Heron Island beach rock.     

Cyanobacterial life at low O(2): community genomics and function reveal metabolic versatility and extremely low diversity in a Great Lakes sinkhole mat

Cyanobacteria are renowned as the mediators of Earth's oxygenation. However, little is known about the cyanobacterial communities that flourished under the low-O(2) conditions that characterized most of their evolutionary history. Microbial mats in the submerged Middle Island Sinkhole of Lake Huron provide opportunities to investigate cyanobacteria under such persistent low-O(2) conditions. Here, venting groundwater rich in sulfate and low in O(2) supports a unique benthic ecosystem of purple-colored cyanobacterial mats. Beneath the mat is a layer of carbonate that is enriched in calcite and to a lesser extent dolomite. In situ benthic metabolism chambers revealed that the mats are net sinks for O(2), suggesting primary production mechanisms other than oxygenic photosynthesis. Indeed, (14)C-bicarbonate uptake studies of autotrophic production show variable contributions from oxygenic and anoxygenic photosynthesis and chemosynthesis, presumably because of supply of sulfide. These results suggest the presence of either facultatively anoxygenic cyanobacteria or a mix of oxygenic/anoxygenic types of cyanobacteria. Shotgun metagenomic sequencing revealed a remarkably low-diversity mat community dominated by just one genotype most closely related to the cyanobacterium Phormidium autumnale, for which an essentially complete genome was reconstructed. Also recovered were partial genomes from a second genotype of Phormidium and several Oscillatoria. Despite the taxonomic simplicity, diverse cyanobacterial genes putatively involved in sulfur oxidation were identified, suggesting a diversity of sulfide physiologies. The dominant Phormidium genome reflects versatile metabolism and physiology that is specialized for a communal lifestyle under fluctuating redox conditions and light availability. Overall, this study provides genomic and physiologic insights into low-O(2) cyanobacterial mat ecosystems that played crucial geobiological roles over long stretches of Earth history.     

Impact of carbon metabolism on 13C signatures of cyanobacteria and green non-sulfur-like bacteria inhabiting a microbial mat from an alkaline siliceous hot spring in Yellowstone National Park (USA)

Alkaline siliceous hot spring microbial mats in Yellowstone National Park are composed of two dominant phototropic groups, cyanobacteria and green non-sulfur-like bacteria (GNSLB). While cyanobacteria are thought to cross-feed low-molecular-weight organic compounds to support photoheterotrophic metabolism in GNSLB, it is unclear how this could lead to the heavier stable carbon isotopic signatures in GNSLB lipids compared with cyanobacterial lipids found in previous studies. The two groups of phototrophs were separated using percoll density gradient centrifugation and subsequent lipid and stable carbon isotopic analysis revealed that we obtained fractions with a approximately 60-fold enrichment in cyanobacterial and an approximately twofold enrichment in GNSLB biomass, respectively, compared with the mat itself. This technique was used to study the diel cycling and 13C content of the glucose pools in and the uptake of 13C-bicarbonate by the cyanobacteria and GNSLB, as well as the transfer of incorporated 13C from cyanobacteria to GNSLB. The results show that cyanobacteria have the highest bicarbonate uptake rates and accumulate glucose during the afternoon in full light conditions. In contrast, GNSLB have relatively higher bicarbonate uptake rates compared with cyanobacteria in the morning at low light levels. During the night GNSLB take up carbon that is likely derived through fermentation of cyanobacterial glucose enriched in 13C. The assimilation of 13C-enriched cyanobacterial carbon may thus lead to enriched 13C-contents of GNSLB cell components.     

Halophilic algal-bacterial and cyanobacterial communities and their role in carbonate precipitation

This work studies the diversity of cyanobacterial and algal-bacterial communities of saline water bodies in the Crimean Peninsula and Altai Region. Plant-bacterial communities are described for the first time. The dependence of the production and destruction on the season and salinity of the water body is shown. The development of planktonic cyanobacteria is related to the presence of zooplankton, the development of which is controlled by hydrogen sulfide. The high hydrogen sulfide tolerance of benthic cyanobacteria secures the integrity of cyanobacterial communities. Observations in nature and laboratory modeling show that the formation of mineral layers is restricted to conditions of supersaturation with mineral components. Carbonate precipitation can take place in cyanobacterial communities under conditions of mixing sea water enriched with Ca and Mg with continental water enriched with sodium carbonate. Cyanobacteria are able to form and transform various Ca-Mg-carbonates. Dolomite formation is a derived process that occurs in cyanobacterial mats in the presence of sulfate-reducing bacteria. Carbonatization of cyanobacterial cells is considered using the example of the unicellular halophilic-alkaliphilic cyanobacterium Euhalothece sp. The accomplished study is of certain interest for interpretation of geological and paleontological data in the context of the supposed analogy between cyanobacterial mats and ancient stromatolites.     

Biodiversity and monitoring of colorless filamentous bacteria in sulfide aquatic systems of North Caucasus region

Bacterial mats in sulfide aquatic systems of North Caucasus are basically composed by the species of genera Thiothrix and Sphaerotilus. Additionally, several non-filamentous sulfur-oxidizing bacteria were isolated from the mats and several minor 16S rRNA phylotypes were found in clone libraries from these mats. The minor components were affiliated with Proteobacteria, Chlorobia, Cyanobacteria and Firmicutes. Even in an individual mat population heterogeneity of Thiothrix spp. was revealed by analysis of 16S rRNA gene and RAPD-PCR. Five Thiothrix isolates were described as new species Thiothrix caldifontis sp. nov. and Thiothrix lacustris sp. nov. In the Thiothrix-Sphaerotilus type of bacterial mat the proportion of dominant organisms might be influenced by sulfide concentration in the spring water. The higher sulfide concentration (more than 10 mg/l) in the spring water is more favorable for the development of bacterial mats with dominant Thiothrix organisms than for Thiothrix-Sphaerotilus type of sulfur mat.     

Multi-trichomous cyanobacterial microfossils from the Mesoproterozoic Gaoyuzhuang Formation, China: Paleoecological and taxonomic implications

Populations of the multi-trichomous microbial fossil Eoschizothrix composita n.gen. et sp. are preserved in growth position in silicified stratiform stromatolites of the Gaoyuzhuang Formation, Hebei Province, northern China. The microbial fossils consist predominantly of preserved sheaths, although several specimens retain shriveled remains of trichomes within sheaths. Comparisons with modern morphological counterparts, including shape, growth habit and orientation, degradational sequences, and habitat, support the interpretation of the multi-trichomous microfossils as cyanobacteria, which acted as frame-builders of ancient stromatolites. The distribution and orientation of multi-trichomous microfossils within a synsedimentary context reveal their behavioral responses to sedimentation regime. Horizontally spread, interwoven mats formed during periods of sedimentary stasis. During periods of rapid sediment influx, the filaments assumed an upright orientation, possibly to avoid accumulating particles. This is the first record of fossil stromatolite-building multi-trichomous cyanobacterial which underscores early morphological and functional diversification in cyanobacterial evolution.     

Composition and diversity of nifH genes of nitrogen-fixing cyanobacteria associated with boreal forest feather mosses

Recent studies have revealed that nitrogen fixation by cyanobacteria living in association with feather mosses is a major input of nitrogen to boreal forests. We characterized the community composition and diversity of cyanobacterial nifH phylotypes associated with each of two feather moss species (Pleurozium schreberi and Hylocomium splendens) on each of 30 lake islands varying in ecosystem properties in northern Sweden. Nitrogen fixation was measured using acetylene reduction, and nifH sequences were amplified using general and cyanobacterial selective primers, separated and analyzed using density gradient gel electrophoresis (DGGE) or cloning, and further sequenced for phylogenetic analyses. Analyses of DGGE fingerprinting patterns revealed two host-specific clusters (one for each moss species), and sequence analysis showed five clusters of nifH phylotypes originating from heterocystous cyanobacteria. For H. splendens only, N(2) fixation was related to both nifH composition and diversity among islands. We demonstrated that the cyanobacterial communities associated with feather mosses show a high degree of host specificity. However, phylotype composition and diversity, and nitrogen fixation, did not differ among groups of islands that varied greatly in their availability of resources. These results suggest that moss species identity, but not extrinsic environmental conditions, serves as the primary determinant of nitrogen-fixing cyanobacterial communities that inhabit mosses.     

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.     

Formation and diagenesis of modern marine calcified cyanobacteria

Calcified cyanobacterial microfossils are common in carbonate environments through most of the Phanerozoic, but are absent from the marine rock record over the past 65 Myr. There has been long-standing debate on the factors controlling the formation and temporal distribution of these fossils, fostered by the lack of a suitable modern analog. We describe calcified cyanobacteria filaments in a modern marine reef setting at Highborne Cay, Bahamas. Our observations and stable isotope data suggest that initial calcification occurs in living cyanobacteria and is photosynthetically induced. A single variety of cyanobacteria, Dichothrix sp., produces calcified filaments. Adjacent cyanobacterial mats form well-laminated stromatolites, rather than calcified filaments, indicating there can be a strong taxonomic control over the mechanism of microbial calcification. Petrographic analyses indicate that the calcified filaments are degraded during early diagenesis and are not present in well-lithified microbialites. The early diagenetic destruction of calcified filaments at Highborne Cay indicates that the absence of calcified cyanobacteria from periods of the Phanerozoic is likely to be caused by low preservation potential as well as inhibited formation.