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.     

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.     

Carbonate fabrics in the modern microbialites of Pavilion Lake: two suites of microfabrics that reflect variation in microbial community morphology,growth habit,and lithification

Modern microbialites in Pavilion Lake, BC, provide an analog for ancient non‐stromatolitic microbialites that formed from in situ mineralization. Because Pavilion microbialites are mineralizing under the influence of microbial communities, they provide insights into how biological processes influence microbialite microfabrics and mesostructures. Hemispherical nodules and micrite–microbial crusts are two mesostructures within Pavilion microbialites that are directly associated with photosynthetic communities. Both filamentous cyanobacteria in hemispherical nodules and branching filamentous green algae in micrite–microbial crusts were associated with calcite precipitation at microbialite surfaces and with characteristic microfabrics in the lithified microbialite. Hemispherical nodules formed at microbialite surfaces when calcite precipitated around filamentous cyanobacteria with a radial growth habit. The radial filament pattern was preserved within the microbialite to varying degrees. Some subsurface nodules contained well‐defined filaments, whereas others contained only dispersed organic inclusions. Variation in filament preservation is interpreted to reflect differences in timing and amount of carbonate precipitation relative to heterotrophic decay, with more defined filaments reflecting greater lithification prior to degradation than more diffuse filaments. Micrite–microbial crusts produce the second suite of microfabrics and form in association with filamentous green algae oriented perpendicular to the microbialite surface. Some crusts include calcified filaments, whereas others contained voids that reflect the filamentous community in shape, size, and distribution. Pavilion microbialites demonstrate that microfabric variation can reflect differences in lithification processes and microbial metabolisms as well as microbial community morphology and organization. Even when the morphology of individual filaments or cells is not well preserved, the microbial growth habit can be captured in mesoscale microbialite structures. These results suggest that when petrographic preservation is extremely good, ancient microbialite growth structures and microfabrics can be interpreted in the context of variation in community organization, community composition, and lithification history. Even in the absence of distinct microbial microfabrics, mesostructures can capture microbial community morphology.     

Evidence of microbiocoenosis in the formation of laminae in modern stromatolites

The first appearance of laminated stromatolites in sedimentary rocks around 3.5 billion years ago is generally accepted as evidence for the earliest advanced ecosystem. However, the bio-mechanisms leading to the construction of stromatolites continue to be poorly understood. Using a combination of microscopic and advanced spectroscopic techniques (STXM/NEXAFS/EELS) to analyze lithified laminae, living biomass, reference microorganisms and their byproducts, it is demonstrated that lithified laminae contain the bio-molecular signatures of sulphide oxidizers and sulphate reducers, as well as cyanobacterial extracellular polymeric substances. From this, it is concluded that a combination of microbial processes, such as the degradation of cyanobacterial biomass, phototrophic sulphide oxidation, and sulphate reduction, results in the formation of laminae, a process here called amalgamation. In summary, the described microbialite formation process is biochemically controlled with minimal external particle input and serves as an alternative biotic explanation for the trapping and binding model of lamina formation.     

Lithostratigraphic analysis of a new stromatolite–thrombolite reef from across the rise of atmospheric oxygen in the Paleoproterozoic Turee Creek Group,Western Australia

This study describes a previously undocumented dolomitic stromatolite–thrombolite reef complex deposited within the upper part (Kazput Formation) of the c. 2.4–2.3 Ga Turee Creek Group, Western Australia, across the rise of atmospheric oxygen. Confused by some as representing a faulted slice of the younger c. 1.8 Ga Duck Creek Dolomite, this study describes the setting and lithostratigraphy of the 350‐m‐thick complex and shows how it differs from its near neighbour. The Kazput reef complex is preserved along 15 km of continuous exposure on the east limb of a faulted, north‐west‐plunging syncline and consists of 5 recognisable facies associations (A–E), which form two part regressions and one transgression. The oldest facies association (A) is characterised by thinly bedded dololutite–dolarenite, with local domical stromatolites. Association B consists of interbedded columnar and stratiform stromatolites deposited under relatively shallow‐water conditions. Association C comprises tightly packed columnar and club‐shaped stromatolites deposited under continuously deepening conditions. Clotted (thrombolite‐like) microbialite, in units up to 40 m thick, dominates Association D, whereas Association E contains bedded dololutite and dolarenite, and some thinly bedded ironstone, shale and black chert units. Carbon and oxygen isotope stratigraphy reveals a narrow range in both δ¹³C_carb values, from ?0.22 to 0.97‰ (VPDB: average = 0.68‰), and δ¹⁸O values, from ?14.8 to ?10.3‰ (VPDB), within the range of elevated fluid temperatures, likely reflecting some isotopic exchange. The Kazput Formation stromatolite–thrombolite reef complex contains features of younger Paleoproterozoic carbonate reefs, yet is 300–500 Ma older than previously described Proterozoic examples worldwide. Significantly, the microbial fabrics are clearly distinct from Archean stromatolitic marine carbonate reefs by way of containing the first appearance of clotted microbialite and large columnar stromatolites with complex branching arrangements. Such structures denote a more complex morphological expression of growth than previously recorded in the geological record and may link to the rise of atmospheric oxygen.     

Patterns of metal distribution in hypersaline microbialites during early diagenesis: Implications for the fossil record

The use of metals as biosignatures in the fossil stromatolite record requires understanding of the processes controlling the initial metal(loid) incorporation and diagenetic preservation in living microbialites. Here, we report the distribution of metals and the organic fraction within the lithifying microbialite of the hypersaline Big Pond Lake (Bahamas). Using synchrotron‐based X‐ray microfluorescence, confocal, and biphoton microscopies at different scales (cm–μm) in combination with traditional geochemical analyses, we show that the initial cation sorption at the surface of an active microbialite is governed by passive binding to the organic matrix, resulting in a homogeneous metal distribution. During early diagenesis, the metabolic activity in deeper microbialite layers slows down and the distribution of the metals becomes progressively heterogeneous, resulting from remobilization and concentration as metal(loid)‐enriched sulfides, which are aligned with the lamination of the microbialite. In addition, we were able to identify globules containing significant Mn, Cu, Zn, and As enrichments potentially produced through microbial activity. The similarity of the metal(loid) distributions observed in the Big Pond microbialite to those observed in the Archean stromatolites of Tumbiana provides the foundation for a conceptual model of the evolution of the metal distribution through initial growth, early diagenesis, and fossilization of a microbialite, with a potential application to the fossil record.     

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.     

Formation of stromatolite lamina at the interface of oxygenic–anoxygenic photosynthesis

In modern stromatolites, mineralization results from a complex interplay between microbial metabolisms, the organic matrix, and environmental parameters. Here, we combined biogeochemical, mineralogical, and microscopic analyses with measurements of metabolic activity to characterize the mineralization processes and products in an emergent (<18 months) hypersaline microbial mat. While the nucleation of Mg silicates is ubiquitous in the mat, the initial formation of a Ca‐Mg carbonate lamina depends on (i) the creation of a high‐pH interface combined with a major change in properties of the exopolymeric substances at the interface of the oxygenic and anoxygenic photoautotrophic layers and (ii) the synergy between two major players of sulfur cycle, purple sulfur bacteria, and sulfate‐reducing bacteria. The repetition of this process over time combined with upward growth of the mat is a possible pathway leading to the formation of a stromatolite.     

Lacustrine Nostoc (Nostocales) and associated microbiome generate a new type of modern clotted microbialite

Microbialites are mineral formations formed by microbial communities that are often dominated by cyanobacteria. Carbonate microbialites, known from Proterozoic times through the present, are recognized for sequestering globally significant amounts of inorganic carbon. Recent ecological work has focused on microbial communities dominated by cyanobacteria that produce microbial mats and laminate microbialites (stromatolites). However, the taxonomic composition and functions of microbial communities that generate distinctive clotted microbialites (thrombolites) are less well understood. Here, microscopy and deep shotgun sequencing were used to characterize the microbiome (microbial taxa and their genomes) associated with a single cyanobacterial host linked by 16S sequences to Nostoc commune Vaucher ex Bornet & Flahault, which dominates abundant littoral clotted microbialites in shallow, subpolar, freshwater Laguna Larga in southern Chile. Microscopy and energy‐dispersive X‐ray spectroscopy suggested the hypothesis that adherent hollow carbonate spheres typical of the clotted microbialite begin development on the rigid curved outer surfaces of the Nostoc balls. A surface biofilm included >50 nonoxygenic bacterial genera (taxa other than Nostoc) that indicate diverse ecological functions. The Laguna Larga Nostoc microbiome included the sulfate reducers Desulfomicrobium and Sulfospirillum and genes encoding all known proteins specific to sulfate reduction, a process known to facilitate carbonate deposition by increasing pH. Sequences indicating presence of nostocalean and other types of nifH, nostocalean sulfide:ferredoxin oxidoreductase (indicating anoxygenic photosynthesis), and biosynthetic pathways for the secondary products scytonemin, mycosporine, and microviridin toxin were identified. These results allow comparisons with microbiota and microbiomes of other algae and illuminate biogeochemical roles of ancient microbialites.     

Deep‐water microbialites of the Mesoproterozoic Dismal Lakes Group: microbial growth,lithification, and implications for coniform stromatolites

Offshore facies of the Mesoproterozoic Sulky Formation, Dismal Lakes Group, arctic Canada, preserve microbialites with unusual morphology. These microbialites grew in water depths greater than several tens of meters and correlate with high‐relief conical stromatolites of the more proximal September Lake reef complex. The gross morphology of these microbial facies consists of ridge‐like vertical supports draped by concave‐upward, subhorizontal elements, resulting in tent‐shaped cuspate microbialites with substantial primary void space. Morphological and petrographic analyses suggest a model wherein penecontemporaneous upward growth of ridge elements and development of subhorizontal draping elements initially resulted in a buoyantly supported, unlithified microbial form. Lithification began via precipitation within organic elements during microbialite growth. Mineralization either stabilized or facilitated collapse of initially neutrally buoyant microbialite forms. Microbial structures and breccias were then further stabilized by precipitation of marine herringbone cement. During late‐stage diagenesis, remaining void space was occluded by ferroan dolomite cement. Cuspate microbialites are most similar to those found in offshore facies of Neoarchean carbonate platforms and to unlithified, buoyantly supported microbial mats in modern ice‐covered Antarctic lakes. We suggest that such unusual microbialite morphologies are a product of the interaction between motile and non‐motile communities under nutrient‐limiting conditions, followed by early lithification, which served to preserve the resultant microbial form. The presence of marine herringbone cement, commonly associated with high dissolved inorganic carbon (DIC), low O₂ conditions, also suggests growth in association with reducing environments at or near the seafloor or in conjunction with a geochemical interface. Predominance of coniform stromatolite forms in the Proterozoic—across a variety of depositional environments—may thus reflect a combination of heterogeneous nutrient distribution, potentially driven by variable redox conditions, and an elevated carbonate saturation state, which permits preservation of these unusual microbialite forms.     

Quantifying CaCO3 Microprecipitates Within Developing Surface Mats of Marine Stromatolites Using GIS and Digital Image Analysis

The unique geochemical coupling of organic molecules and mineral CaCO₃ provides a fluorescence signature detectable using conventional confocal scanning laser microscopy (CSLM). The surface microbial mats of open-water marine stromatolites (Bahamas) exist in a continuum of states ranging from a Type 1 (i.e., nonlithifying) to Type 2 (i.e., lithified micritic laminae present) to Type 3 (i.e., fused grain layer). An approach was developed here, that utilizes geographical information systems (GIS) and digital image analysis, coupled with CSLM to estimate concentrations of calcium carbonate precipitates in developing marine stromatolites. We propose that the area occupied by particles within each image can be used to estimate concentrations of precipitates. Fluorescent polymeric microbeads and bacteria were used to calibrate the approach. We used this approach to demonstrate that CaCO₃ precipitates in lithifying layers were quantifiable and significantly different (p < 0.0001) from those in nonlithifying layers. The approach provided a useful tool for the unambiguous assessment of relative changes in microbial precipitates occurring over small ( μ m to mm) spatial scales, and that characterize the formation of lithified layers (micritic laminae) in open-water marine stromatolites.     

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.     

Processes Forming Daily Lamination in a Microbe-Rich Travertine Under Low Flow Condition at the Nagano-yu Hot Spring,Southwestern Japan

A daily rhythm of microbial processes, in terms of sub-mm order lamination, was identified for a microbe-rich aragonite travertine formed at a low-flow site of the Nagano-yu Hot Spring in Southwestern Japan. Continuous observation and sampling clearly showed that the lamination consisted of diurnal microbe-rich layers (M-layers) and nocturnal crystalline layers (C-layers). The M-layers originated from biofilm formed by growth and upward migration of filamentous cyanobacteria related to Microcoleus sp., which can rapidly glide and secrete extracellular polymeric substances (EPS). During the daytime, cyanobacterial biofilm development inhibited aragonite precipitation on the travertine surface due to the calcium-binding ability of EPS. After sunset, aragonite precipitation started on the surface where aerobic heterotrophic bacteria decomposed EPS, which induced precipitation of micritic crystals. This early stage of C-layer formation was followed by abiotic precipitation of fan-shaped aragonite aggregates. Despite their major role in lamina formation, the cyanobacteria were readily degraded within 6–10 days after embedding, and the remaining open spaces in the M-layers were sparsely filled with crystal clots. These lamina-forming processes were different from those observed in a high-flow site where the travertine has a dense texture of aragonite crystals. The microbial travertine at Nagano-yu is similar to some Precambrian stromatolites in terms of in situ mineral precipitation, regular sub-mm order lamination, and arrangement of filamentous microbes; therefore, the lamination of these stromatolites possibly occur with a daily rhythm. The microbial processes demonstrated in this study may revise the interpretation of ancient stromatolite formation.     

Prokaryote populations of extant microbialites along a depth gradient in Pavilion Lake,British Columbia,Canada

Pavilion Lake in British Columbia, Canada, is home to modern‐day microbialites that are actively growing at multiple depths within the lake. While microbialite morphology changes with depth and previous isotopic investigations suggested a biological role in the formation of these carbonate structures, little is known about their microbial communities. Microbialite samples acquired through the Pavilion Lake Research Project (PLRP) were first investigated for phototrophic populations using Cyanobacteria‐specific primers and 16S rRNA gene cloning. These data were expounded on by high‐throughput tagged sequencing analyses of the general bacteria population. These molecular analyses show that the microbial communities of Pavilion Lake microbialites are diverse compared to non‐lithifying microbial mats also found in the lake. Phototrophs and heterotrophs were detected, including species from the recently described Chloroacidobacteria genus, a photoheterotroph that has not been previously observed in microbialite systems. Phototrophs were shown as the most influential contributors to community differences above and below 25 meters, and corresponding shifts in heterotrophic populations were observed at this interface as well. The isotopic composition of carbonate also mirrored this shift in community states. Comparisons to previous studies indicated this population shift may be a consequence of changes in lake chemistry at this depth. Microbial community composition did not correlate with changing microbialite morphology with depth, suggesting something other than community changes may be a key to observed variations in microbialite structure.     

Bacterial Insights into the Formation of Opaline Stromatolites from the Chimalacatepec Lava Tube System,Mexico

Abstract

Cave lithifying systems are excellent models to study biomineralization in the dark. The Chimalacatepec Lava Tube System in Mexico harbors diverse biospeleothems where previous studies suggest that the formation of opaline terrestrial stromatolites is related to microorganisms in contiguous mats. However, there is no information regarding their characterization and their role in mineral formation. In this study, we characterized the bacterial and archaeal composition of microbial mats and stromatolites and suggested the main processes involved in the genesis of opaline stromatolites. Our results showed that the microbial mats and stromatolites have a similar 16S rRNA gene composition, but stromatolites contain more Actinobacteria, which have been previously found in other lava tubes together with other key bacteria. Microorganisms found here belonged to groups with the potential to fix carbon and degrade organic matter. We propose that the synergic interaction of autotrophic and heterotrophic microorganisms that thrive in the dark might be inducing carbonate precipitation within the Ca-enriched extracellular polymeric substances (EPS), generating opal-A and calcite laminae. The similar 16S rRNA gene fingerprint and the presence of potential pathways that induce carbonate precipitation in opaline stromatolites and microbial mats suggest that microbial mats lithify and contribute to the stromatolite biotic genesis.     

Development of albian microbialites and microbialite reefs at marginal platform areas of the Vasco-Cantabrian Basin (Soba reef area,Cantabria, N. Spain)

The Middle Albian sequence from the western marginal area of the Vasco-Cantabrian Basin contains calcified microbialites in different marine depositional environments, individually well defined by microstructure, lamina characteristics and mode of formation. Microbialites may form the primary framework of reefs, which occur as composite stacks in mid to lower slope environments or as isolated bodies in small intraplatform basins. In most areas microbialite reef growth was initiated below the photic zone. Stratiform intercalations of microbialites and composite microbialite/foraminifer oncoids are restricted to well bedded carbonate platform deposits (Urgonian). Three basis types of microbialites are recognized:

Irradiance Regulation of Photosynthesis and Respiration in Modern Marine Microbialites Built by Benthic Cyanobacteria in a Tropical Lagoon (New Caledonia)

Microbialites are organosedimentary deposits that have built up as a result of the growth and binding of detrital sediment by a benthic microbial community. This study focuses on microbialites built by monospecific populations of cyanobacteria in the south-west lagoon of New Caledonia, where they have been observed down to 20–25 m depth. The aim was to study their photosynthetic and respiratory responses to various light intensities. The Phormidium sp. TK1 microbialite was collected at 19 m depth and the P. crosbyanum (Tilden) microbialite was collected at 0.5 and 13 m depth. Phormidium sp. TK1 showed all the characteristic features of a low-light adapted species. The initial slope of the Photosynthesis versus Irradiance curve for this microbialite was close to the maximum quantum yield indicating an efficient light absorption and utilization at low light. The photosynthesis maximum was located 0.2–0.4 mm below the surface and did not shift with changing light intensity. Respiration rates were low and not enhanced by light; photoinhibition was observed at higher light intensities. In Phormidium crosbyanum (Tilden) microbialites, the photosynthesis maximum shifted downward to lower depths with increasing light, probably as a result of phototactic migration of cyanobacterial filaments, and light-enhanced respiration was observed at light intensities above light saturation. The photosynthetic para- meters measured in P. crosbyanum indicate that P. crosbyanum is capable of photo-acclimation at high light intensities. The gross productivity of the different microbialites was comparable to values measured in cyanobacterial stromatolites observed in other shallow environments. However, the microbialites studied here were characterized by a lower respiration / production ratio which indicates a higher growth efficiency.     

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.     

The Girvanella-like remains from Messinian marine deposits (Sardinia,Italy): Lagerstätten paradigm for microbial biota?

The lower Messinian marine sediments of the Capo San Marco Formation in the Sinis area (Sardinia, Italy) contain extensive carbonate buildups mainly made of microbialites. These microbialites exhibit general thrombolitic fabric and occur in meso-macroscopic scale as dominant cauliflower-like structures, digitations and encrusting rings. All the microbialites are here associated with serpulid tubes and bryozoan colonies. Examination of thin sections from microbialite samples reveal the presence of dense flexuous, not ramified and erect tubular micritic structures, with an external diameter ranging from 30 to 40 μm, all characters being very close to those of the Girvanella-type filaments. Although all microbialites show quite similar structural aspects, only two levels contain clearly visible networks of such filaments. The associated marine biota is diverse (cemented, borers, burrowers) related to the available biotopes (hard substrates, fine grained sediment, cavities…). The general scarcity of microbial remains in Messinian microbialites points out to the problem of taphonomic processes allowing a good preservation of microbial structures. The concept of Lagerstätten could well be extended to the preservation of microscopic organisms in the carbonate material. The discovery of Girvanella-like filaments demonstrates the implication of cyanobacterial organisms in the construction processes of the Messinian thrombolitic buildups. Furthermore, it is the first time that Girvanella-like microbialites are documented from Upper Miocene marine rocks.     

Microbialite concretions in a dolostone crust at the Permian–Triassic boundary of the Xishan section in Jiangsu Province,South China

Carbonate concretions with structures and fossil groups associated with microbialite developed in a dolostone crust at the Permian–Triassic boundary of the Xishan section in Jiangsu Province, South China. These structures include clotted fabrics and laminated carbonate needles, as well as abundant carbonate crystal fans. Fossil groups associated with microbialite include microconchids, small gastropods, and small foraminifers. These fabrics and fossils suggest that the concretions are carbonate microbialite blocks developed in the dolostone crust. On the basis of the analysis of the microfabrics and the fossil groups together with a comparison to modern analogues, we attribute the formation of the micritic patches in the microbialite concretions to the calcification of cyanobacterial mats via carbonate nanoparticles and we attribute the carbonate crystal fans to the direct recrystallization of micritic carbonates. The sparitic patches were interpreted as either the direct recrystallization of micritic carbonates or the precipitation of carbonate spars in the inter-/intra-spaces of metazoan shells together with the recrystallization of these shells. The similarities to modern stromatolites, both in morphology and in internal texture, suggest that the laminated carbonate needles are stromatolite laminae built by filamentous cyanobacteria. The preservation of these microbialite microfabrics indicates that early lithification by carbonate precipitation was widespread and intense following the end-Permian boundary events. The weak development of microbialites as small concretions may be attributed to the deeper water depth and the lower water energy in the Xishan area during the earliest Triassic.