The importance of groundwater flow to the formation of modern thrombolitic microbialites

Modern microbialites are often located within groundwater discharge zones, yet the role of groundwater in microbialite accretion has yet to be resolved. To understand relationships between groundwater, microbialites, and associated microbial communities, we quantified and characterized groundwater flow and chemistry in active thrombolitic microbialites in Lake Clifton, Western Australia, and compared these observations to inactive thrombolites and lakebed sediments. Groundwater flows upward through an interconnected network of pores within the microstructure of active thrombolites, discharging directly from thrombolite heads into the lake. This upwelling groundwater is fresher than lake water and is hypothesized to support microbial mat growth by reducing salinity and providing limiting nutrients in an osmotically stressful and oligotrophic habitat. This is in contrast to inactive thrombolites that show no evidence of microbial mat colonization and are infiltrated by hypersaline lake water. Groundwater discharge through active thrombolites contrasts with the surrounding lakebed, where hypersaline lake water flows downward through sandy sediments at very low rates. Based on an appreciation for the role of microorganisms in thrombolite accretion, our findings suggest conditions favorable to thrombolite formation still exist in certain locations of Lake Clifton despite increasing lake water salinity. This study is the first to characterize groundwater flow rates, paths, and chemistry within a microbialite‐forming environment and provides new insight into how groundwater can support microbial mats believed to contribute to microbialite formation in modern and ancient environments.     

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.     

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.     

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.     

Microbialite formation in southern Sinai (Egypt)

Ongoing microbialite formation is described at two previously unreported sites in southern Sinai, Egypt. Samples were collected in the peritidal tropical environment of Nabq Bay and Hidden Bay (southern Sinai, Egypt). Field observations and sample analyses show evidence of both sediment trapping and biostabilization in bacterial mucilaginous sheaths and microbially induced mineralization, producing a suite of increasingly lithified material: from agglomerated, consolidated sand to lithified crusts and oncoids. Thin-sections show evidence of bacteria (cyanobacteria and sulphate-reducing bacteria) among the constituent grains in the form of gelatinous filaments (green and red sheaths), and microalgal colonies along the outer edge, accompanied by a very high grade of clast alteration. The alternation of planar to irregular dark, superposed layers, and clastic layers is visible at the surface and/or inside some crusts. Widespread filaments of Schizothrix among grains have been identified, as well as extracellular polymeric substances (EPS), sheaths stabilizing particles, and calcareous tubular encrustations around cyanobacteria filaments. Carbonate precipitates include diffuse micrite, microcrystals of high-Mg calcite precipitating in the EPS matrix, and acicular aragonite as isopachous rims around grains. Cementation is accompanied by partial dissolution and progressive alteration of original grain boundaries. We describe four microbialite categories on the basis of their macroscopic morphology combined with different texture and lithification grade. The occurrence of the southern Sinai microbialites is explained by the interplay of local sedimentary dynamics and accommodation space in a peritidal tropical environment undergoing large temperature and salinity variations.     

Microbialite response to an anthropogenic salinity gradient in Great Salt Lake,Utah

A railroad causeway across Great Salt Lake, Utah (GSL), has restricted water flow since its construction in 1959, resulting in a more saline North Arm (NA; 24%–31% salinity) and a less saline South Arm (SA; 11%–14% salinity). Here, we characterized microbial carbonates collected from the SA and the NA to evaluate the effect of increased salinity on community composition and abundance and to determine whether the communities present in the NA are still actively precipitating carbonate or if they are remnant features from prior to causeway construction. SSU rRNA gene abundances associated with the NA microbialite were three orders of magnitude lower than those associated with the SA microbialite, indicating that the latter community is more productive. SSU rRNA gene sequencing and functional gene microarray analyses indicated that SA and NA microbialite communities are distinct. In particular, abundant sequences affiliated with photoautotrophic taxa including cyanobacteria and diatoms that may drive carbonate precipitation and thus still actively form microbialites were identified in the SA microbialite; sequences affiliated with photoautotrophic taxa were in low abundance in the NA microbialite. SA and NA microbialites comprise smooth prismatic aragonite crystals. However, the SA microbialite also contained micritic aragonite, which can be formed as a result of biological activity. Collectively, these observations suggest that NA microbialites are likely to be remnant features from prior to causeway construction and indicate a strong decrease in the ability of NA microbialite communities to actively precipitate carbonate minerals. Moreover, the results suggest a role for cyanobacteria and diatoms in carbonate precipitation and microbialite formation in the SA of GSL.     

Microbial diversity and biomarker analysis of modern freshwater microbialites from Laguna Bacalar,Mexico

Laguna Bacalar is a sulfate‐rich freshwater lake on the Yucatan Peninsula that hosts large microbialites. High sulfate concentrations distinguish Laguna Bacalar from other freshwater microbialite sites such as Pavilion Lake and Alchichica, Mexico, as well as from other aqueous features on the Yucatan Peninsula. While cyanobacterial populations have been described here previously, this study offers a more complete characterization of the microbial populations and corresponding biogeochemical cycling using a three‐pronged geobiological approach of microscopy, high‐throughput DNA sequencing, and lipid biomarker analyses. We identify and compare diverse microbial communities of Alphaproteobacteria, Deltaproteobacteria, and Gammaproteobacteria that vary with location along a bank‐to‐bank transect across the lake, within microbialites, and within a neighboring mangrove root agglomeration. In particular, sulfate‐reducing bacteria are extremely common and diverse, constituting 7%–19% of phylogenetic diversity within the microbialites, and are hypothesized to significantly influence carbonate precipitation. In contrast, Cyanobacteria account for less than 1% of phylogenetic diversity. The distribution of lipid biomarkers reflects these changes in microbial ecology, providing meaningful biosignatures for the microbes in this system. Polysaturated short‐chain fatty acids characteristic of cyanobacteria account for <3% of total abundance in Laguna Bacalar microbialites. By contrast, even short‐chain and monounsaturated short‐chain fatty acids attributable to both Cyanobacteria and many other organisms including types of Alphaproteobacteria and Gammaproteobacteria constitute 43%–69% and 17%–25%, respectively, of total abundance in microbialites. While cyanobacteria are the largest and most visible microbes within these microbialites and dominate the mangrove root agglomeration, it is clear that their smaller, metabolically diverse associates are responsible for significant biogeochemical cycling in this microbialite system.     

Specific carbonate–microbe interactions in the modern microbialites of Lake Alchichica (Mexico)

The role of microorganisms in microbialite formation remains unresolved: do they induce mineral precipitation (microbes first) or do they colonize and/or entrap abiotic mineral precipitates (minerals first)? Does this role vary from one species to another? And what is the impact of mineral precipitation on microbial ecology? To explore potential biogenic carbonate precipitation, we studied cyanobacteria–carbonate assemblages in modern hydromagnesite-dominated microbialites from the alkaline Lake Alchichica (Mexico), by coupling three-dimensional imaging of molecular fluorescence emitted by microorganisms, using confocal laser scanning microscopy, and Raman scattering/spectrometry from the associated minerals at a microscale level. Both hydromagnesite and aragonite precipitate within a complex biofilm composed of photosynthetic and other microorganisms. Morphology and pigment-content analysis of dominant photosynthetic microorganisms revealed up to six different cyanobacterial morphotypes belonging to Oscillatoriales, Chroococcales, Nostocales and Pleurocapsales, as well as several diatoms and other eukaryotic microalgae. Interestingly, one of these morphotypes, Pleurocapsa-like, appeared specifically associated with aragonite minerals, the oldest parts of actively growing Pleurocapsa-like colonies being always aragonite-encrusted. We hypothesize that actively growing cells of Pleurocapsales modify local environmental conditions favoring aragonite precipitation at the expense of hydromagnesite, which precipitates at seemingly random locations within the biofilm. Therefore, at least part of the mineral precipitation in Alchichica microbialites is most likely biogenic and the type of biominerals formed depends on the nature of the phylogenetic lineage involved. This observation may provide clues to identify lineage-specific biosignatures in fossil stromatolites from modern to Precambrian times.     

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.     

Reconstruction of limnology and microbialite formation conditions from carbonate clumped isotope thermometry

Quantitative tools for deciphering the environment of microbialite formation are relatively limited. For example, the oxygen isotope carbonate‐water geothermometer requires assumptions about the isotopic composition of the water of formation. We explored the utility of using ‘clumped’ isotope thermometry as a tool to study the temperatures of microbialite formation. We studied microbialites recovered from water depths of 10–55 m in Pavilion Lake, and 10–25 m in Kelly Lake, spanning the thermocline in both lakes. We determined the temperature of carbonate growth and the ¹⁸O/¹⁶O ratio of the waters that microbialites grew in. Results were then compared to current limnological data from the lakes to reconstruct the history of microbialite formation. Modern microbialites collected at shallow depths (11.7 m) in both lakes yield clumped isotope‐based temperatures of formation that are within error of summer water temperatures, suggesting that clumped isotope analyses may be used to reconstruct past climates and to probe the environments in which microbialites formed. The deepest microbialites (21.7–55 m) were recovered from below the present‐day thermoclines in both lakes and yield radioisotope ages indicating they primarily formed earlier in the Holocene. During this time, pollen data and our reconstructed water ¹⁸O/¹⁶O ratios indicate a period of aridity, with lower lake levels. At present, there is a close association between both photosynthetic and heterotrophic communities, and carbonate precipitation/microbialite formation, with biosignatures of photosynthetic influences on carbonate detected in microbialites from the photic zone and above the thermocline (i.e., depths of generally <20 m). Given the deeper microbialites are receiving <1% of photosynthetically active radiation (PAR), it is likely these microbialites primarily formed when lower lake levels resulted in microbialites being located higher in the photic zone, in warm surface waters.     

Bacterial diversity and carbonate precipitation in the giant microbialites from the highly alkaline Lake Van, Turkey

Lake Van harbors the largest known microbialites on Earth. The surface of these huge carbonate pinnacles is covered by coccoid cyanobacteria whereas their central axis is occupied by a channel through which neutral, relatively Ca-enriched, groundwater flows into highly alkaline (pH ~9.7) Ca-poor lake water. Previous microscopy observations showed the presence of aragonite globules composed by rounded nanostructures of uncertain origin that resemble similar bodies found in some meteorites. Here, we have carried out fine-scale mineralogical and microbial diversity analyses from surface and internal microbialite samples. Electron transmission microscopy revealed that the nanostructures correspond to rounded aragonite nanoprecipitates. A progressive mineralization of cells by the deposition of nanoprecipitates on their surface was observed from external towards internal microbialite areas. Molecular diversity studies based on 16S rDNA amplification revealed the presence of bacterial lineages affiliated to the Alpha-, Beta- and Gammaproteobacteria, the Cyanobacteria, the Cytophaga-Flexibacter-Bacteroides (CFB) group, the Actinobacteria and the Firmicutes. Cyanobacteria and CFB members were only detected in surface layers. The most abundant and diverse lineages were the Firmicutes (low GC Gram positives). To the exclusion of cyanobacteria, the closest cultivated members to the Lake Van phylotypes were most frequently alkaliphilic and/or heterotrophic bacteria able to degrade complex organics. These heterotrophic bacteria may play a crucial role in the formation of Lake Van microbialites by locally promoting carbonate precipitation.     

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.     

Mineralisation of filamentous cyanobacteria in Lake Thetis stromatolites,Western Australia

Stromatolites are cited as some of the earliest evidence for life on Earth, but problems remain in reconciling the paucity of microfossils in ancient carbonate examples with the abundance of microbes that help construct modern analogues. Here, we trace the mineralisation pathway of filamentous cyanobacteria within stromatolites from Lake Thetis, Western Australia, providing new insights into microfossil preservation in carbonate stromatolites. Lake Thetis cyanobacteria exhibit a spectrum of mineralisation processes that include early precipitation of Mg‐silicates, largely controlled by the morphochemical features of the cyanobacteria, followed by aragonite formation that is inferred to be driven by heterotrophic activity. Fossilised cyanobacteria with high‐quality morphological preservation are characterised by a significant volume of authigenic Mg‐silicates, which have preferentially nucleated in/on extracellular organic material and on cell walls, and now replicate the region once occupied by the cyanobacterial sheath. In such specimens, aragonite is restricted to the outer sheath margin and parts of the cell interior. Cyanobacteria that display more significant degradation appear to possess a higher ratio of aragonite to Mg‐silicate. In these specimens, aragonite forms micronodules in the sheath zone and is spatially associated with the inferred remains of heterotrophic bacteria. Aragonite also occurs as an advancing front from the outer margin of the sheath where it is commonly intergrown with Mg‐silicates. Where there is no evidence of Mg‐silicates within filaments, the fidelity of microfossil preservation is poor. In these cases, individual filaments may no longer be visible under light microscopy, and little organic material remains, but filament traces remain detectable using electron microscopy due to variations in aragonite texture. These data provide further evidence that authigenic silicate minerals play a crucial role in the fossilisation of micro‐organisms; in their absence, carbonate crystal growth potentially mediated by heterotrophic microbial decay may largely obliterate morphological evidence for life within stromatolites, although mineralogical traces may still be detectable using electron microscopy.     

Hydrochemistry and microbialites of the alkaline crater lake Alchichica,Mexico

The structure, mineralogy, and accretion processes of the modern and subfossil cyanobacterial microbialites from the alkaline crater lake Alchichica (Puebla, Mexico) were studied, along the lake’s bathymetry and hydrochemistry. The recent lowering of the lake level had exposed microbialitic carbonate mounds and crusts, which emerged up to 2 m above the water surface, while accreting cyanobacterial microbialites were present down to a depth of ~15 m. Morphological and molecular analysis found that the living cyanobacterial mats were composed of diverse filamentous and coccoid cyanobacteria (Nostocales, Chroococcales, Oscillatoriales, and Pleurocapsales). The emerged subfossil microbialites comprised two generations: “white” (domes and crusts composed mainly of hydromagnesite with an admixture of huntite and calcite, ²³⁸U/²³⁰Th age of ~2.8 ka BP), and “brown” (chimneys, columns and laminated crusts composed of aragonite with an admixture of Mg-calcite, ²³⁸U/²³⁰Th age of ~1.1 ka BP). The significant age, structural, mineralogical, and isotopic differences suggest that the two generations were formed in different environmental conditions: the “white” during a dry period, and the “brown” in wet climate associated with high water level and intense inflow of ground water, which lowered the Mg/Ca ratio resulting in formation of aragonite instead of hydromagnesite. The hydromagnesite, replacing the primary aragonite precipitated in the living cyanobacterial biofilm, frequently undergoes silicification, which obliterates both the primary structure of the carbonate and the enclosed remains of cyanobacterial microbiota. This process helps to explain the abundant formation of dolomites and cherts in an allegedly highly alkaline Early Precambrian ocean. Thus, Lake Alchichica represents a modern alkaline environment where biosedimentary structures resembling Precambrian deposits are generated.     

Microatoll microbialites of Lake Clifton,Western Australia: Morphological analogues ofCryptozoön proliferum Hall,the first formally-named stromatolite

Summary The Linnaean nameCryptozo?n proliferum Hall was proposed in 1883 for a previously undescribed life-form preserved in spectacular exposures of Cambrian limestones in New York State, USA. It is now recognised that these are exposures of stromatolitic microbialites, laminated organosedimentary structures formed from interaction between a benthic microbial community (BMC) and the environment. Microbialites are neither fossil organisms nor trace fossils. These complex structures are the products of dissipative, self-organising systems involving a BMC, the external environment and the accreting microbialite. Functionally analogous BMCs of different species compositions may build similar structures in similar environments in quite separate periods. The type exposures ofCryptozo?n proliferum show objects composed of complex, concentric rings, up to a metre in diameter, that have grown laterally without any restriction other than that provided by neighbouring structures. They are not the relicts of domes truncated by penecontemporaneous erosion or Pleistocene glaciation, but depositional forms in which upward growth was restricted. Analogous modern structures occur on a reef platform along the north east shore of hyposaline Lake Clifton, Western Australia. These are tabular thrombolitic microbialites that vary lakeward across the reef platform from low, compound structures to discrete, concentric structures up to 50 cm high. The Lake Clifton forms are, in turn, morphological analogues of microatolls found on coral reef platforms. Coral microatolls are coral colonies with flat, dead tops and living perimeters in which upward growth is constrained by the sea surface. In shallow water they form circular rims of laterally growing coral around a dead centre. In deeper water they form coral heads that develop flat tops on reaching sea level. It is concluded that both the tabular microbialites of Lake Clifton and the type exposures ofCryptozo?n proliferum are analogous to coral microatolls in both form and origin-structures that have been able to grow laterally, but in which upward growth is restricted by subaerial exposure. Similar microatoll microbialites have been described from other modern environments, including Great Salt Lake, Utah, USA and Stocking Island, Exuma Cays, Bahamas. Ancient examples may include some of the “tufa” deposits of the Basal Purbeck Formation in Dorset, UK, as well as the coalesced domal bioherms of the Upper Cambrian Arrinthrunga Formation of the Georgina Basin, Central Australia, and the “washbowl” structures described from the B?tsfjord Formation of the Varanger Peninsula, north Norway. Progress towards a reliable interpretation of ancient microbialites depends on an understanding of modern environments in which analogous structures are forming. This study of microatolls has demonstrated that other sessile life forms may create colonial ecomorphs that, used cautiously, can serve as analogues for understanding the factors controlling the growth and form of microbialites. The surprising lack of pre-Pleistocene examples of microatolls recorded to date has simply been due to their lack of recognition in the geological record. They occur in sequences from the Proterozoic onwards, and provide powerful environmental indicators of ancient reef platforms on which biological growth was adjusted to contemporary sea level.     

Carbonate organo-mineral micro- and ultrastructures in sub-fossil stromatolites: Marion lake, South Australia

Sub-fossil stromatolites (5000-3000 years old) occur on the marginal flat surrounding Marion Lake (South Australia). A micrite/microsparite crystal fabric characterises these fine-grained, well-laminated stromatolites, which lack trapped grains. The internal lamination is characterised by a sub-millimetric alternation of porous and dense laminae. The microfabric of the laminae is ubiquitously composed of a fine (10-20 μm) peloidal texture, with many thinner aphanitic layers. Aggregates of very fine, low-Mg calcite and aragonite constitute both peloidal and aphanitic micrite, which is coated, respectively, by spherulitic and fringing acicular microspar. Micrite, with a high organic matter content, is formed of coalescing nanospheres grading into small polyhedrons, probably composed mainly of aragonite, with less calcite enriched in Mg, Sr, Na and S. Bacteria-like microfossils and relics of extracellular polymeric substance (EPS) occur abundantly within this micritic framework. The former consist of empty moulds and mineralised bodies of coccoid forms, whereas EPS relics consist of sheet-like or filamentous structures that appear both mineralised and more often still preserved as a C-enriched dehydrated substance that represents the main organic matter component of the deposit. Acicular crystals, which show a prismatic elongate shape, are composed of Mg-depleted aragonite that lacks fossils or organic relicts. Degrading EPS and micro-organisms appear gradually to be replaced and entombed by the nanospherical precipitates, implying the existence of processes of organo-mineralisation within an original syn-sedimentary microbial community. Succeeding micron-scale crystals merge to form isolated or connected micritic aggregates (the peloids), followed by the gradual formation of the acicular crystals as purely inorganic precipitates.     

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.     

A freshwater analog for the production of Epiphyton‐like microfossils

Calcified microbial microfossils—often interpreted as cyanobacteria—were important components of Precambrian and Paleozoic limestones, but their paucity in modern marine environments complicates our ability to make conclusive interpretations about their taxonomic affinity and geologic significance. Freshwater spring‐associated limestones (e.g., travertine and tufa) serve as terrestrial analogs to investigate mineralization in and around aquatic biofilms on observable timescales. We document the diagenesis of calcite fabrics associated with the freshwater algae Oocardium stratum, an epiphytic colonial green algae (desmid) known for producing stalks of extracellular polymeric substances (EPS) and passively producing a bifurcating tubular calcite monocrystal. Bifurcating EPS stalks produced by Oocardium colonies can become calcified and preserved in ancient carbonate deposits. Calcified micritic EPS stalks have a filamentous morphology, show evidence of branching, and maintain uniformity in diameter thickness throughout the mm‐scale colony, much like the enigmatic calcimicrobe Epiphyton. We provide a mechanism by which calcification associated with a colonial semispherical micro‐organism produces microfossils that deceptively resemble filamentous forms. These findings have implications for the use of morphological traits when assigning taxonomic affinities to extinct microfossil groups and highlight the utility of calcifying freshwater modern environments to investigate microbial taphonomy.     

Modern carbonate microbialites from an asbestos open pit pond, Yukon, Canada

Microbialites were discovered in an open pit pond at an abandoned asbestos mine near Clinton Creek, Yukon, Canada. These microbialites are extremely young and presumably began forming soon after the mine closed in 1978. Detailed characterization of the periphyton and microbialites using light and scanning electron microscopy was coupled with mineralogical and isotopic analyses to investigate the mechanisms by which these microbialites formed. The microbialites are columnar in form (cm scale), have an internal spherulitic fabric (mm scale), and are mostly made of aragonite, which is supersaturated in the subsaline pond water. Initial precipitation is seen as acicular aragonite crystals nucleating onto microbial biomass and detrital particles. Continued precipitation entombs benthic diatoms (e.g. Brachysira vitrea), filamentous algae (e.g. Oedogonium sp.), dinoflagellates, and cyanobacteria. The presence of phototrophs at spherulite centers strongly suggests that these microbes play an important initial role in aragonite precipitation. Substantial growth of individual spherulites occurs abiotically through periodic precipitation of aragonite that forms concentric laminations around spherulite centers while pauses in spherulite growth allow for colonization by microbes. Aragonite associated with biomass (δ(13)C = -4.6‰ VPDB) showed a (13)C-enrichment of 0.8‰ relative to aragonite exhibiting no biomass (δ(13)C = -5.4‰ VPDB), which suggests a modest removal of isotopically light dissolved inorganic carbon by phototrophs. The combination of a low sedimentation rate, high calcification rate, and low microbial growth rate appears to result in the formation of these microbialites. The formation of microbialites at an historic mine site demonstrates that an anthropogenically constructed environment can foster microbial carbonate formation.