The role of microbes in reef-building communities of the Cannindah limestone (Mississippian), Monto region, Queensland, Australia

Reefs in the Cannindah Limestone at Old Cannindah Homestead, Monto region, Queensland, are exceptional in Eastern Australian Mississippian (Carboniferous) build-ups because of their largest dimension and differentiated microbial fabrics. Calcimicrobes and microbial carbonates, which represent a marine reefal environment occupied by both corals and sponges, are particularly abundant in the reef framework fabrics compared to other Mississippian build-ups in the world. They contributed significantly to the rigidity of the reefs on a crinoidal bank setting. Metazoans and calcimicrobes coexisted and played different roles in reef construction. Reef-building and cavity-dwelling microbes include Renalcis, Palaeomicrocodium, Girvanella, problematic Aphralysia, Ortonella, Shamovella-like, Rothpletzella-like, Wetheredella-like, and some problematic calcimicrobes, which occur in inter-corallite infillings of fasciculate rugose corals, in thrombolitic textures, in or within deposits between microdigitate stromatolite and laminated microbialites, and in reef cavities. Some reef intervals are entirely formed by Renalcis, Palaeomicrocodium, problematic calcimicrobes, and cement. Girvanella, as an encrusting calcimicrobe, generally bound bioclasts and micrite, or together with cement, formed boundstone. Microbial carbonates, including thrombolites, microencrusters, microdigitate stromatolite, laminated and tabular microbialite, irregular layers of self-encrusting vesicles, and microbial micrite, occur commonly in reef framestone and boundstone. The role of microbes and relevant microbial carbonates in the Cannindah reef limestone highlighted a significant account of microbial facies complexes associated with the Mississippian reefs.     

Palaeoenvironmental significance of cool‐water microbialites in the Darriwilian (Middle Ordovician) of Sweden

Well‐developed oncoids and centimetre‐sized stromatolites are reported for the first time from the Darriwilian (Middle Ordovician) cool‐water ‘orthoceratite limestone’ at Kinnekulle, Västergötland, Sweden. The characteristics and stratigraphical distribution of these microbialites show an apparent relationship to fluctuations in relative sea level. The most abundant and well‐developed oncoids occur in stratigraphical intervals that are characterized by notable sea‐level lowstands. Stromatolites, which share many compositional characteristics with the oncoids, are essentially confined to a single bed associated with an especially prominent lowstand. Stromatolite‐like lamination also occurs in the uppermost part of the studied succession, but this feature may be of abiogenic origin. The microbialites appear to be originally calcareous, but synsedimentary iron‐ and/or phosphate‐enriched laminae are conspicuous, and secondary substitution by coarse calcite and barite is common. Iron staining is most prominent in poorly preserved specimens. Diagenesis has occluded the identity of the producers of these microbialites, but characteristics of associated endolithic borings suggest that they were formed in photic waters. The laminated fabrics of the documented microbialites record a depositional environment sensitive to high‐frequency environmental change. Most significantly, the microbialites have provided important information about the depositional environment of their enigmatic host limestone, and the collective observations challenge the notion that the studied strata were deposited in a deep shelf to basinal environment – rather, it appears that they are to a large extent, shallow‐water deposits, formed in waters only a few tens of metres deep.     

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.     

Formation and stability of oxygen-rich bubbles that shape photosynthetic mats

Gas release in photic-zone microbialites can lead to preservable morphological biosignatures. Here, we investigate the formation and stability of oxygen-rich bubbles enmeshed by filamentous cyanobacteria. Sub-millimetric and millimetric bubbles can be stable for weeks and even months. During this time, lithifying organic-rich laminae surrounding the bubbles can preserve the shape of bubbles. Cm-scale unstable bubbles support the growth of centimetric tubular towers with distinctly laminated mineralized walls. In environments that enable high photosynthetic rates, only small stable bubbles will be enclosed by a dense microbial mesh, while in deep waters extensive microbial mesh will cover even larger photosynthetic bubbles, increasing their preservation potential. Stable photosynthetic bubbles may be preserved as sub-millimeter and millimeter-diameter features with nearly circular cross-sections in the crests of some Proterozoic conical stromatolites, while centrimetric tubes formed around unstable bubbles provide a model for the formation of tubular carbonate microbialites that are not markedly depleted in ¹³C.     

Microfacies and diagenesis of an upper oxfordian carbonate buildup in Mydlniki (Cracow Area,Southern Poland)

Summary A carbonate buildup near the top of the Upper Jurassic limestone sequence in the Cracow area with a rigid framework built ofTubiphytes and thrombolites, and some fragments of encrusted siliceous sponges and serpules is described. The limestones form a dome-like elevation at the eastern wall of a 15 m high quarry flanked on both sides by stratified limestones with cherts. Six microfacies have been distinguished within the buildup: (1)Tubiphytes/thrombolite boundstone and (2) bioclasticTubiphytes/thrombolite wackestone dominate in the central and bottom part of the buildup. They gradually replace the cyanobacterial crusts and siliceous sponges (3. sponge-algal boundstone), which are sporadically the rock-forming elements in the basal part of the buildup as well as the top. Serpules randomly distributed within the buildup also form small cm-sized structures with a rigid framework (4. serpula-peloid boundstone). (5) tuberoid-peloid wackestone/floatstone and (6) ooid intraclastic grainstone exhibit no significant distributional pattern. Bioclastic-peloidal packstone comprising material derived from the destruction of the buildup occurs in the highest part of the outcrop, overlying the buildup. The sediments of the buildup were subject to rapid lithification, evidence by borings and neptunian microdykes filled with internal sediments, as well as by fracturedTubiphytes. Numerous petrographic features indicate probable episodic emergence of the buildup during its growth; these include asymmetric dissolution textures, asymmetric cements, vadose crystal silt and calcite pseudomorphs after gypsum. Upper Oxfordian carbonate buildups in the Cracow area display various stages of evolution. The carbonate buildup in Mydlniki most closely resembles classical Upper Jurassic reefs.     

Youngest early carboniferous (late Visean) shallow-water patch reefs in eastern Australia (Rockhampton Group, Queensland): Combining quantitative micro- and macro-scale data

Summary Although skeletal organisms have received most of the emphasis in studies on Phanerozoic roef history, the roles of non-skeletal (non-enzymatic) carbonates (e.g., synsedimentary cements, automicrite, microbialite, etc.) in reef framework construction are becoming increasingly better understood. One problem in understanding the role of non-enzymatic carbonates in reef construction has been the difficulty in recognizing them in reef facies. Whereas skeletal organisms commonly can be recognized and documented in the field, non-enzymatic carbonates may be recognizable only in thin section. This paper describes the application of a new sampling technique that allows the quantitative comparison of skeletal macrofauna and flora with associated non-enzymatic carbonates and other microfaunal/microfloral constituents. The technique involves the point counting of thin sections made from small diameter cores that are systematically recovered from grids and line transects that cover a reasonable area (m²) of reef facies. Small, shallow-water patch reefs are abundant in scattered oolitic intervals in the Lower Carboniferous strata of eastern Australia. The youngest known Carboniferous reefs in eastern Australia occur in uppermost Visean strata (limestone FC5) near the top of the Rockhampton Group, approximately 50 km west-northwest of Rockhampton, Queensland. The largest sampled reef was 15 m thick and 42 m in diameter, with synsedimentary relief above the sea floor of at least 2 m during the primary growth phase. The reef occurs within bioclasticoolitic grainstones representing a shallow shelf setting and consists of eight common framework microfacies: 1) coral boundstone; 2) bryozoan boundstone; 3) mixed crinoid-bryozoan boundstone; 4) tubular problematica boundstone; 5) sponge-automicrite boundstone; 6) encrusted thrombolite boundstone; 7) mixed automicrite boundstone; and 8) thrombolitic wackestone-packstone. Reef growth was initiated by automicrite-producing biofilms, sponges and a tubular problematic organism. Primary relief building was accomplished by automicrite-dominated frameworks and lithistid sponges, crinoids, and corals. Large cerioidAphrophyllum coral colonies had a heterogeneous distribution through the reef. The framework of the main relief-bearing portion of the reef consists on average of 44.4% automicrite and automicrite-bound detritus, excluding automicrite-bound sponge body fossils, and at most 19.6% skeletal organisms in growth position (minimum of 7.2%). If sponge body fossils are included as automicrite framework, because they are preserved only as a result of automicrite formation, the percentage of automicrite and bound sediment is 54.9%. A smaller sampled reef consisting of the same basic facies had 39.5% automicrite and automicrite-bound sediment in its fremework (50.2% including sponges) and, at most, 33.4% skeletal organisms in growth position (minimum of 22.7%). The greater volume of skeletal framework in the small reef reflects a greater proportion of large corals. Of framebuilding skeletal organisms, automicrite-preserved lithistid and other sponges and cerioid rugose corals provided the greatest volume. However, crinoid holdfasts were the most widespread skeletal framework components. The dominant framework facies are sponge-automicrite boundstone, encrusted thrombolite, boundstone, mixed automicrite boundstone, and coral boundstone. The reefs are similar in overall framework construction and ecological succession to slightly older Visean reefs in eastern Australia and to some of the late Visean reefs of northern England. Surprisingly, framework similarities also exist between the reefs and certain thrombolite-lithistid-coral reefs of the European Jurassic.     

Prokaryotic and eukaryotic community structure in field and cultured microbialites from the alkaline Lake Alchichica (Mexico)

The geomicrobiology of crater lake microbialites remains largely unknown despite their evolutionary interest due to their resemblance to some Archaean analogs in the dominance of in situ carbonate precipitation over accretion. Here, we studied the diversity of archaea, bacteria and protists in microbialites of the alkaline Lake Alchichica from both field samples collected along a depth gradient (0-14 m depth) and long-term-maintained laboratory aquaria. Using small subunit (SSU) rRNA gene libraries and fingerprinting methods, we detected a wide diversity of bacteria and protists contrasting with a minor fraction of archaea. Oxygenic photosynthesizers were dominated by cyanobacteria, green algae and diatoms. Cyanobacterial diversity varied with depth, Oscillatoriales dominating shallow and intermediate microbialites and Pleurocapsales the deepest samples. The early-branching Gloeobacterales represented significant proportions in aquaria microbialites. Anoxygenic photosynthesizers were also diverse, comprising members of Alphaproteobacteria and Chloroflexi. Although photosynthetic microorganisms dominated in biomass, heterotrophic lineages were more diverse. We detected members of up to 21 bacterial phyla or candidate divisions, including lineages possibly involved in microbialite formation, such as sulfate-reducing Deltaproteobacteria but also Firmicutes and very diverse taxa likely able to degrade complex polymeric substances, such as Planctomycetales, Bacteroidetes and Verrucomicrobia. Heterotrophic eukaryotes were dominated by Fungi (including members of the basal Rozellida or Cryptomycota), Choanoflagellida, Nucleariida, Amoebozoa, Alveolata and Stramenopiles. The diversity and relative abundance of many eukaryotic lineages suggest an unforeseen role for protists in microbialite ecology. Many lineages from lake microbialites were successfully maintained in aquaria. Interestingly, the diversity detected in aquarium microbialites was higher than in field samples, possibly due to more stable and favorable laboratory conditions. The maintenance of highly diverse natural microbialites in laboratory aquaria holds promise to study the role of different metabolisms in the formation of these structures under controlled conditions.     

Commensal symbiosis between agglutinated polychaetes and sulfate‐reducing bacteria

Pendant bioconstructions occur within submerged caves in the Plemmirio Marine Protected Area in SE Sicily, Italy. These rigid structures, here termed biostalactites, were synsedimentarily lithified by clotted‐peloidal microbial carbonate that has a high bacterial lipid biomarker content with abundant compounds derived from sulfate‐reducing bacteria. The main framework builders are polychaete serpulid worms, mainly Protula with subordinate Semivermilia and Josephella. These polychaetes have lamellar and/or fibrillar wall structure. In contrast, small agglutinated terebellid tubes, which are a minor component of the biostalactites, are discontinuous and irregular with a peloidal micritic microfabric. The peloids, formed by bacterial sulfate reduction, appear to have been utilized by terebellids to construct tubes in an environment where other particulate sediment is scarce. We suggest that the bacteria obtained food from the worms in the form of fecal material and/or from the decaying tissue of surrounding organisms and that the worms obtained peloidal micrite with which to construct their tubes, either as grains and/or as tube encompassing biofilm. Peloidal worm tubes have rarely been reported in the recent but closely resemble examples in the geological record that extend back at least to the early Carboniferous. This suggests a long‐lived commensal relationship between some polychaete worms and heterotrophic, especially sulfate‐reducing, bacteria.     

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.     

Coexisting living stromatolites and infaunal metazoans

Microbialites, bioaccretionary structures formed during the growth and metabolism of microorganisms (principally cyanobacteria) were the dominant lifeform in shallow late-Archean and Proterozoic oceans. During the Cambrian radiation of metazoan life, which began ~540 Mya, microbialite abundance and diversity further declined following a peak in the Mesoproterozoic. Notwithstanding contention, grazing and bioturbation effects of metazoans have been hypothesized as the dominant driver of modern microbialite scarcity. However, this metazoan–microbialite exclusion has not been fully explored in the few extant microbialites. Here we provide further evidence showing that living marine layered microbialites (stromatolites) coexist with a persistent assemblage of benthic macro-invertebrates, as has previously been demonstrated in some thrombolitic (clotted) microbialites. Surprisingly, these metazoans have active habits, such as burrowing, which should be expected to disrupt the layered matrix. As other studies have shown, through a network of burrows, metazoans can exploit local diurnal oxygen refugia within microbialites as well as escape predation. Our results, therefore, add novel evidence in support of the hypotheses that geologically, metazoans are not always incompatible with stromatolites, while ecologically, microbialites may act as micro-refugia for modern metazoans and historically have performed a similar inferred role in past ecosystems.     

Legacies of recent environmental change in the benthic communities of Lake Joyce, a perennially ice-covered Antarctic lake

Many Antarctic lakes provide habitat for extensive microbial mats that respond on various timescales to environmental change. Lake Joyce contains calcifying microbialites and provides a natural laboratory to constrain how environmental changes influence microbialite development. In Lake Joyce, depth-specific distributions of calcitic microbialites, organic carbon, photosynthetic pigments and photosynthetic potential cannot be explained by current growth conditions, but are a legacy of a 7-m lake level rise between 1973 and 2009. In the well-illuminated margins of the lake, photosynthetically active benthic communities colonised surfaces submerged for just a few years. However, observed increases in accumulated organic material with depth from 5 to 20 m (2-40 mg ash-free dry weight cm(-2)) and the presence of decimetre-scale calcite microbialites at 20-22 m depth, apparently related to in situ photosynthetic growth, are inconsistent with the current distributions of irradiance, photosynthetic pigments and mat photosynthetic potential (as revealed by pulse-amplitude-modulated fluorometry). The microbialites appeared photosynthetically active in 1986 and 1997, but were outside the depth zone where significant phototrophic growth was possible and were weakly photosynthetically competent in 2009. These complex microbial structures have persisted after growth has ceased, demonstrating how fluctuating environmental conditions and the hysteresis between environmental change, biological response and microbialite development can be important factors to consider when interpreting modern, and by inference ancient, microbially mediated structures.     

Evaluation of the predatory wasp,Ancistrocerus gazella,for biological control of leafrollers in Otago fruit crops. II. Wasp phenology and seasonal changes in prey composition

The phenology of the predatory wasp, Ancistrocerus gazella (Panzer), colonising artificial nests was studied over 3 years in seven Study Areas in or adjacent to mixed pome and stonefruit orchards in Otago, New Zealand. Each Study Area had three to four nest sites comprising groups of wooden blocks drilled with 6 mm diameter nest tubes 75–150 mm in length. The timing of nest tube colonisation was recorded by observing the presence of nest closures (mud plugs) and their subsequent opening during adult emergence. The nest tubes from some entire nest sites were dissected to determine the stages of development of the wasps and their subsequent emergence, and samples of nest tubes were dissected weekly from a range of nest sites to determine changes in prey composition over the season. A. gazella was found to be primarily bivoltine, but part of each generation entered diapause and a small number of second generation wasps also emerged before winter in warm sites. The adult foraging of each generation was well synchronised with late larval instars of the two generations of three pest leafroller species. However, very low numbers of leafroller larvae were collected because A. gazella was highly polyphagous, predominantly exploiting a sequence of other lepidopterous species. It was concluded that management of A. gazella for leafroller control would be uneconomic but it provides a useful component of the natural enemy complex of pest leafroller species.     

Late Smithian microbial deposits and their lateral marine fossiliferous limestones (Early Triassic,Hurricane Cliffs,Utah, USA)

Recurrent microbialite proliferations during the Early Triassic are usually explained by ecological relaxation and abnormal oceanic conditions. Most Early Triassic microbialites are described as single or multiple lithological units without detailed ecological information about lateral and coeval fossiliferous deposits. Exposed rocks along Workman Wash in the Hurricane Cliffs (southwestern Utah, USA) provide an opportunity to reconstruct the spatial relationships of late Smithian microbialites with adjacent and contemporaneous fossiliferous sediments. Microbialites deposited in an intertidal to subtidal interior platform are intercalated between inner tidal flat dolosiltstones and subtidal bioturbated fossiliferous limestones. Facies variations along these fossiliferous deposits and microbialites can be traced laterally over a few hundreds of meters. Preserved organisms reflect a moderately diversified assemblage, contemporaneous to the microbialite formation. The presence of such a fauna, including some stenohaline organisms (echinoderms), indicates that the development of these late Smithian microbial deposits occurred in normal-marine waters as a simple facies belt subject to relative sea-level changes. Based on this case study, the proliferation of microbialites cannot be considered as direct evidence for presumed harsh environmental conditions.     

ATM activation in the presence of oxidative stress

The Ataxia-Telangiectasia mutated (ATM) kinase is regarded as the major regulator of the cellular response to DNA double strand breaks (DSBs). In response to DSBs, ATM dimers dissociate into active monomers in a process promoted by the Mre11-Rad50-Nbs1 (MRN) complex. ATM can also be activated by oxidative stress directly in the form of exposure to H₂O₂. The active ATM in this case is a disulfide-crosslinked dimer containing two or more disulfide bonds. Mutation of a critical cysteine residue in the FATC domain involved in disulfide bond formation specifically blocks ATM activation by oxidative stress. Here we show that ATM activation by DSB s is inhibited in the presence of H₂O₂ because oxidation blocks the ability of MRN to bind to DNA . However, ATM activation via direct oxidation by H₂O₂ complements the loss of MRN/DSB-dependent activation and contributes significantly to the overall level of ATM activity in the presence of both DSB s and oxidative stress.Key words: ATM, DNA repair, double-strand break, oxidative stress, ROS     

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.     

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.     

Genesis of microbialites as contemporaneous framework components of deglacial coral reefs, Tahiti (IODP 310)

Deglacial reefs from Tahiti (IODP 310) feature a co-occurrence of zooxanthellate corals with microbialites that compose up to 80 vol% of the reef framework. The notion that microbialites tend to form in more nutrient-rich environments has previously led to the concept that such encrustations are considerably younger than the coral framework, and that they have formed in deeper storeys of the reef edifice, or that they represent severe disturbances of the reef ecosystem. As indicated by their repetitive interbedding with coralline red algae, the microbialites of this reef succession of Tahiti, however, formed immediately after coral growth under photic conditions. Clearly, the deglacial reef microbialites present in the IODP 310 cores did not follow disturbances such as drowning or suffocation by terrestrial material, and are not “disaster forms”. Given that the corals and the microbialites developed in close spatial proximity, highly elevated nutrient levels caused by fluvial or groundwater transport from the volcanic hinterland are an unlikely cause for the exceptionally voluminous development of microbialites. That voluminous deglacial reef microbialites generally are restricted to volcanic islands, however, implies that moderately, and possibly episodically elevated nutrient levels favored this type of microbialite formation.     

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.     

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.