International audienceMicrobialitesareorgano-sedimentaryrocksfoundinabundancethroughoutthegeologicalrecordbackto∼3.5Ga.Interpretationsofthebiologicalandenvironmentalconditionsunderwhichtheyformedrelyoncomparisonswithmodernmicrobialites.Therefore,abettercharacterizationofdiversemodernmicrobialitesiscrucialtoimprovesuchinterpretations.Here,westudiedmodernmicrobialitesfromthreeMexicanalkalinecraterlakes:Quechulac,LaPreciosaandAtexcac.Thegeochemicalanalysesofwatersolutionsshowedthattheyweresupersaturatedtovaryingextentswithseveralmineralphases,includingaragonite,calcite,hydromagnesite,aswellashydratedMg-silicates.Consistently,X-raydiffraction(XRD)andFouriertransforminfraredspectroscopy(FTIR)analysesrevealedthatmicrobialitesarecomposedofadiversityofmineralphasesincludingaragoniteandsometimescalcite,hydromagnesite,andmoreinterestingly,apoorly-crystallinehydratedsilicatephase.Couplingofscanningelectronmicroscopy(SEM)withenergydispersiveX-rayspectrometrymicroanalysesonpolishedsectionsshowedthatthislatterphaseisabundant,authigenic,magnesium-richandsometimesassociatedwithironandmanganese.Thismineralphaseissimilartokerolite,ahydratedpoorlycrystallinetalc-likephase(Mg3Si4O10(OH)2·nH2O).Diversemicrofossilswerepermineralizedbythissilicatephase.Someofthemwereimagedin3DbyFIB-tomographyshowingthattheirmorphologywasexquisitelypreserveddowntothefewnm-scale.ThestructuralandchemicalfeaturesofthesefossilswerefurtherstudiedusingacombinationoftransmissionelectronmicroscopyandscanningtransmissionX-raymicroscopy(STXM)atthecarbonandmagnesiumK-edgesandironL2,3-edges.Theseresultsshowedthatorganiccarbonispervasivelyassociatedwithkerolite.Overall,itissuggestedthatthepoorly-crystallinehydratedmagnesium-richsilicateformsinmanyalkalinelakesandhasastrongpotentialforfossilizationofmicrobesandorganicmatter.Moreover,thefrequentoccurrenceofsuchanauthigenicsilicatephaseinmodernlacustrinemicrobialitescallsforareappraisalofitspotentialpresenceinancientrocks

Cyanobacteria are thought to play a key role in carbonate formation due to their metabolic activity, but other organisms carrying out oxygenic photosynthesis (photosynthetic eukaryotes) or other metabolisms (e.g., anoxygenic photosynthesis, sulfate reduction), may also contribute to carbonate formation. To obtain more quantitative information than that provided by more classical PCR-dependent methods, we studied the microbial diversity of microbialites from the Alchichica crater lake (Mexico) by mining for 16S/18S rRNA genes in metagenomes obtained by direct sequencing of environmental DNA. We studied samples collected at the Western (AL-W) and Northern (AL-N) shores of the lake and, at the latter site, along a depth gradient (1, 5, 10, and 15 m depth). The associated microbial communities were mainly composed of bacteria, most of which seemed heterotrophic, whereas archaea were negligible. Eukaryotes composed a relatively minor fraction dominated by photosynthetic lineages, diatoms in AL-W, influenced by Si-rich seepage waters, and green algae in AL-N samples. Members of the Gammaproteobacteria and Alphaproteobacteria classes of Proteobacteria, Cyanobacteria, and Bacteroidetes were the most abundant bacterial taxa, followed by Planctomycetes, Deltaproteobacteria (Proteobacteria), Verrucomicrobia, Actinobacteria, Firmicutes, and Chloroflexi. Community composition varied among sites and with depth. Although cyanobacteria were the most important bacterial group contributing to the carbonate precipitation potential, photosynthetic eukaryotes, anoxygenic photosynthesizers and sulfate reducers were also very abundant. Cyanobacteria affiliated to Pleurocapsales largely increased with depth. Scanning electron microscopy (SEM) observations showed considerable areas of aragonite-encrusted Pleurocapsa-like cyanobacteria at microscale. Multivariate statistical analyses showed a strong positive correlation of Pleurocapsales and Chroococcales with aragonite formation at macroscale, and suggest a potential causal link. Despite the previous identification of intracellularly calcifying cyanobacteria in Alchichica microbialites, most carbonate precipitation seems extracellular in this system.

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International audienceInterpreting the environmental conditions of ancient microbialites rely on comparisons with modern analogues. Yet, we lack a detailed reference framework relating the chemical and mineralogical composition of modern lacustrine microbialites with the physical and chemical parameters prevailing in the lakes where they form. Here we performed geochemical analyses of water solutions and mineralogical analyses of microbialites in 12 Mexican crater lakes. We found a large diversity of microbialites in terms of mineralogical composition, with occurrence of diverse carbonate phases such as magnesian calcite, monohydrocalcite, aragonite, hydromagnesite, and dolomite as well as authigenic magnesium silicate phases. In parallel, the chemical compositions of the lakes differed particularly by their alkalinity, their concentration of ortho-silicic acid (H 4 SiO 4) and their Mg/Ca ratio. From this study, we infer a minimum alkalinity value for the formation of lacustrine microbialites, as well as several constraints given by the presence of mineral phases on the chemical composition of the lakes in which microbialites formed. Finally, we observe a general correlation between the alkalinity and the sodium content of the lakes. This may relate to variations in evaporation intensity and provide a historical model for lacustrine microbialite formation: microbialite start forming only when the lake is sufficiently old/evaporated

Microbial mats have been repeatedly suggested to promote early fossilization of macroorganisms. Yet, experimental simulations of this process remain scarce. Here, we report results of 5 year-long experiments performed onfish carcasses to document the influence of microbial mats on mineral precipitation during early fossilization. Carcasses were initially placed on top of microbial mats. After 2 weeks, fish became coated by the mats forming a compact sarcophagus, which modified the microenvironment close to the corpses. Our results showed that these conditions favored the precipitation of a poorly crystalline silicate phase rich in magnesium. This talc-like mineral phase has been detected in three different locations within the carcasses placed in microbial mats for more than 4 years: (1) within inner tissues, colonized by several bacillary cells; (2) at the surface of bones of the upper face of the corpse buried in the mat; and (3) at the surface of several bones such as the dorsal fin which appeared to be gradually replaced by the Mg-silicate phase. This mineral phase has been previously shown to promote bacteria fossilization. Here we provide first experimental evidence that such Mg-rich phase can also be involved in exceptional preservation of animals.

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