International audienceAragonitic microbialites, characterized by a reticulate fabric,were discovered beneath lacustrine microbial mats on the atoll ofKiritimati, Republic of Kiribati, Central Pacific. The microbialmats, with cyanobacteria as major primary producers, grow inevaporated seawater modified by calcium carbonate and gypsumprecipitation and calcium influx via surface and/or groundwaters.Despite the high aragonite supersaturation and a high photosyntheticactivity, onlyminor aragonite precipitates are observed in thetop parts of the microbial mats. Instead, major aragonite precipitationtakes place in lower mat parts at the transition to the anoxiczone. The prokaryotic community shows a high number of phylotypesclosely related to halotolerant taxa and/or taxa with preferenceto oligotrophic habitats. Soil- and plant- inhabiting bacteriaunderline a potential surface or subsurface influx from terrestrialareas, while chitinase-producing representatives coincide with theoccurrence of insect remains in the mats. Strikingly, many of theclones have their closest relatives inmicroorganisms either involvedin methane production or consumption ofmethane or methyl compounds.Methanogens, represented by the methylotrophic genusMethanohalophilus, appear to be one of the dominant organisms inanaerobic mat parts. All this points to a significant role of methaneand methyl components in the carbon cycle of the mats. Nonetheless,thin sections and physicochemical gradients through themats,as well as the 12C-depleted carbon isotope signatures of carbonatesindicate that spherulitic components of the microbialites initiatein the photosynthesis-dominated orange mat top layer, and furthergrow in the green and purple layer below. Therefore, thesespherulites are considered as product of an extraordinary highphotosynthesis effect simultaneous to a high inhibition by pristineexopolymers. Then, successive heterotrophic bacterial activityleads to a condensation of the exopolymer framework, and finallyto the formation of crevice-like zones of partly degraded exopolymers.Here initiation of horizontal aragonite layers and verticalaragonite sheets of the microbialite occurs, which are consideredas a product of high photosynthesis at decreasing degree of inhibition.Finally, at low supersaturation and almost lack of inhibition,syntaxial growth of aragonite crystals at lamellae surfaces leadsto thin fibrous aragonite veneers. While sulfate reduction, methylotrophy,methanogenesis and ammonification play an importantrole in element cycling of the mat, there is currently no evidencefor a crucial role of them in CaCO3 precipitation. Instead, photosynthesisand exopolymer degradation sufficiently explain theobserved pattern and fabric of microbialite formation

The atoll of Christmas Island (now known as Kiritimati) in the Kiribati Republic (Central Pacific) lies at about 2°N in the intertropical convergence zone. Much of the surface area of the atoll (ca. 360 km2) is occupied by numerous lakes in which carbonate, evaporite (calcium sulfate, halite) and organic layers are deposited. Observations suggest that deposition of these different laminae is controlled by climatic and biologic factors. It is thought that periodic climatic variations, such as El Niño-Southern Oscillations (ENSO) events which bring heavy rainfall to the atoll, result in the succession of the precipitation of carbonate minerals (during periods after dilution of hypersaline waters by heavy rains), followed by evaporitic minerals (carbonate, calcium sulfate, halite) when salinity increases through evaporation. Thick (up to 5 cm) microbial (essentially cyanobacterial) mats develop continuously on the lake bottom surfaces providing the sediment with an important (total organic carbon 2–5%) organic contribution in the form of an internal, geometrically structured, network in which the authigenic minerals precipitate. The high bioproductivity of these microbial populations is reflected in low δ13C values of sedimentary organic carbon (−14 to −17‰), interpreted as being the result of high atmospheric CO2 demand (Geochim. Cosmochim. Acta, 56 (1992) 335). The well-laminated organic layers present in the sediment profile result from the death and burial of microbial populations at the time of severe climatic events (storms, heavy rainfall). These lagoonal lakes provide a model for the deposition of carbonate and organic matter in an evaporitic environment. The high ratio of deposited carbonate vs. sulfate+chloride, when compared to low ratio in evaporitic salinas, results from both a lack of limitation of calcium, magnesium and carbonate ions (in a carbonate reef environment) and active processes of high-Mg calcite precipitation (organomineralization)

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