Photosynthetic carbon assimilation is commonly invoked as the cause of calcium carbonate precipitation in cyanobacterial biofilms that results in the formation of calcareous stromatolites. However, biofilm calcification patterns in recent lakes and simulation of photosynthetically induced rise in calcium carbonate supersaturation demonstrate that this mechanism applies only in settings low in dissolved inorganic carbon and high in calcium. Taking into account paleo-partial pressure curves for carbon dioxide, we show that Phanerozoic oceans sustaining calcified cyanobacteria must have had considerably higher calcium concentrations than oceans of today. In turn, the enigmatic lack of calcified cyanobacteria in stromatolite-bearing Precambrian sequences can now be explained as a result of high dissolved inorganic carbon concentrations.

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On the Kiritimati atoll, several lakes exhibit microbial mat-formation under different hydrochemical conditions. Some of these lakes trigger microbialite formation such as Lake 21, which is an evaporitic, hypersaline lake (salinity of approximately 170‰). Lake 21 is completely covered with a thick multilayered microbial mat. This mat is associated with the formation of decimeter-thick highly porous microbialites, which are composed of aragonite and gypsum crystals. We assessed the bacterial and archaeal community composition and its alteration along the vertical stratification by large-scale analysis of 16S rRNA gene sequences of the nine different mat layers. The surface layers are dominated by aerobic, phototrophic, and halotolerant microbes. The bacterial community of these layers harbored Cyanobacteria (Halothece cluster), which were accompanied with known phototrophic members of the Bacteroidetes and Alphaproteobacteria. In deeper anaerobic layers more diverse communities than in the upper layers were present. The deeper layers were dominated by Spirochaetes, sulfate-reducing bacteria (Deltaproteobacteria), Chloroflexi (Anaerolineae and Caldilineae), purple non-sulfur bacteria (Alphaproteobacteria), purple sulfur bacteria (Chromatiales), anaerobic Bacteroidetes (Marinilabiacae), Nitrospirae (OPB95), Planctomycetes and several candidate divisions. The archaeal community, including numerous uncultured taxonomic lineages, generally changed from Euryarchaeota (mainly Halobacteria and Thermoplasmata) to uncultured members of the Thaumarchaeota (mainly Marine Benthic Group B) with increasing depth.

The crater lake of the small volcanic island Satonda, Indonesia, is unique for its red-algal microbial reefs thriving in marinederived water of increased alkalinity. The lake is a potential analogue for ancient oceans sustaining microbialites under open-marine conditions. Current reef surfaces are dominated by living red algae covered by non-calcified biofilms with scattered cyanobacteria and diatoms. Minor CaCO 3 precipitates are restricted to the seasonally flooded reef tops, which develop biofilms up to 500 m thick dominated by the cyanobacteria Pleurocapsa, Calothrix, Phormidium, and Hyella. Microcrystalline aragonite patches form within the biofilm mucilage, and fibrous aragonite cements grow in exopolymer-poor spaces such as the inside of dead, lysed green algal cells, and reef framework voids. Cementation of lysed hadromerid sponge resting bodies results in the formation of ''Wetheredella-like'' structures. Hydrochemistry data and model calculations indicate that CO 2 degassing after seasonal mixis can shift the carbonate equilibrium to cause CaCO 3 precipitation. Increased concentrations of dissolved inorganic carbon limit the ability of autotrophic biofilm microorganisms to shift the carbonate equilibrium. Therefore, photosynthesis-induced cyanobacterial calcification does not occur. Instead, passive, diffusioncontrolled EPS-mediated permineralization of biofilm mucus at contact with the considerably supersaturated open lake water takes place. In contrast to extreme soda lakes, the release of Ca 2؉ from aerobic degradation of extracellular polymeric substances does not support CaCO 3 precipitation in Satonda because the simultaneously released CO 2 is insufficiently buffered.Subfossil reef parts comprise green algal tufts encrusted by microstromatolites with layers of fibrous aragonite and an amorphous, unidentified Mg-Si phase. The microstromatolites probably formed when Lake Satonda evolved from seawater to Ca 2؉ -depleted raised-alkalinity conditions because of sulfate reduction in bottom sediments and pronounced seasonality with deep mixing events and strong CO 2 degassing. The latter effect caused rapid growth of fibrous aragonite, while Mg-Si layers replaced the initially Mg-calcite-impregnated biofilms. This could be explained by dissolution of siliceous diatoms and sponge spicules at high pH, followed by Mg-calcite dissolution and Mg-silica precipitation at low pH due to heterotrophic activity within the entombed biofilms.

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