Several species of cyanobacteria biomineralizing intracellular amorphous calcium carbonates (ACC) were recently discovered. However, the mechanisms involved in this biomineralization process and the determinants discriminating species forming intracellular ACC from those not forming intracellular ACC remain unknown. Recently, it was hypothesized that the intensity of Ca uptake (i.e., how much Ca was scavenged from the extracellular solution) might be a major parameter controlling the capability of a cyanobacterium to form intracellular ACC. Here, we tested this hypothesis by systematically measuring the Ca uptake by a set of 52 cyanobacterial strains cultured in the same growth medium. The results evidenced a dichotomy among cyanobacteria regarding Ca sequestration capabilities, with all strains forming intracellular ACC incorporating significantly more calcium than strains not forming ACC. Moreover, Ca provided at a concentration of 50 μM in BG‐11 was shown to be limiting for the growth of some of the strains forming intracellular ACC, suggesting an overlooked quantitative role of Ca for these strains. All cyanobacteria forming intracellular ACC contained at least one gene coding for a mechanosensitive channel, which might be involved in Ca influx, as well as at least one gene coding for a Ca2+/H+ exchanger and membrane proteins of the UPF0016 family, which might be involved in active Ca transport either from the cytosol to the extracellular solution or the cytosol toward an intracellular compartment. Overall, massive Ca sequestration may have an indirect role by allowing the formation of intracellular ACC. The latter may be beneficial to the growth of the cells as a storage of inorganic C and/or a buffer of intracellular pH. Moreover, high Ca scavenging by cyanobacteria biomineralizing intracellular ACC, a trait shared with endolithic cyanobacteria, suggests that these cyanobacteria should be considered as potentially significant geochemical reservoirs of Ca.
Better understanding the conditions of formation of authigenic Mg‐silicates and their reactivity is key to interpret the palaeoenvironmental message carried by the sedimentary record and evaluate the effect of reverse weathering, a process involved in long‐term climate evolution. Microbialites from most alkaline crater lakes in Mexico contain Mg‐silicates except those in Lake Alchichica, where concentration of orthosilicic acid is low (<26 μm). This study investigated the first metre of sediments in Lake Alchichica in order to check how their mineralogy compared with that of shoreline microbialites. The mineralogy and chemistry of the sediment column were determined, together with the pore water chemistry, providing insights on the processes occurring during early diagenesis. Below ca 3 cm in depth, diatom frustules are progressively pseudomorphized into Al‐poor Mg‐silicates with a composition corresponding to stevensite. This diagenetic process is massive and the resulting silicate represents between 30 and 53 wt.% of the sediment content at all depths. This observation questions the possibility to infer lake palaeochemistry from the presence/absence of Mg‐silicates in the sedimentary record. Moreover, it allowed refinement of the conditions under which Mg‐silicates authigenesis occurs: the saturation of the solution should be higher or equal to the solubility of a Mg‐silicate phase close to that of ‘amorphous sepiolite’. Although the solubility of authigenic silicates is a key parameter of reverse weathering modelling during geological times, it is still debated. In this study, a solubility constant deduced from a natural system is proposed that should be considered when modelling the formation of Mg‐silicates in a natural environment. The proportion of reverse weathering associated with this solubility constant could be higher than previously predicted based on experiments and thus have a greater impact on climate stability over geological timescales.
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