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International audience; Bacteria synthesize a wide range of intracellular submicrometer-sized inorganic precipitates of diverse chemical compositions and structures, called biominerals. Their occurrences, functions and ultrastructures are not yet fully described despite great advances in our knowledge of microbial diversity. Here, we report bacteria inhabiting the sediments and water column of the permanently stratified ferruginous Lake Pavin, that have the peculiarity to biomineralize both intracellular magnetic particles and calcium carbonate granules. Based on an ultrastructural characterization using transmission electron microscopy (TEM) and synchrotron-based scanning transmission X-ray microscopy (STXM), we showed that the calcium carbonate granules are amorphous and contained within membrane-delimited vesicles. Single-cell sorting, correlative fluorescent in situ hybridization (FISH), scanning electron microscopy (SEM) and molecular typing of populations inhabiting sediments affiliated these bacteria to a new genus of the Alphaproteobacteria. The partially assembled genome sequence of a representative isolate revealed an atypical structure of the magnetosome gene cluster while geochemical analyses indicate that calcium carbonate production is an active process that costs energy to the cell to maintain an environment suitable for their formation. This discovery further expands the diversity of organisms capable of intracellular Ca-carbonate biomineralization. If the role of such biomineralization is still unclear, cell behaviour suggests that it may participate to cell motility in aquatic habitats as magnetite biomineralization does.

International audience; 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.

The metal excretion and metal sorption are two important microbial processes in the regulation of intracellular metal concentration. However, the contribution of freshwater phototrophic biofilm to metal efflux in the environment and its related isotopic fractionation remain poorly known. In this study, Cu efflux from a mature phototrophic biofilm and related Cu isotopic fractionation between the biofilm and aqueous solution were studied over 4 days in closed batch and open drip-flow reactors (BR and DFR, respectively). The impact of a 48-h drying event on Cu efflux from the biofilm and its isotopic signature was quantified in the BR. Cu excretion from the biofilm depended on Cu concentration in the biomass and did not show a latent phase. For the wet biofilm, in DFR, the Cu efflux rate decreased over time whereas in BR, Cu efflux exhibited fluctuations with a net re-sorption phase. During short-term early excretion over the first 4 h (BR) and 24 h (DFR), the enrichment of solution in heavy isotopes with a Δ65Cu(sol–biofilm) of +0.7 ± 0.2‰ could be explained by a combination of i) desorption via competition between H+ and Cu2+ for surface sites and ii) release of isotopically heavy Cu(II)-organic complexes. In addition, there was a retention of isotopically light Cu (I) within the cells which is consistent with the biofilm ability to reduce Cu. With further exposure of the wet biofilm to the solution, a progressive decrease of Δ65Cu(sol–biofilm), down to 0 and −0.36‰ respectively in BR and DFR after 96 h, could be explained by an active efflux of Cu(I) and by a passive diffusion of Cu2+, both of these processes favoring light isotopes. The re-hydrated and wet biofilms showed distinctly different behavior during excretion, in terms of magnitude of the flux and direction of the fractionation for long term exposure. Indeed, the efflux from the re-hydrated biofilm was ten times higher than the one from the wet biofilm and the re-hydrated biofilm did not exhibit a decrease of Δ65Cu(sol–biofilm) after 24 h of reaction. This could be related to partial devitalization of the biofilm during drying, producing a release of isotopically heavy Cu and decreasing the intensity of Cu(I) active efflux of light isotopes. Taken together, the Cu isotopic signature in natural aquatic environments containing phototrophic biofilms varies up to 1‰ on a daily scale.

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 Ca

International audience; The cyanobacterium Gloeomargarita lithophora forms intracellular amorphous calcium carbonates and has been recently shown to accumulate Ca, Sr and Ba at a high level with a preference for Ba > Sr > Ca. This is of interest to the design of bioremediation strategies for Sr and/or Ba pollutions in aquatic systems. However, this preference has so far been evidenced at high dissolved Sr and Ba concentrations only, i. e. conditions rarely encountered in the environment. Here, we followed the uptake of alkaline earth metals (Mg, Ca, Sr and Ba) by G. lithophora in alkaline solutions with more environmentally-relevant, low Sr and Ba concentrations. Two settings were analyzed: 1) batch cultures in which alkaline earth metals were added only at the beginning; 2) cultures supplemented continuously with fragments of alkaline earth element-containing carbonate microbialites. The growth of G. lithophora was higher in the presence of microbialites, suggesting that some limiting elements, possibly P and/or Ca, were supplied by their dissolution. In the two settings, the concentration of dissolved Ca was in the micromolar range, while it was more than three orders of magnitude lower for Ba and Sr. In cultures with microbialite fragments, these low Sr and Ba concentrations resulted from an input by aragonite dissolution balanced by high Sr and Ba uptake by the cyanobacterial cells with high affinity. Thereby, the continuous supply of alkaline earth metals by microbialite dissolution increased the Sr and Ba content of the cyanobacterial intracellular carbonates over time. G. lithophora incorporated preferentially Ba and Sr over Ca even at very low Ba and Sr concentrations and despite the presence of Ca at a much higher concentration. Overall, this shows the capability of G. lithophora to impact the geochemical cycles of Sr and Ba by buffering dissolved Sr/Ca and Ba/Ca ratios at low levels.