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The biomineralization of intracellular amorphous carbonates by bacteria
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Edité par CCSD -
International audience. Bacteria forming intracellular amorphous carbonates (iAC) have been increasingly discovered, including diverse Proteobacteria, among which some magnetotactic bacteria, as well as Cyanobacteria. The iAC grains can be massive and fill a large proportion of the cell volume. They are found in very diverse environments from marine to freshwater, anoxic sediments, soils or thermal water. Bacteria can form iAC even in aqueous solutions significantly undersaturated with all CaCO3 phases. Sometimes, these bacteria can be locally abundant such as in the case of Microcystis, a cosmopolitan bloom-forming cyanobacterium. This suggests a potentially important environmental impact, although efforts to better quantify this are in progress. These biocarbonates can have major and/or trace element composition at odd with what is expected for abiotic carbonates precipitated in the same environment. In particular, one iAC-forming cyanobacterium efficiently sequesters the radioactive 226Ra isotope intracellularly, even in Ra-diluted solutions and in the presence of competing cations such as Ca, Ba and Sr. This may benefit the future development of efficient 226Ra bioremediation strategies. The mechanisms involved in the formation of bacterial intracellular carbonates are under study. They likely differ among the diverse bacteria. Interestingly, a gene with no homologue of known function was discovered, which serves as a good marker of the capability of cyanobacteria to form iAC. Phylogenetic reconstructions suggest that this gene was present in ancestral cyanobacteria with losses in various lineages along the evolution of Cyanobacteria, indicating that this biomineralization pathway is ancient.
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