Calcium speciation and coordination environment in intracellular amorphous calcium carbonate (ACC) formed by cyanobacteria

An increasing number of bacteria has been shown to form intracellular amorphous calcium carbonates (ACC), as previously observed for biomineralizing eukaryotes. Yet, the short-range order of these bacterial intracellular ACC remains largely unexplored. Because ACC in bacteria is easily lost upon sample preparation, the use of in situ techniques is required to probe ACC within intact cells at the atomic scale. Here, we show the application of X-ray absorption near-edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) spectroscopies to study the calcium (Ca) speciation and coordination environment in intact cells of diverse ACC-forming cyanobacteria. Our results show that XANES was effective in detecting ACC in intact cells. Between 60 and 85% of the total Ca in ACC-forming cyanobacteria strains is contained in the ACC phase and the rest in a single phase or a mixture of amorphous phases, possibly Ca complexed by organic molecules. Moreover, the short-range order of cyanobacterial ACC was different from eukaryotic ACC. This short-range order was similar to monohydrocalcite. These findings shed light on the fate of cyanobacterial ACC and the role of ACC-forming cyanobacteria in the Ca biogeochemical cycle.