Widespread formation of intracellular calcium carbonates by the bloom-forming cyanobacterium Microcystis.

The formation of intracellular amorphous calcium carbonates (iACC) has been recently observed in a few cultured strains of Microcystis, a potentially toxic bloom-forming cyanobacterium found worldwide in freshwater ecosystems. If iACC-forming Microcystis are abundant within blooms, they may represent a significant amount of particulate Ca. Here, we investigate the significance of iACC biomineralization by Microcystis. First, the presence of iACC-forming Microcystis cells has been detected in several eutrophic lakes, indicating that this phenomenon occurs under environmental conditions. Second, some genotypic (presence/absence of ccyA, a marker gene of iACC biomineralization) and phenotypic (presence/absence of iACC) diversity have been detected within a collection of strains isolated from one single lake. This illustrates that this trait is frequent but also variable within Microcystis even at a single locality. Finally, one-third of publicly available genomes of Microcystis were shown to contain the ccyA gene, revealing a wide geographic and phylogenetic distribution within the genus. Overall, the present work shows that the formation of iACC by Microcystis is common under environmental conditions. While its biological function remains undetermined, this process should be further considered regarding the biology of Microcystis and implications on the Ca geochemical cycle in freshwater environments.

Detection of biogenic amorphous calcium carbonate (ACC) formed by bacteria using FTIR spectroscopy.

While the formation of intracellular amorphous calcium carbonate (ACC) by living organisms is widespread, its detection in prokaryotes remains difficult owing to its susceptibility to transform or dissolve upon sample preparation. Because of these challenges, a large number of ACC-forming prokaryotes may have been undetected and their abundance in the natural environment is possibly underestimated. This study identifies diagnostic spectral markers of ACC-forming prokaryotes that facilitate their detection in the environment. Accordingly, ACC formed by cyanobacteria was characterized using Fourier transform infrared (FTIR) spectroscopy in near-IR, mid-IR, and far-IR spectral regions. Two characteristic FTIR vibrations of ACC, at âˆ¼ 860 cm-1and âˆ¼ 306 cm-1, were identified as reliable spectral probes to rapidly detect prokaryotic ACC. Using these spectral probes, several Microcystis strains whose ACC-forming capability was unknown, were tested. Four out of eight Microcystis strains were identified as possessing ACC-forming capability and these findings were confirmed by scanning electron microscopy (SEM) observations. Overall, our findings provide a systematic characterization of prokaryotic ACC that facilitate rapid detection of ACC forming prokaryotes in the environment, a prerequisite to shed light on the role of ACC-forming prokaryotes in the geochemical cycle of Ca in the environment.