Diel changes in the expression of a marker gene and candidate genes for intracellular amorphous CaCO3 biomineralization in Microcystis

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إنجليزي. Phylogenetically diverse cyanobacteria biomineralize intracellular amorphous calcium carbonate (iACC) inclusions. This includes several genotypes of the Microcystis genus, a potentially toxic, bloom-forming cyanobacterium foundworldwide in freshwater ecosystems.Whilewe ignore the biological function of iACC and the molecular mechanisms driving their formation, this process may impact local geochemical cycles and/or be used for bioremediation strategies. Recently, a marker gene of this biomineralization pathway, named ccyA, was discovered. However, the function of the calcyanin protein encoded by ccyA remains unknown. Here, based on an RNA-Seq approach, we assess the expression of the ccyA gene in Microcystis aeruginosa PCC 7806 during a 24 h day/night cycle. The ccyA gene shows a clear day/night expression pattern with maximumtranscript abundances during the second half of the night. This is consistent with the assumption that iACC biomineralization is related with photosynthesis and may therefore follow a day/night cycle as well. Moreover, several genes directly co-localized upstream and downstream of ccyA, on the same DNA strand show a similar expression pattern, including a cax gene encoding a calcium/proton exchanger and a gene encoding a protein with a domain also present in the Nterminal region of calcyanins in many iACC-forming cyanobacteria. This suggests that they all could be part of an operon, and may play a concerted role in iACC formation. Last, several other genes involved in carbon concentrating mechanisms and calcium transport show an expression pattern similar to that of ccyA. Overall, this study provides a list of candidate genes that may beinvolved in the biomineralization of iACC by cyanobacteria and whose role could be, in the future, analyzed by biochemistry and genetics approaches.