In this interesting study by Bruley et al. (2024), the cyanobacterium Microcystis aeruginosa PCC7806 is taken as a model organism for intracellular CaCO3 precipitation in Cyanobacteria, i.e. in the form of intracellular amorphous calcium carbonates (iACC). This phenomenon, which was first described in 2012, is an example of genetically controlled biomineralization in bacteria. Hence, a gene coding for the protein calcyanin (ccyA) has been documented in iACC biomineralizing cyanobacteria. Nevertheless, so far, the functioning of the calcyanin protein remains unknown. As a first step to elucidate its role in iACC biomineralization the authors study the diel variations of ccyA expression. An approximately 2.5-fold variation in abundance of ccyA transcripts has been observed with the highest values of ccyA expression observed during the second half of the dark period. In addition, the authors made a thorough investigation of transcriptomics data, to detect gene-expressions with temporal patterns that positively or negatively correlate with ccyA. A particular focus was made on neighboring genes (both upstream and downstream) to detect a possible operon gathering ccyA with other genes. Very interestingly, the authors discovered that some neighboring genes coding for Ca2+/H+ antiporter systems, showed transcripts with abundances that correlate with that of ccyA. 

This study raises many interesting questions on genetically controlled biomineralization in bacteria and more particularly the function of iACC biomineralization in Cyanobacteria. As the authors write, iACC biomineralization could be involved in carbon-concentrating mechanisms (CCM), intracellular pH buffering, and create “ballast” for buoyancy and floatability regulation. Nevertheless, these roles would require mechanisms for the dissolution of iAAC in concert with its precipitation ; fine-tuning of both resulting in homeostasis or cyclic temporal patterns of iAAC increase/decrease. As a perspective, the response of Microcystis to fluctuations in calcium and/or pCO2 levels could provide valuable insights into the molecular mechanisms underlying the biomineralization of iACC, as well as comparisons with non-iACC biomineralizing strains or with a mutant of PCC 7806 with a deactivated/deleted ccyA gene.

Reference:

Bruley A, Gaëtan J, Gugger M, Pancrace C, Millet M, Gaschignard G, Dezi M, Humbert J-F, Leloup J, Skouri-Panet F, Callebaut I, Benzerara K and Duprat E (2024) Diel changes in the expression of a marker gene and candidate genes for intracellular amorphous CaCO3 biomineralization in Microcystis. bioRxiv, ver.3 peer-reviewed and recommended by PCI Microbiol. https://doi.org/10.1101/2024.07.07.602159

In this interesting study by Bruley et al. (2024), the cyanobacterium Microcystis aeruginosa PCC7806 is taken as a model organism for intracellular CaCO3 precipitation in Cyanobacteria, i.e. in the form of intracellular amorphous calcium carbonates (iACC). This phenomenon, which was first described in 2012, is an example of genetically controlled biomineralization in bacteria. Hence, a gene coding for the protein calcyanin (ccyA) has been documented in iACC biomineralizing cyanobacteria. Nevertheless, so far, the functioning of the calcyanin protein remains unknown. As a first step to elucidate its role in iACC biomineralization the authors study the diel variations of ccyA expression. An approximately 2.5-fold variation in abundance of ccyA transcripts has been observed with the highest values of ccyA expression observed during the second half of the dark period. In addition, the authors made a thorough investigation of transcriptomics data, to detect gene-expressions with temporal patterns that positively or negatively correlate with ccyA. A particular focus was made on neighboring genes (both upstream and downstream) to detect a possible operon gathering ccyA with other genes. Very interestingly, the authors discovered that some neighboring genes coding for Ca2+/H+ antiporter systems, showed transcripts with abundances that correlate with that of ccyA. 

This study raises many interesting questions on genetically controlled biomineralization in bacteria and more particularly the function of iACC biomineralization in Cyanobacteria. As the authors write, iACC biomineralization could be involved in carbon-concentrating mechanisms (CCM), intracellular pH buffering, and create “ballast” for buoyancy and floatability regulation. Nevertheless, these roles would require mechanisms for the dissolution of iAAC in concert with its precipitation ; fine-tuning of both resulting in homeostasis or cyclic temporal patterns of iAAC increase/decrease. As a perspective, the response of Microcystis to fluctuations in calcium and/or pCO2 levels could provide valuable insights into the molecular mechanisms underlying the biomineralization of iACC, as well as comparisons with non-iACC biomineralizing strains or with a mutant of PCC 7806 with a deactivated/deleted ccyA gene.

Reference:

Bruley A, Gaëtan J, Gugger M, Pancrace C, Millet M, Gaschignard G, Dezi M, Humbert J-F, Leloup J, Skouri-Panet F, Callebaut I, Benzerara K and Duprat E (2024) Diel changes in the expression of a marker gene and candidate genes for intracellular amorphous CaCO3 biomineralization in Microcystis. bioRxiv, ver.3 peer-reviewed and recommended by PCI Microbiol. https://doi.org/10.1101/2024.07.07.602159