Synechocystis sp. PCC 6803 shows high cell cycle dynamics reflected by an extraordinary genome copy number variation

Several cyanobacterial strains are known to contain a high number of genome copies and elevated cell cycle dynamics. We systematically investigated the ploidy of the model cyanobacterium Synechocystis sp. PCC 6803 during growth under conditions differing in type (light, CO2, phosphate) and extent of growth limitation. The results obtained via flow cytometry with DAPI-stained cells and quantitative PCR revealed a direct correlation between growth rate and genome copy number (GCN), shedding light on the widely varying copy numbers reported in previous studies. The highest GCNs of up to 72 in average and well above 100 for individual cells were detected during unlimited growth. The lowest GCNs were measured upon limitation by CO2, light, and/or phosphate. Prolonged limitation by multiple factors led to the development of a small population with a GCN of 1 prone to facilitate genetic engineering. Additionally, under phosphate limitation, we found the development of distinct subpopulations differing in cell size, cell shape, as well as pigment and storage compound content, indicating a type of differentiation to survive stress conditions. This study deciphers the high GCN dynamics in Synechocystis sp. PCC 6803, thereby provides knowledge useful to determine the growth state of cells and for strain engineering (transformation, pathway engineering, etc.), and gives novel insights into physiological adaptations of cyanobacteria upon variation of environmental conditions.