Effects of salinity on cellular growth and exopolysaccharide production of freshwater Synechococcus strain CCAP1405

The picocyanobacterium Synechococcus is ubiquitous in freshwater and high-saline marine ecosystems. The cellular response of freshwater Synechococcus to salinity variations is not well understood. Herein, we compare cellular physiology, cell division and exopolysaccharide production of freshwater Synechococcus CCAP1405 when grown under different salinity conditions. A comparable growth rate between salinity 0 and 10 indicates that this strain is capable of adapting to such moderate salinity change. The photosynthetic efficiency and the growth were significantly lower at salinity 20 and 30 suggesting that cells experienced stress above salinity 10. For the first time, we record bimodal DNA distribution pattern for a freshwater Synechococcus strain with cell division synchronized to light-dark cycle. Pre-replication (G1) and synthesis (S) phase durations were significantly longer at high salinity, pointing to slower growth. However, post-replication (G2) phase duration was not influenced by a change in salinity. The soluble and attached exopolysaccharide production increased with salinity and age of the culture. Increased exopolysaccharide production in the vicinity of the cell-boundary facilitated its survival and microaggregate formation under salinity stress conditions. These findings indicate that flux of freshwater Synechococcus into estuarine and marine conditions would influence its growth, exopolysaccharide production and picophytoplankton carbon flow in the food web.