Effects of ambient silicic acid concentration on the physiology of marine cyanobacterial Synechococcus

The marine cyanobacterium Synechococcus can accumulate significant amounts of silicon (Si) within its cells, potentially contributing to the vertical transport of biogenic Si in the ocean. This has prompted interest in understanding the uptake, subcellular distribution, and chemical forms of Si in Synechococcus. The surprisingly high intracellular Si concentration could have important impacts on Synechococcus physiology, but these effects have been relatively unexplored. In this study, we investigated the effects of external Si concentrations, [Si(OH)₄], on the cell physiology of two strains of Synechococcus (CC9311 and WH7803). Increased ambient [Si(OH)₄] significantly enhanced total cellular and water-soluble Si content in Synechococcus, but had little effect on their growth. However, the cell size, volume and photosynthetic performance were significantly reduced in strain WH7803, indicating some harmful effects in the cell. Increases in ambient [Si(OH)₄] also altered the Derjaguin-Muller-Toporov (DMT) modulus, adhesion, surface potential and elemental composition of the Synechococcus cells. Notably, significant changes in area-specific adsorbed metals and volume-specific metal content were observed at higher [Si(OH)₄] levels, suggesting Si availability in seawater could markedly impact the interactions between Synechococcus cell and environment. Overall, this study provides new insights into the physiological adaptations of marine cyanobacterium Synechococcus to varying environmental silicic acid concentrations.