Physiological changes in marine picocyanobacterial Synechococcus strains exposed to elevated CO2 partial pressure

Unicellular marine cyanobacteria are abundant in both coastal and oligotrophic environments, where they contribute substantially to primary production. The physiological effect of future increases in atmospheric CO2 concentrations on the marine picocyanobacteria is still poorly known. We studied the physiological changes in marine phycocyanin (PC)-rich and phycoerythrin (PE)-rich Synechococcus strains under different CO2 partial pressures (350, 600 and 800 ppm). The PE strain showed no significant change in growth rate over the experimental CO2 range. A significant increase (25.4%) in carbohydrate was observed at 800 ppm CO2, but no significant change in protein and RNA/DNA ratio was observed in any CO2 treatment. The PC strain showed a significant increase (36.7%) in growth rate at 800 ppm CO2, but no significant change in carbohydrate or protein content was observed over the entire CO2 range. The RNA/DNA ratio increased with increasing CO2 concentration and was positively correlated with growth rate. Cellular red fluorescence and orange fluorescence of the PE strain tended to decline in all CO2 treatments. However, no such decline was observed at higher CO2 treatments in the PC strain. Our results suggest that the PC strain would probably benefit more than the PE strain from future increases in atmospheric CO2 concentrations.