Effects of elevated CO2 and nitrogen supply on the growth and photosynthetic physiology of a marine cyanobacterium, Synechococcus sp PCC7002

Ocean acidification due to increasing atmospheric CO2 concentration and coastal eutrophication are growing global threats to affect marine organisms and ecosystem health. However, little is known about their interactive impacts on marine picocyanobacteria which contribute to a large proportion of primary production. In this study, we cultivated the cyanobacterium Synechococcus sp. PCC7002 at ambient (380 ppmv) and high CO2 (1000 ppmv), across a range of nitrogen levels (LN, 10 mu M NO3 (-); MN, 35 mu M NO3 (-); HN, 110 mu M NO3 (-)). In LN media, elevated CO2 significantly decreased cellular chlorophyll a, but insignificantly affected growth rate, photosynthetic efficiency (F (v) /F (m) ) and maximum relative electron transport rate (rETR(max)). Nitrogen (N)-supply positively increased the growth, F (v) /F (m) , dissolved organic carbon (DOC) and cellular carotenoids/Chl a ratios, but decreased the rETR(max) in both ambient and elevated CO2 conditions. The cellular C/N ratios were significantly increased by either elevated CO2 or N-supply, and the cell size was significantly enhanced by elevated CO2, not by N-supply. In addition, we found the N-supply alone had no significant effects on the four main components of chromophoric dissolved organic matter (cDOM) in ambient CO2, while the N-supply interacted with elevated CO2 significantly decreasing the cDOM contents in the cultures. Our results indicated that elevated CO2 and N-supply interacted to alter the physiology and cellular biochemistry of Synechococcus sp. PCC7002, providing useful information for understanding the environmental adaptability of Synechococcus to coastal ocean acidification and eutrophication.