Carbon dioxide regulation of nitrogen and phosphorus in four species of marine phytoplankton

Abstract

The concentration of carbon dioxide in seawater may affect phytoplankton physiology and ecology and their role in marine biogeochemical cycles. In order to assess the effects of CO 2 on the elemental composition of marine phytoplankon, carbon, nitrogen, and phosphorus quotas were measured in 4 species of marine phytoplankton acclimated to 150 to 1500 ppm CO 2 (5 to 50 μM) in semi-continuous cultures. Nitrogen quotas declined steeply with increasing CO 2 in the centric diatoms Thalassiosira pseudonana and T. weissflogii acclimated to 150 to 380 ppm (5 to 13 μM), but more slowly as the CO 2 increased from 380 to 1500 ppm (13 to 50 μM). Nitrogen demand varied little with CO 2 in the pennate diatom Phaeodactylum tricornutum, but was positively correlated with CO 2 over the range of 150 to 770 ppm in the prymnesiophyte Isochrysis galbana. Based on the nitrogen-CO 2 trends in centric diatoms, relief from carbon-nitrogen colimitation could lead to 2-fold larger cells as CO 2 increases from 150 to 380 ppm, but only 15% larger cells from 380 to 770 ppm CO 2. Phosphorus quotas in the 3 diatoms decreased as CO 2 increased from 150 to 380 ppm. As previously observed in these and other species, C:N, C:P, and N:P ratios increased with increasing CO 2, but the present results show that much of this variation was due to differences in nitrogen and phosphorus rather than carbon quotas. Marine phyto - plankton could provide a negative feedback against increasing CO 2 over the pCO 2 range of 150 to 380 ppm by supporting larger cells or higher biomass, but would support a smaller carbon sink as atmospheric CO 2 rises above 380 ppm. © Inter-Research 2012.