MEPS 466:57-67 (2012)  -  DOI: https://doi.org/10.3354/meps09905

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 CO₂ 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 CO₂ (5 to 50 µM) in semi-continuous cultures. Nitrogen quotas declined steeply with increasing CO₂ in the centric diatoms Thalassiosira pseudonana and T. weissflogii acclimated to 150 to 380 ppm (5 to 13 µM), but more slowly as the CO₂ increased from 380 to 1500 ppm (13 to 50 µM). Nitrogen demand varied little with CO₂ in the pennate diatom Phaeodactylum tricornutum, but was positively correlated with CO₂ over the range of 150 to 770 ppm in the prymnesiophyte Isochrysis galbana. Based on the nitrogen−CO₂ trends in centric diatoms, relief from carbon−nitrogen co-limitation could lead to 2-fold larger cells as CO₂ increases from 150 to 380 ppm, but only 15% larger cells from 380 to 770 ppm CO₂. Phosphorus quotas in the 3 diatoms decreased as CO₂ 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 CO₂, but the present results show that much of this variation was due to differences in nitrogen and phosphorus rather than carbon quotas. Marine phytoplankton could provide a negative feedback against increasing CO₂ over the pCO₂ range of 150 to 380 ppm by supporting larger cells or higher biomass, but would support a smaller carbon sink as atmospheric CO₂ rises above 380 ppm.

KEY WORDS: Nitrogen · Phosphorus · Carbon dioxide · Phytoplankton