Temperature But Not CO2 Stimulates Growth in Southern Ocean Phytoplankton Over a Range of Light and Fe Conditions

Model projections for the Southern Ocean indicate that light and iron availability, temperature, and carbon dioxide (CO₂) will change into the future due to climate change. We explored the potential responses of Southern Ocean phytoplankton to these changes by culturing the haptophyte Phaeocystis antarctica and the diatom Chaetocerous flexuosus under various combinations of light and Fe, temperature and CO₂. The growth of both phytoplankton species increased under future conditions (warming, increased irradiance, high CO₂) under Fe-replete conditions compared to present-day conditions. Under Fe-limiting future conditions C. flexuosus growth increased while growth decreased for P. antarctica. Warming under present-day CO₂ levels increased the growth of both species. Surprisingly, CO₂ generally had a negative impact on the photochemical efficiency of photosystem II (PSII; F_v/F_m) in both species. Carbon (C) and nitrogen (N) content of both species was variable, as was the C:N ratio. Despite the lack of a distinguishable CO₂ fertilisation effect, our results suggest that increased CO₂, when combined with future warming and increased light due to ocean stratification, will stimulate primary productivity, but only under Fe replete conditions. Consequently, Fe bioavailability will continue to play a key role in regulating carbon export in the Southern Ocean.