Photoinhibition of phytoplankton and ice algae in the West Antarctic Peninsula

Variations in Southern Ocean light environments appear to influence phytoplankton community composition, thereby influencing carbon fixation and food web structure. Climate change alters circulation, stratification, and ice cover, which in turn changes the underwater light environment. Phytoplankton adapt to the highly variable light by optimizing their photosynthetic apparatus for carbon fixation, while minimizing photodamage from high irradiance. Here we investigate how different phytoplankton taxa are photosynthetically adapting to various light environments, and how this might affect their relative success within a community. Photodamage and photoprotection were measured and compared to productivity in natural phytoplankton assemblages and ice algae in the West Antarctic Peninsula during NBP14-09 (Oct-Nov 2014). Phytoplankton and ice algae were briefly exposed to surface irradiance, after which the decline and recovery of F_v/F_m (maximum efficiency of Photosystem II) was measured. After further incubation at in situ light levels, their productivity and species composition was quantified. This series of measurements was carried out under various light treatments: natural light (includes UVR), No-UVB, and No-UVR, to assess the effects of UVR on photodamage and photorepair relative to the effects of high light. In general, the light shock decreased productivity in phytoplankton but not in ice algae, which appeared to be better photoprotected and less photodamaged. Relative to the No-UVR treatment, UVR (natural light) increased photodamage. Phytoplankton productivity declined equally under both treatments. These results suggest that ice algae is more acclimated to high light than phytoplankton, but that UVR is not responsible for suppressing productivity at this time of year.