PHOTOSYNTHESIS AND REGULATION OF RUBISCO ACTIVITY IN NET PHYTOPLANKTON FROM DELAWARE BAY

Net phytoplankton (> 20 mu m) comprised 51 +/- 9% of the total chlorop hyll (Chl) in a Skeletonema costatum-dominated spring bloom in Delawar e Bay. The net phytoplankton had low C:N and high protein: carbohydrat e ratios, indicating that their growth was nutrient-replete. Their pho tosynthetic responses were characterized by low specific absorption, l ow light-limited and light-saturated rates of photosynthesis, and high quantum yields, indicative of acclimation to low irradiance and inter nal self-shading. High fucoxanthin: Chl ratios also indicated low ligh t acclimation, but high photoprotective xanthophyll: Chl ratios sugges ted a high capacity for photoprotective energy dissipation. Ribulose-1 ,5-bisphosphate carboxylase/oxygenase (Rubisco) could be activated and deactivated in response to changes in irradiance and was fully activa ted at the surface of the water column and fully deactivated in aphoti c deep water. Maximum Rubisco activity was correlated with Rubisco con tent and bulk protein content of the phytoplankton and with light-satu rated rates of photosynthesis measured in short (< 20-min) incubations . Long (60-min) incubations caused a decrease in the light-saturated r ate of photosynthesis, possibly because of feedback limitation. While feedback limitation is unlikely to occur in the water column, it shoul d be considered when estimating productivity in well-mixed waters from fixed light-depth incubations.