Increased sensitivity to ultraviolet radiation in nitrogen-limited dinoflagellates: Photoprotection and repair

Nitrogen (N) limitation significantly increased the sensitivity of photosynthesis to inhibition by ultraviolet radiation (UV) in two estuarine dinoflagellates, Akashiwo sanguinea (5 Gymnodinium sanguineum) and Gymnodinium (5 Gyrodinium) cf. instriatum. Biological weighting functions (BWFs) and the kinetics of photosynthetic response to UV indicated that the main mechanism for the increase in sensitivity was less efficient repair. A decrease in cell size and in the concentration of the photoprotective mycosporine-like amino acids also elevated sensitivity. The BWFs predict that increased UV-B due to ozone depletion would cause a more than 1.5-fold greater additional inhibition of N-limited compared to nutrient-sufficient dinoflagellates. The BWFs of the N-limited cultures are similar to those measured for natural assemblages of phytoplankton in the Chesapeake Bay under low N availability. Solar ultraviolet radiation (UV) (290–400 nm) is a significant stressor in aquatic environments. Its adverse effects on phytoplankton, the major primary producers in most aquatic ecosystems, are well documented: UV inhibits photosynthesis (Lorenzen 1979; Cullen et al. 1992) and growth (Ekelund 1990), and damages DNA (Karentz et al. 1991). Climate change and ozone depletion modify the intensity of UV stress (Pienitz and Vincent 2000), but predicting the effects of changing UV exposure is complicated by variable phytoplankton sensitivity (Neale et al. 1998b) resulting from differences in community composition or physiological state (Karentz et al. 1994), such as nutritional status. Nitrogen (N) limitation can potentially increase phytoplankton susceptibility to UV by constraining defenses against UV as well as the ability to counteract photodamage. Phytoplankton repair UV-induced damage using several mechanisms, many of which involve N-requiring enzymes and/or protein cofactors (Roy 2000). Other enzymes are important in detoxifying UV-induced reactive oxygen species (Lesser and Shick 1989). Nitrogen is also required for UVscreening compounds such as mycosporine-like amino acids (MAAs). Another mechanism that protects photosynthetic electron transport is the dissipation of excess reductant through nonassimilatory reduction of NO (Lomas and Gli3 bert 1999). In addition, N limitation can lead to a cell size reduction (Doucette and Harrison 1990), which decreases the pathlength for UV attenuation in the cell and thus increases UV-induced damage (Garcia-Pichel 1994). The importance of N availability to many mechanisms of 1 Present address: IMCS, Rutgers University, 71 Dudley Road, New Brunswick, New Jersey 08901. 2 Present address: Unidad Académica Puerto Morelos, Instituto de Ciencias del Mar y Limnologı́a, Universidad Nacional Autónoma De México, Apartado Postal 1152, Cancún, 77500, México.