Acclimation of marine phytoplankton to ultraviolet radiation

Abstract

The ability of marine phytoplankton to acclimate to ultraviolet radiation (UVR) was examined. Monocultures of a subtropical diatom, Chaetoceros gracilis, were maintained under photosynthetically available radiation (PAR-only) and PAR plus UVR (PAR+UVR) for a 48 h exposure period. Growth rates, pigment concentrations, carbon fixation capability, variable fluorescence, and activity of 1, 5 ribulose biphosphate oxygenase/carboxylase (Rubisco) were initially reduced by UVR. By 24 h, and for the remainder of the 48 h exposure period, growth rate, pigment concentrations, Rubisco activity and carbon fixation capability were not affected by PAR+UVR. After 48 h of UVR exposure, (acclimation assumed complete), turnover rates of the putative D1 protein of photosystem II (PSII) and Chlorophyll (Chl) a were higher than controls, suggesting continual damage by UVR. Maximum rate of oxygen evolution and the efficiency of PSII increased following acclimation to UVR. The maximum rate of carbon fixation was not affected on a per cell basis and decreased on a per Chl a basis following UVR acclimation. Chlorophyll a specific photosynthesis over a 5 h exposure period was equal between the two acclimation treatments (PAR-only and PAR+UV). Transfer of PAR-only acclimated cells to PAR+UVR for the same 5 h period lead to a reduction in Chl a specific photosynthesis, indicating acute inhibition of photosynthesis by UVR. Chlorophyll a specific photosynthesis of cells acclimated to PAR+UVR and transferred to PAR-only was 24% higher than cells maintained in PAR+UVR during the determination of photosynthesis, indicating enhancement of productivity following the removal of UVR. Effect of ambient subtropical UVR on natural phytoplankton populations was examined. Natural phytoplankton assemblages from Kane'ohe Bay, Hawai'i were maintained for ten days at ambient temperature and nutrient levels. Rates of primary production by assemblages exposed to PAR-only and PAR+UVR were equal. Assemblages acclimated to PAR+UVR and then transferred to PAR-only fixed 67% more carbon per Chl a than assemblages acclimated to PAR+UVR and maintained in PAR+UVR. Acclimation to ambient PAR+UVR resulted in a 171% increase in Chl a concentration compared to assemblages maintained under PAR-only conditions.