Effects of ultraviolet radiation on photosynthesis in the subtropical marine diatom, Chaetoceros gracilis (Bacillariophyceae)

Acclimation to ambient ultraviolet radiation (UVR) was examined in a subtropical marine diatom, Chaetoceros gracilis Schutt. Short-term exposure to UVR (<24 h) reduced the efficiency of photosynthetic energy conversion, carbon fixation, activity of 1,5-bisphosphate carboxylase-loxygenase (RUBISCO), and the rapid turnover of the putative D1 reaction center (32 kDa) protein, whereas longer term exposure to ambient UVR (24-48 h) revealed a steady-state acclimation, defined as recovery of carbon fixation and RUBISCO activity to rates equivalent to treatments without exposure to UVR. The turnover of D1 and chlorophyll a (Chl a) remained high during exposure to UVR. Efficiency of energy conversion by photosystem II, measured with double flash (pump and probe) fluorometry increased by 24 % in cells acclimated to UVR. Acclimation to UVR had no detectable effect on the functional absorption cross-section or cellular concentrations of Chl a, Chl c, or total carotenoids. However, the maximum rate of carbon fixation was reduced by UVR on a Chl a basis but remained unaffected on a per-cell basis. Response to UVR exposure in this subtropical diatom has two components: a short-term inhibitory response and a longer-tem acclimation process that ameliorates the inhibition of carbon fixation.