ACCLIMATION OF PHYTOPLANKTON TO UV-B RADIATION - OXIDATIVE STRESS ANDPHOTOINHIBITION OF PHOTOSYNTHESIS ARE NOT PREVENTED BY UV-ABSORBING COMPOUNDS IN THE DINOFLAGELLATE PROROCENTRUM MICANS

Experiments on the temperate marine dinoflagellate Prorocentrum micans showed that cultures acclimated to moderate intensities (120 mu mol q uanta m(-2) s(-1)) of visible radiation and supplemental ultraviolet ( UV) radiation exhibited significant inhibition of photosynthesis. This inhibition of photosynthesis caused a significant 30 % decrease in sp ecific growth rates for those cells exposed to UV radiation by the end of the 21 d culture. The mechanism for this decrease in chlorophyll s pecific photosynthetic rate does not appear to have been damage to pho tosystem II, as suggested for many acute exposure experiments. Rather, significant decreases in chlorophyll per cell and the specific activi ties of the carboxylating enzyme, Rubisco, explain the observed decrea se in photosynthesis. The decrease in cellular chlorophyll and Rubisco activities occurs despite the presence and accumulation of mycosporin e-like amino acids, whose UV absorbing properties have been suggested as an important protective mechanism against the deleterious effects o f UV radiation. Our results also implicate oxidative stress, most like ly a result of photodynamic interactions, as the cause for the decreas e in Rubisco activities. Action spectra generated from these experimen ts show a significant decrease in the wavelength-dependent effects of UV radiation in cultures exposed to UV radiation, suggesting that UV-a bsorbing compounds do provide some, if not complete, protection. Previ ous predictions about specific changes in the shape of action spectra were centered around the absorption maximum of individual UV-absorbing compounds. The observed changes in the overall shape of the UV action spectra for photosynthesis in P. micans can be attributed to the broa d overlapping absorption spectra of the suite of UV-absorbing compound s.