Acclimation of phytoplankton to UV-B radiation:oxidative stress and photoinhibition of photosynthesis are not prevented by UV-absorbing compounds in the dinoflagellate Prorocentrum micans

Experiments on the temperate marine dinoflagellate Prorocentrum mjcans showed that cultures acclimated to moderate intensities (120 pm01 quanta m-2 S-') of visible radiation and supplemental ultraviolet (UV) ra&ation exhibited significant lnhibltlon of photosynthesis. This inhibltlon of photosynthesis caused a significant 30% decrease in s p e c ~f ~c growth rates for those cells exposed to UV radiation by the end of the 21 d culture. The mechanism for t h ~s decrease in chlorophyll specific photosynthetic rate does not appear to have been damage to photosystem 11, as suggested for many acute exposure expenments Rather, significant decreases in chlorophyll per cell and the specific act~vities of the carboxylating enzyme, Rublsco, explain the observed decrease in photosynthesis. The decrease in cellular chlorophyll and Rubisco activities occurs despite the presence and accumulation of mycosporine-like amino acids, whose UV absorbing properties have been suggested as a n important protective mechanism against the deleterious effects of UV radiation. Our results also implicate oxidative stress, most likely a result of photodynamic interactions, a s the cause for the decrease in Rubisco activities. Action spectra generated from these experiments show a significant decrease in the wavelength-dependent effects of UV radiation in cultures exposed to UV radiation, suggesting that UVabsorbing compounds do provide some, if not complete, protection. Previous 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 mlcans can be attributed to the broad overlapping absorption spectra of the suite of UV-absorbing compounds.