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.

Colonies of the hermatypic coral Pocillopora damcornis were collected from the shallow reefs of Kaneohe Bay, Hawaii, to assess the wavelength-dependent effects of ultraviolet (UV) radiation on photosynthesis. Measurements of photosynthesis and respiration were made while corals were exposed to different UV irradiances, keeping visible radiation constant, using long-band pass filters. A differential action spectrum (biological weighting function) for the inhibition of photosynthesis by UV radiation was then determined for P darnicornis The action spectrum revealed an increase in the wavelength-dependent effects of UV radiation on photosynthesis between 290 and 310 nm that is greater than those increases reported for action spectra on natural assemblages and unialgal cultures of marine microalgae. The greater effect at these wavelengths is a result of the high biologically effective doses of UV radiation experienced by these corals on shallow reefs, and the decrease in the absorbance of UV radiation by UV absorbing compounds found in the host tissues and algal symbionts between 290 and 310 nm. The irradiances of wavelengths in the region between 290 and 310 nm are those which will increase in the event of any decrease in stratosphenc ozone over equatonal regions. If the observed sensitivity of P. damicorn~s in this spectral region is common in other species, it may have important consequences for growth, reproduction, and occurrence of the bleaching phenomenon for shallow water corals.

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