In discussing their study of the effects of atmospheric CO2 enrichment on photosynthesis, lipid peroxidation and the activities of antioxidative enzymes of the marine microalgae Platymonas subcordiformis, Yu et al. (2004) note that "oxidative stress is potentially experienced by all aerobic life when exposed to UV-B radiation," and that "elevated CO2 can enhance the capacity of plants to resist stress-induced oxidative damage," citing the study of Ren et al. (2001) who worked with terrestrial plants.  Consequently, Yu et al. set about to see if this was also the case with marine phytoplankton, which they describe as "the single most important ecosystem on our planet."

The five scientists grew P. subcordiformis in the laboratory at ambient levels of atmospheric CO2 concentration and UV-B radiation flux density as well as at elevated levels of 5000 ppm CO2 and a UV-B radiation intensity characteristic of that anticipated to result from a 25% stratospheric ozone depletion under clear sky conditions in summer.  They report that the elevated UV-B treatment "significantly decreased [microalgal] dry weight, photosynthetic rate, chlorophyll a and carotenoid contents," while the elevated CO2 treatment by itself "enhanced dry weight and photosynthetic rate."  They also report that elevated UV-B by itself significantly increased production of the toxic superoxide anion and hydrogen peroxide, as well as malonyldialdehyde, which is an end product of lipid peroxidation, whereas elevated CO2 by itself did just the opposite.  In addition, in the treatment consisting of both elevated UV-B and elevated CO2, the concentrations of these three substances were lower than those observed in the elevated UV-B and ambient CO2 treatment.  Finally, they report that elevated CO2 decreased the levels of several antioxidative enzymes found in the microalgae, reflective of their reduced need for detoxification of reactive oxygen species in the elevated CO2 treatment.

Yu et al. say their results suggest that "CO2 enrichment could reduce oxidative stress of reactive oxygen species to P. subcordiformis, and reduce the lipid peroxidation damage of UV-B to P. subcordiformis."  They also say that "CO2 enrichment showed a protective effect against the oxidative damage of UV-B-induced stress," and that "elevated CO2 can be [in] favor of enhancing the capacity of stress resistance."  Put more simply, they say in their concluding paragraph that "algae grown under high CO2 would better overcome the adverse impact of environmental stress factor[s] that act via generation of activated oxygen species."

Turning our attention to corals, Meehan and Ostrander (1997) note that different species of coral zooxanthellae possess different levels of resistance to UV-B radiation, and that prolonged exposure -- primarily due to an increase in water clarity -- may induce bleaching in corals whose zooxanthellae "cannot increase concentrations of UV-blocking pigments quickly enough to adapt to such sudden changes."  If, however, coral symbionts respond to atmospheric CO2 enrichment as do marine phytoplankton such as P. subcordiformis, this deleterious effect of UV-B radiation should become less and less significant as the air's CO2 content continues to rise.

In a study of the protective mucus coverings of the corals Montastraea faveolata and Colpophyllia natans, Lyons et al. (1998) discovered, in their words, "the first evidence of UV-induced DNA damage in the microbial communities of the coral-surface microlayer."  Based on this finding, and noting that "the potential role of UV radiation in the global decline of coral reefs is receiving increased attention," they further state that "increases in environmental levels of ultraviolet radiation, specifically UV-B, may lead to changes in reef-building coral abundance," which deleterious effects would likely be lessened, however, as the atmosphere's CO2 concentration climbs ever higher and better protects the algal symbionts that provide nourishment for their coral hosts that produce the protective mucus coverings.

In a somewhat different context, Fine et al. (2002) determined that ultraviolet radiation may at times prevent bleaching in the Mediterranean coral Oculina patagonica.  In this specific case, the causative agent of the bleaching was a bacterium (Vibrio shiloi); and they observed that "when O. patagonica was infected with V. shiloi in laboratory aquaria and subsequently exposed to sunlight, the intracellular bacteria were rapidly killed, aborting the infection and preventing bleaching," but that "when the infected corals were protected from ultraviolet (UV) light, the intracellular V. shiloi multiplied and the coral bleached."

So, it all depends on what the problem is.  In cases where bacteria are responsible for the bleaching of corals, some extra UV-B radiation may be a ticket to renewed coral health.  In a much broader context, however, too much UV-B radiation may be lethal to the other components of the coral-zooxanthellae symbiosis; and in these situations higher atmospheric CO2 concentrations will definitely be of help.

References
Fine, M., Banin, E., Israely, T., Rosenberg, E. and Loya, Y.  2002.  Ultraviolet radiation prevents bleaching in the Mediterranean coral Oculina patagonica.  Marine Ecology Progress Series 226: 249-254.

Lyons, M.M., Aas, P., Pakulski, J.D., Van Waasbergen, L., Miller, R.V., Mitchell, D.L. and Jeffrey, W.H.  1998.  DNA damage induced by ultraviolet radiation in coral-reef microbial communities.  Marine Biology 130: 537-543.

Meehan, W.J. and Ostrander, G.K.  1997.  Coral bleaching: a potential biomarker of environmental stress.  Journal of Toxicology and Environmental Health 50: 529-552.

Yu, J., Tang, X-X., Zhang, P-Y., Tian, J-Y. and Cai, H-J.  2004.  Effects of CO2 enrichment on photosynthesis, lipid peroxidation and activities of antioxidative enzymes of Platymonas subcordiformis subjected to UV-B radiation stress.  Acta Botanica Sinica 46: 682-690.