Background
Yu et al. state 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.  Hence, they 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."

What was done
The marine microalgae Platymonas subcordiformis (Wille) Hazen was grown 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 UV-B radiation characteristic of that anticipated to result from a 25% stratospheric ozone depletion under clear sky conditions in summer.

What was learned
By itself, the authors 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, but chlorophyll a content and carotenoid content had no major difference compared with those of ambient UV-B and ambient CO2."  They also report that elevated UV-B by itself significantly increased the 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.

What it means
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, therefore, that "elevated CO2 can be [in] favor of enhancing the capacity of stress resistance."  Put more simply, they say in their concluding paragraph that "we have shown 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."

Reference
Ren, H.X., Chen, X. and Wu, D.X.  2001.  Effects of elevated CO2 on photosynthesis and antioxidative ability of broad bean plants grown under drought condition.  Acta Agronomica Sinica 27: 729-736.