Abstract:

When the CO₂ concentration was kept at 360 mL/L (ambient) and 5 000 mL/L (elevated), the effects of UV-B stress on photosynthesis, lipid peroxidation and antioxidative enzymes of marine microalgae Platymonas subcordiformis (Wille) Hazen were examined. The experiment indicated that: (1) UV-B alone significantly decreased dry weight, photosynthetic rate, chlorophyll a (Chl a) and carotenoid (Car.) contents; CO₂ enrichment alone enhanced dry weight and photosynthetic rate, but Chl a content and Car. content had no major difference compared with those of ambient UV-B and ambient CO₂; the dry weight and photosynthetic rate of P. subcordiformis grown under the combination of UV-B and CO₂ had no major difference compared with that under ambient UV-B and ambient CO₂; while the Chl a content and Car. content significantly decreased compared to those of P. subcordiformis grown under ambient UV-B and ambient CO₂. (2) Both UV-B alone and CO₂ enrichment alone significantly decreased soluble protein content, when UV-B and CO₂ were in combination, the soluble protein content was higher than that of UV-B alone. Changes in soluble protein content of algae grown in high CO₂ could be largely due to a decline in Rubisco protein. (3) UV-B alone significantly increased the rate of O₂-. production, H₂O₂ and malondialdehyde (MDA) content, CO₂ enrichment alone significantly decreased the rate of O₂-. production, H₂O₂ and MDA contents, when UV-B and CO₂ were in combination, the rate of O₂-. production, H₂O₂ and MDA contents were significantly lower than those of UV-B alone. The results suggested 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. And (4) UV-B alone significantly increased SOD, POD, CAT, GR and GP_X activities, CO₂ enrichment alone significantly decreased the activities of SOD, POD and GR, while the CAT and GP_X activities were decreased a little but not changed significantly compared to ambient UV-B and ambient CO₂. The SOD, POD, CAT, GR and GP_X activities of P. subcordiformis grown under the combination of UV-B and CO₂ were much lower than those of P. subcordiformis grown under UV-B alone. The results indicated that CO₂ enrichment showed a protective effect against the oxidative damage of UV-B-induced stress. Therefore, elevated CO₂ can be favor of enhancing the capacity of stress resistance.

Key words: CO₂ enrichment, UV-B radiation, Platymonas subcordiformis , photosynthesis, lipid peroxidation, antioxidative enzymes