DEPTH-DEPENDENT RESPONSES TO SOLAR ULTRAVIOLET-RADIATION AND OXIDATIVE STRESS IN THE ZOOXANTHELLATE CORAL ACROPORA-MICROPHTHALMA

Colonies of Acropora microphthalma (Verrill 1869) were transferred fro m depths of 2 to 3, 10, 20, and 30 m to UV-transparent and UV-opaque r espirometry chambers placed at 1 m depth at Bowl Reef, Great Barrier R eef, in March 1989. Peak rates of photosynthesis in colonies originati ng at 2 and 10 m were unaffected by solar ultraviolet (UV) radiation a t 1 m, whereas photosynthesis showed 30 and 38% inhibition in colonies transferred from 20 and 30 m, respectively. This differential sensiti vity of corals to UV radiation was consistent with the five- to ten-fo ld higher concentrations of UV-absorbing, mycosporine-like amino acids (MAAs, putative defenses against UV) in 2- and 10-m colonies compared with 20- and 30-m colonies. Photosynthesis in zooxanthellae freshly i solated from 2- and 10-m corals, however, was inhibited by UV, indicat ing that the host's tissues, which contain 95% of the total MAAs in co rals at these depths, are the first line of defense against solar UV a nd provide protection to their endosymbiotic algae. The general bathym etric decline in the activities of the antioxidant enzyme superoxide d ismutase (SOD) in the host, and SOD, catalase, and ascorbate peroxidas e in the zooxanthellae, is related to the decrease in potential for ph otooxidative stress with increasing depth.