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

Colonies of Acropora microphthalma (Verrill 1869) were transferred from depths of 2 to 3, 10, 20, and 30 m to UV-transparent and UV-opaque respirometry chambers placed at 1 m depth at Bowl Reef, Great Barrier Reef, in March 1989. Peak rates of photosynthesis in colonies originating at 2 and 10 m were unaffected by solar ultraviolet (UV) radiation at 1 m, whereas photosynthesis showed 30 and 38% inhibition in colonies transferred from 20 and 30 m, respectively. This differential sensitivity of corals to UV radiation was consistent with the five- to ten-fold 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 isolated from 2- and 10-m corals, however, was inhibited by UV, indicating that the host's tissues, which contain 95% of the total MAAs in corals at these depths, are the first line of defense against solar UV and provide protection to their endosymbiotic algae. The general bathymetric decline in the activities of the antioxidant enzyme superoxide dismutase (SOD) in the host, and SOD, catalase, and ascorbate peroxidase in the zooxanthellae, is related to the decrease in potential for photooxidative stress with increasing depth.