Photoadaption and defenses against oxygen toxicity in zooxanthellae from natural populations of symbiotic cnidarians

Abstract

The enzymes Superoxide dismutase, catalase and ascorbate peroxidase, which provide protection against toxic forms of oxygen, consistently show a positive correlation between specific activities and irradiance zooxantellae freshly isolated from several symbiotic cnidarians in Bermuda. This evidence shows that photoadaptive changes in spectral quantity and quality also involve the protection of photosynthetic components from potentially damaging forms of active oxygen produced photodynamically in the presence of high oxygen concentrations and chlorophyll, a potent photosensitizing agent. Additionally, a reversal in the level of enzyme activities occurs when individuals of the sea anemone Aiptasia pallida (Verrill) (Anthozo: Actiniaria) from bright and dim areas are reciprocally transplanted, showing considerable plasticity in the regulation of these enzymes when exposed to different irradiances. The population genetics of A. pallida in Walsingham Pond, Bermuda, suggests that several asexually reproducing clones are resident in the pond. The chlorophyll content of zooxanthellae from nontransplanted bright and dim individuals of A. pallida is not significantly different while the enzymes Superoxide dismutase and catalase but not ascorbate peroxidase do differ significantly. Maintaining higher activities of Superoxide dismutase and catalase may be a result of the temporally variable levels of short-term changes in irradiance, known as “sunflecks”, produced by the mangrove canopy of the pond. These short-term exposures to high irradiance would likely cause an increase in the production of active forms of oxygen against which protection must be provided for the continuation of maximum photosynthetic capacity.