PHOTOBIOLOGY OF THE SYMBIOTIC SEA ANEMONE, ANTHOPLEURA ELEGANTISSIMA: DEFENSES AGAINST PHOTODYNAMIC EFFECTS, AND SEASONAL PHOTOACCLIMATIZATION.

The sea anemone Anthopleura elegantissima, which contains photosynthetic symbionts (zooxanthellae), responds both biochemically and behaviorally to the combined environmental stresses of exposure to sunlight and photosynthetically generated hyperbaric O2. Activities of the enzymes superoxide dismutase (SOD) and catalase, which act in concert as defenses against oxygen toxicity, parallel the distribution of chlorophyll. A. elegantissima shows a finely controlled contraction behavior which shades the zooxanthellae and reduces O2 production, but which leaves the body column tissues directly exposed to sunlight. However, the body column contains disproportionately high SOD and catalase activities as defenses against photodynamic damage. This additional role of SOD is demonstrated by shade-adapted aposymbiotic anemones in which SOD and catalase activities increase by 590% and 100% respectively following a 7 day exposure to sunlight. In response to elevated levels of O2 and sunlight exposure, A. elegantissima attaches gravel and other debris to its body surface which serves as a sunscreen that effectively reduces zooxanthella expulsion during exposure to bright sunlight. Finally, anemone chlorophyll content fluctuates on a seasonal basis, varying inversely with mean solar radiation. These seasonal changes are not due to corresponding changes in the number of algal cells, but rather to changes in the chlorophyll content and chlorophyll a:c2 ratio of a fairly uniform standing crop of zooxanthellae.