THE ENZYMATIC RESPONSE OF THE SYMBIOTIC DINOFLAGELLATE SYMBIODINIUM-MICROADRIATICUM (FREUDENTHAL) TO GROWTH-INVITRO UNDER VARIED OXYGEN-TENSIONS

Symbiotic dinoflagellates colonize microhabitats characterized by large fluctuations in oxygen tension. To test the hypothesis that oxygen detoxifying enzymes are activated in response to elevated oxygen tension (pO2), the symbiotic dinoflagellate Symbiodinium microadriaticum Freudenthal was grown in vitro under experimentally manipulated oxygen tensions representing hyperoxic, normoxic and hypoxic conditions. The specific activities, normalized to protein and to cell numbers, of three key enzymes believed to be involved in oxygen detoxification; superoxide dismutase (SOD), catalase and ascorbate peroxidase, were assayed in cell-free extracts. Activities of all three enzymes were found in varying amounts in supernatant and pelleted fractions after aqueous extraction. The pellet fraction has been overlooked in the past. We found a direct relation between O2 tension and SOD activity per mg protein; SOD activities expressed per cell were similar in hyperoxic and normoxic treatments, and both were higher than hypoxic treatments. Although catalase activities (normalized to either protein or cell numbers) were proportional to O2 tension in cells grown under hyperoxic and normoxic conditions, cells grown under hypoxic conditions demonstrated higher activities than cells grown under normoxic conditions. The specific activities per protein of ascorbate peroxidase generally decreased as a function of decreasing O2, but the specific activities per cell showed the reverse relation.