The oxygen microenvironment adjacent to the tissue of the scleractinian Dichocoenia stokesii and its effects on symbiont metabolism

The biology of symbiotic scleractinians is profoundly influenced by their i ntracellular zooxanthellae, and many studies have focused on the mechanisti c basis of this influence. This has usually been accomplished by examining the metabolism of zooxanthellae under physical conditions measured in the o pen reef and assumed to be similar to conditions in hospite. Recent advance s in the measurement of conditions near and within coral tissue suggests th at this assumption may result in substantial errors. To address this possib ility, the role of water flow in determining oxygen saturation adjacent to the tissue of Dichocoenia stokesii was investigated, and the effect of thes e measured oxygen saturations on the respiration and photosynthesis of zoox anthellae isolated from the same species was quantified. Using a microelect rode (700 mu m diam), we measured oxygen saturations above (less than or eq ual to 4 mm) the tissue in two flow speeds over 24 h periods in a flume rec eiving sunlight at in situ levels. The results were used as a proxy for eco logically relevant intracellular oxygen saturations, which were applied to zooxanthellae in vitro to assess their effect on symbiont metabolism. Micro environment oxygen saturations (% air saturation) ranged from 74-159% in sl ow how (2.7 cm s(-1)) to 88-110% in faster flow (7.5 cm s(-1)) over day-nig ht cycles. Therefore, the metabolic rates of zooxanthellae were measured at 50 to 54% (hypoxia), 98 to 102% (normoxia) and 146 to 150% (hyperoxia) oxy gen saturation. Oxygen saturation significantly affected the metabolism of zooxanthellae, with gross photosynthesis increasing 1.2-fold and dark respi ration increasing 2-fold under hyperoxia compared to hypoxia. These results suggest that the metabolism of zooxanthellae in hospite is affected marked ly by their microenvironment which, in turn, is influenced by flow-mediated mass transfer.