BHAGOOLI, R.; ARCHER, J.; ZIELKE, S.; TRAPIDO-ROSENTHAL, H.*: Zooxanthellae Released from the Coral Stephanocoenia intersepta During Heat-Induced Bleaching Have Elevated Levels of Nitric Oxide Synthase Activity

Zooxanthellae (zoox) released from the coral Stephanocoenia intersepta during heat-induced bleaching possess elevated levels of nitric oxide synthase activity. Coral bleaching, in which corals lose their photosynthetic symbionts, is a phenomenon that is recognized as one of fundamental ecological importance. However, mechanisms by which bleaching occurs are not at all well understood. In some cases, it is thought that bleaching is a result of zoox leaving their host, rather than being expelled by the host. In such cases, the free zoox often remain physiological viable. Zoox living in a symbiotic association with cnidarians have been shown to have nitric oxide synthase (NOS) activity, the enzyme that generates the multifunctional signal molecule nitric oxide (NO). We monitored the activity of zoox NOS as an indicator of the physiological viability of zoox found in seawater after corals had been heat-stressed. Specimens of S. intersepta were placed in seawater at either 28�C or 32�C. After 24 h, zoox were found in the 32�C seawater, and were collected by centrifugation. Zoox remaining in the experimental and control corals were collected by airbrushing, filtration and centrifugation. Zoox were homogenized and NOS activity in the homogenate was determined by measuring the conversion of arginine to citrulline. The striking result was that zoox released from the heat-stressed corals had NOS activities over 100-fold greater than zoox isolated from the tissue of either control or experimental corals. We are now testing hypotheses that zoox in heat-stressed corals use high concentrations of NO to emigrate from their hosts.