Studies on diversity and stress tolerance of coral-zooxanthella symbiotic system.

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To investigate bleaching mechanisms of coral-zooxanthella symbiotic system at cellular or tissue level, we have established an experimental system using coral cell aggregates (tissue balls). Tissue balls prepared from dissociated cells of Fungia sp. and Pavona divaricata were exposed to either 31°C or 25°C. Survival curves of tissue balls were markedly different between 31°C and 25°C. There was a significant negative correlation between the survival time and zooxanthella density of tissue balls at 31°C, while no significant relationship was found at 25°C. The present results showed that zooxanthellae produced harmful substances and cause damage to coral ce Is under high temperature stress. The finding that the antioxidants extended the survival time of tissue balls at high temperature suggests that zooxanthellae produce reactive oxygen species under the stress condition.To examine whether damage to zooxanthellar photosystem II (PSII) is the primary step of thermally induced coral bleaching, we assessed the relationship between the maximum quantum yield of PSII (Fv/Fm) or active PSII centers (Fv/Fo) and maximum electron transport rate (ETRmax) in the symbiotic algae of the coral Pachyseris rugosa. The I h high temperature treatment in darkness produced a significant, but reversible decline in ETRmax without any change in Fv/Fm. A minimum of 60% of the Fv/Fm or 30% of active PS II centers (Fv/Fo) is required to maintain ETRmax in the symbionts. These results suggest that the primary step of heat-induced damage in the symbiont's photosynthetic apparatus involves a component beyond the PSII, probably at the level of the dark reaction as indicated by reduced ETRmax and the PSII damage is secondary.Planulae or primary polyps of Acropora nobilis and A. palifera could be infected with zooxanthellae from various hosts. We are planning to use this experimental system to assess effects of environmental stresses on corals associated with different types of zooxanthllae.