Spatial heterogeneity in active chlorophyll fluorescence and PSII activity of coral tissues

Chlorophyll-a fluorescence was measured in six species of coral, using pulse-amplitude-modulated fluorometers employing fibre-optic probes with diameters of 8 mm, 1 min and 140 mum. The 8-mm probe integrated responses over a large area, giving more weight to coe-nosarc than polyp tissue for Acropora nobilis. With 1-mm and 140-mum fibre-optic probes, the photosynthetic responses of zooxanthellae in the coenosarc and the polyp tissue of Acropora nobilis were distinguished. The polyp tissue exhibited a lower maximum in relative electron transport rate than did the coenosarc tissue, and was subject to down-regulation at higher irradiances. Coe-nosarc and polyp tissue (both containing zooxanthellae) showed a wide range of responses in the other corals. Down-regulation of photosynthesis in a single polyp of Pocillopora damicornis was followed after exposure to moderate irradiance, with recovery occurring over a further 4 h of shade conditions. All the corals (Acropora millepora, A. nobilis, Cyphastrea serailia, Montipora tuberculosa, Pocillopora damicornis and Porites cylindrica) showed evidence of strong down-regulation of photosynthesis under high irradiance, and little evidence of photoinhibitory damage to photosystem II.