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Chlorophyll fluorescence was used to assess the in situ photosynthesis of a range of reef-dwelling endosymbionts. Such non-intrusive in situ measurements became possible after the recent development of a submersible pulse modulated fluorometer (DIVING-PAM, Walz, Germany). Several corals, a clam (Tridacna maxima) and an anemone (Heteractis sp.) all showed strong chlorophyll a fluorescence signals originating from the dinoflagellate endosymbionts. Quenching analysis by the saturation pulse method revealed high quantum yields and light response curves characteristic of physiologically healthy sun plants. Rapid light curves (RLC) were applied to assess the light saturation behaviour of the different organisms in their rapidly changing natural environment. The 3 corals (Acropora aspera, Goniastrea sp. and Porites sp.), the clam T. maxima, and the anemone Heteractis sp. all showed high photosynthetic activity. The corals had a maximum electron transport rate of 180 to 270 μmol electrons m -2 s -1 , the Heteractis sp. displayed a maximum rate of approximately 120 μmol electrons m -2 s -1 , whilst the T maxima showed no saturation up to 1900 μmol quanta m -2 s -1 , where a rate of 325 μmol electrons m -2 s -1 was observed. Three species of corals showed varying degrees of an apparent mid-day depression, occurring during the summer peak irradiance at low tide. Because quantum efficiency rapidly recovered during the afternoon period, when the irradiance levels decreased again, it is concluded from these preliminary investigations that the depression resulted mainly from down-regulation of photosystem II. Depth did not appear to influence the RLCs of A. aspera growing at 2, 5 and 10 m. Coral morphology influenced the apparent electron transport rate at different locations within a single colony.

In situ underwater measurements of photosynthetic activity of coral zooxanthellae and other reef-dwelling dinoflagellate endosymbionts