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

Chlorophyll fluorescence was used to assess the iu situ photosynthesis of a range of reef-dwelling endosymbionts. Such non-intrusive in situ measureme nts became possible after the recent development of a submersible pulse mod ulated fluorometer (DIVING-PAM, Walt, Germany). Several corals, a clam (Tri dacna maxima) and an anemone (Heteractis rip.) all showed strong chlorophyl l a fluorescence signals originating from the dinoflagellate endosymbionts. Quenching analysis by the saturation pulse method revealed high quantum yi elds and Light response curves characteristic of physiologically healthy su n plants. Rapid light curves (RLC) were applied to assess the light saturat ion 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 photosy nthetic activity. The corals had a maximum electron transport rate of 180 t o 270 mu mol electrons m(-2) s(-1), the Heteractis sp, displayed a maximum rate of approximately 120 mu mol electrons m(-2) s(-1), whilst the T. maxim a showed no saturation up to 1900 mu mol quanta m(-2) s(-1), where a rate o f 325 mu mol electrons m(-2) s(-1) was observed. Three species of corals sh owed varying degrees of an apparent mid-day depression, occurring during th e summer peak irradiance at low tide. Because quantum efficiency rapidly re covered during the afternoon period, when the irradiance levels decreased a gain, it is concluded from these preliminary investigations that the depres sion 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 m orphology influenced the apparent electron transport rate at different loca tions within a single colony.