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Chlorophyll fluorescence was used to assess the in situ photosynthesis of a range of reefdwelling endosymblonts. 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 ( T~d a c n a 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 i ? maxima, and the anemone Heteractis sp. all showed high photosynthetic activity. The corals had a maximum electron transport rate of 180 to 270 pm01 electrons m-2 S-', the Heteractis sp. displayed a maximum rate of approximately 120 pm01 electrons m-2 S-', whilst the T. maxima showed no saturation up to 1900 pm01 quanta m-* S-', where a rate of 325 pm01 electrons m-' S-' 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 11. 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.

While a number of factors have been linked to coral bleaching, such as high light, high temperature, low salinity, and UV exposure, the best explanation for recent coral bleaching events are small temperature excursions of 1 to 2°C above summer sea-surface temperatures in the tropics which induce the dinoflagellate symbionts (zooxanthellae) to be expelled from the host. The mechanism that triggers this expulsion of the algal symbionts is not resolved, but has been attributed to damage to the photosynthetic mechanism of the zooxanthellae. In the present investigation we addressed the question of whether such expelled zooxanthellae are indeed impaired irreversibly in their photosynthesis. We employed a Microscopy Pulse Amplitude-Modulated (PAM) fluorometer, by which individual zooxanthellae can be examined to study photosynthesis in zooxanthellae expelled when corals are subjected to a temperature of 33°C. We show that the expelled zooxanthellae from Cyphastrea serailia were largely unaffected in their photosynthesis and could be heated to 37°C before showing temperature-induced photosynthetic impairment. These results suggest strongly that the early events that trigger temperature-induced expulsion of zooxanthellae involve a dysfunction in the interaction of the zooxanthellae and the coral host tissue, and not a dysfunction in the zooxanthellae per se.

A new high-resolution imaging fluorometer (Imaging-PAM) was used to identify heterogeneity of photosynthetic activity across the surface of corals. Three species were examined: Acropora nobilis Dana (branching), Goniastrea australiensis Edwards & Haime (massive), and Pavona decussata Dana (plate). Images of fluorescence parameters (F, F m 0 , effective quantum yield, optimal quantum yield, electron transport rate, relative photosynthetic rate, and non-photochemical quenching) allowed heterogeneity to be detected in terms of position on colony and indicated that the photosynthetic activity of polyp and coenosarc tissues responded differently to changing light for all three species. The Imaging-PAM offers a special routine, with which images of PAR absorption (absorptivity) are obtained. In this way, for the first time it has become possible to derive images of the relative photosynthesis rate. Polyps had a lower PAR absorptivity than coenosarc tissue for A. nobilis and P. decussata, whereas G. australiensis showed the opposite pattern. Acropora nobilis showed heterogeneity along the longitudinal axis of the branch, which could be differentiated from the effect of variations in illumination across the rugose and curved surface. Diel changes were apparent and influenced the longitudinal heterogeneity along the A. nobilis branch. Images were also obtained showing the degree of photoinhibition caused by high-light stress across a coral surface at a hitherto unobtainable level of resolution.