The early effects of heat stress on the photosynthesis of symbiotic dinoflagellates (zooxanthellae) within the tissues of a reef-building coral were examined using pulseamplitude-modulated (PAM) chlorophyll fluorescence and photorespirometry. Exposure of Stylophora pistillata to 33 and 34°C for 4 h resulted in (1) the development of strong non-photochemical quenching (qN) of the chlorophyll fluorescence signal, (2) marked decreases in photosynthetic oxygen evolution, and (3) decreases in optimal quantum yield (F v /F m ) of photosystem II (PSII). Quantum yield decreased to a greater extent on the illuminated surfaces of coral branches than on lower (shaded) surfaces, and also when high irradiance intensities were combined with elevated temperature (33°C as opposed to 28°C). qN collapsed in heat-stressed samples when quenching analysis was conducted in the absence of oxygen. Collectively, these observations are interpreted as the initiation of photoprotective dissipation of excess absorbed energy as heat (qN) and O 2 -dependent electron flow through the Mehler-Ascorbate-Peroxidase cycle (MAP-cycle) following the point at which the rate of light-driven electron transport exceeds the capacity of the Calvin cycle. A model for coral bleaching is proposed whereby the primary site of heat damage in S. pistillata is carboxylation within the Calvin cycle, as has been observed during heat damage in higher plants. Damage to PSII and a reduction in F v /F m (i.e. photoinhibition) are secondary effects following the overwhelming of photoprotective mechanisms by light. This secondary factor increases the effect of the primary variable, temperature. Potential restrictions of electron flow in heat-stressed zooxanthellae are discussed with respect to Calvin cycle enzymes and the unusual status of the dinoflagellate Rubisco. Significant features of our model are that (1) damage to PSII is not the initial step in the sequence of heat stress in zooxanthellae, and (2) light plays a key secondary role in the initiation of the bleaching phenomena.

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Coral surface temperature was investigated with multiple temperature sensors mounted on hemispherical and branching corals under (a) artificial lighting and controlled flow; (b) natural sunlight and controlled flow; and (c) in situ conditions in a shallow lagoon, under naturally fluctuating irradiance, water flow, and temperature. Under high irradiance and low flow conditions, hemispherical corals were 0.6uC warmer than the surrounding water. Hemispherical corals reached higher temperatures than branching corals, by a measure of 0.2uC to 0.4uC. Microsensor temperature measurements showed the presence of a thermal boundary layer (TBL). The TBL thickness was flow dependent, and under low flow conditions, a TBL up to 3 mm thick limited heat transfer to the ambient water. Combined microsensor measurements of temperature and oxygen showed that the TBL was approximately four times thicker than the diffusive boundary layer, as predicted from heat and mass transfer theory. A simple conceptual model describes coral surface temperature as a function of heat fluxes between coral tissue, skeleton, and surroundings. The slope of the predicted linear relationship between coral temperature and solar irradiance is fixed by the efficiencies of light absorption and the heat losses to the skeleton and the water. Although spectral absorptivity may play a significant role in coral warming, shape-related differences in thermal properties can cause hemispherical corals to reach higher temperatures than branching corals. Shape-related differences in thermal histories may thus help explain differences in susceptibility to coral bleaching between branching and hemispherical coral species.The increasing occurrence of coral bleaching over the last two decades has focused attention on temperature fluctuations on corals reefs (Brown 1997;Berkelmans and Willis 1999;Hoegh-Guldberg 1999). Under mass coral bleaching conditions, small excursions in the ambient water temperature on a coral reef (of just a few degrees Celsius above the normal average temperature maximum) induce the expulsion of the endosymbiotic dinoflagellates (zooxanthellae) and/or the loss of pigments from a wide variety of corals (Glynn 1996;Hoegh-Guldberg 1999). If such high temperature anomalies last for a week or more mass mortality of corals can occur (Glynn 1996; HoeghGuldberg 1999;Coles and Brown 2003).In most of the literature on coral bleaching, temperature of the ambient water is always assumed to be the same as the coral temperature. However, as a result of the shallow nature of many coral reef lagoons, radiant energy reaching the coral surface can increase its temperature relative to the surrounding water. Few studies have considered the 1 Corresponding author (Peter.Ralph@uts.edu.au). AcknowledgmentsWe thank N. Ralph for constructing the flow-through chamber and R. Bilger and J. Kent for valuable discussion of heat transfer theory. We thank M. Ball for discussions of heat transfer in corals at an early stage of this investigation and two anonymous reviewers ...

The cyanobacterium known as Acaryochloris marina is a unique phototroph that uses chlorophyll d as its principal light-harvesting pigment instead of chlorophyll a, the form commonly found in plants, algae and other cyanobacteria; this means that it depends on far-red light for photosynthesis. Here we demonstrate photosynthetic activity in Acaryochloris-like phototrophs that live underneath minute coral-reef invertebrates (didemnid ascidians) in a shaded niche enriched in near-infrared light. This discovery clarifies how these cyanobacteria are able to thrive as free-living organisms in their natural habitat.