Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae

The early effects of heat stress on the photosynthesis of symbiotic dinofla gellates (zooxanthellae) within the tissues of a reef-building coral were e xamined using pulse-amplitude-modulated (PAM) chlorophyll fluorescence and photorespirometry. Exposure of Stylophora pistillata to 33 and 34 degrees C for 4 h resulted in (1) the development of strong non-photochemical quench ing (qN) of the chlorophyll fluorescence signal, (2) marked decreases in ph otosynthetic oxygen evolution, and (3) decreases in optimal quantum yield ( F-v/F-m) of photosystern II (PSII), Quantum yield decreased to a greater ex tent on the illuminated surfaces of coral branches than on lower (shaded) s urfaces, and also when high irradiance intensities were combined with eleva ted temperature (33 degrees C as opposed to 28 degrees C), qN collapsed in heat-stressed samples when quenching analysis was conducted in the absence of oxygen, Collectively, these observations are interpreted as the initiati on of photoprotective dissipation of excess absorbed energy as heat (qN) an d 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 blea ching 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. ph otoinhibition) are secondary effects following the overwhelming of photopro tective 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 e nzymes and the unusual status of the dinoflagellate Rubisco, Significant fe atures of our model are that (1) damage to PSII is not the initial step in the sequence of heat stress in zooxanthellae, acid (2) light plays a key se condary role in the initiation of the bleaching phenomena.