Rj. Jones et al., Temperature-induced bleaching of corals begins with impairment of the CO2 fixation mechanism in zooxanthellae, PL CELL ENV, 21(12), 1998, pp. 1219-1230
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.