Impact of bleaching conditions on the components of non-photochemical quenching in the zooxanthellae of a coral

Mass coral bleaching events are a worldwide phenomenon, which generally occur during periods of elevated sea surface temperature and intense sunlight. These conditions result in a decline in photochemical efficiency of symbiotic microalgae (zooxanthellae) which ultimately leads to the expulsion of these symbionts. The physiological mechanism which triggers the release of the zooxanthellae has yet to be adequately determined. Under bleaching conditions, non-photochemical quenching (NPQ) is used to dissipate excess energy from photosystem II (PSII). NPQ was partitioned into three components, (energy dependent quenching [qE], state transition quenching [qT] and photoinhibitory quenching [qI]), based on relaxation kinetics upon addition of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) and darkening. This investigation revealed that for corals not exposed to bleaching stress, qE was the principle means of energy dissipation (∼60% of the total NPQ). In corals exposed to either high-light (475 μmol photons m^-2 s^-1 and 25°C) or elevated temperature (225 μmol photons m^-2 s ^-1 and 32°C) treatments, the dominant component of NPQ was qE and the relative proportions did not change during the exposure period (1-8 h). When exposed to bleaching conditions (475 μmol photons m^-2 s ^-1 and 32°C) the contribution of the different components changed after 4 h and the total NPQ increased. At this time, the contribution of qT to the total NPQ significantly increased to equal that of qE (40%), suggesting state transitions become more important under such conditions. Throughout the exposure period in all treatments, no change in the proportion of qI was observed.