MEPS 268:43-53 (2004)  -  doi:10.3354/meps268043

ABSTRACT: Differences in the time course of recovery of the photochemical efficiency (F_v/F_m) of PSII and the photosynthetic capacity (relative maximum electron transport rate; rETR_max), of symbiotic dinoflagellates from damage due to thermal and light stresses might have profound effects on the subsequent photodamage of photosynthetic apparatus. The reduction and recovery of F_v/F_m and rETR_max in symbiotic dinoflagellates were investigated at different temperatures (25, 28 and 31°C) after 3 h exposure to high light (1100 µmol photons m^-2 s^-1 of photosynthetically active radiation [PAR]) in 2 corals with different bleaching susceptibility, Pavona divaricata and Montipora digitata. There was a marked difference in both the reduction in rETR_max after high light exposure and the time course of its recovery between P. divaricata and M. digitata. In P. divaricata, the reduction in F_v/F_m and rETR_max during high light exposure was independent of temperature, and their recovery was almost complete within 24 h after stress treatment at all temperatures examined. In M. digitata, while the reduction and recovery of F_v/F_m did not show temperature dependency, rETR_max exhibited a greater decrease and lower recovery at 31°C than at 25 and 28°C. The recovery of rETR_max was slower than that of F_v/F_m in symbiotic dinoflagellates of M. digitata, and this delay of rETR_max recovery increased with increasing temperature during the recovery period. Significant decreases in dinoflagellate density and chlorophyll content were evident only in the 31°C-treated fragments of M. digitata kept at 31°C during the recovery period. These results indicate that it is the difference in the rate of recovery between the PSII function and the dark reaction of symbiotic dinoflagellates that determines whether the photosynthetic ability of algae recovers from the combined effect of thermal and irradiance stresses. The delayed recovery of rETR_max at high temperature made fragments of M. digitata susceptible to photodamage even under moderate light conditions. P. divaricata showed a high ability to recover its rETR_max and consequently was resistant to high light and thermal stresses. It is likely that high temperature affects the rate of recovery of the carbon-fixation cycle and that the ability of symbiotic dinoflagellates to repair damage determines susceptibility of corals to bleaching.

KEY WORDS: Bleaching · Coral · Symbiotic dinoflagellate · PAM · Photosystem II · ETR