Differential recovery of PSII function and electron transport rate in symbiotic dinoflagellates as a possible determinant of bleaching susceptibility of corals

Abstract

2 Institute of Marine Biology, Far East Branch of Russian Academy of Sciences, Vladivostok 690041, Russia ABSTRACT: Differences in the time course of recovery of the photochemical efficiency (Fv /Fm) of PSII and the photosynthetic capacity (relative maximum electron transport rate; rETRmax), 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 Fv /Fm and rETRmax 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 rETRmax after high light exposure and the time course of its recovery between P. divaricata and M. digitata. In P. divaricata, the reduction in Fv /Fm and rETRmax 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 Fv /Fm did not show temperature dependency, rETRmax exhibited a greater decrease and lower recovery at 31°C than at 25 and 28°C. The recovery of rETRmax was slower than that of Fv /Fm in symbiotic dinoflagellates of M. digitata, and this delay of rETRmax recov- ery 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 rETRmax 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 rETRmax 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.