Alternative Photosynthetic Electron Transfers and Bleaching Phenotypes Upon Acute Heat Stress in Symbiodinium and Breviolum spp. (Symbiodiniaceae) in Culture

The breakdown of the symbiosis between cnidarians and Symbiodiniaceae often occurs upon periods of elevated sea surface temperature and gives rise to bleaching events that affect coral reefs worldwide. In this respect, an impairment of photosynthesis would be responsible for light-dependent generation of toxic reactive oxygen species putatively contributing to death of symbionts and/or host cells. In some Symbiodiniaceae species, alternative photosynthetic electron flows (AEF) have been documented to occur upon a shift to high temperature, possibly contributing to photoprotection and to the balance of energetic ratio between photoproduced ATP and NADPH. By using a combination of in vivo spectrofluorimetric and oximetric techniques, we studied the capacity for electron rerouting toward oxygen and cyclic electron flow (CEF) around photosystem I in eight Symbiodiniaceae in culture belonging to Symbiodinium and Breviolum genera upon an acute shift from 25 to 33°C. CEF capacity was determined as the kinetic of PSI primary donor P700 re-reduction in the presence of DCMU, a PSII inhibitor. An active oxygen uptake in light was estimated by comparing net oxygen evolution and relative electron transport rate of PSII at different light intensities. Among strains that showed elevated capacity for both AEF, some were thermotolerant while others were thermosensitive. Conversely, in some thermotolerant strains, capacities for these AEF were low upon acute heat stress. A principal component analysis of these results indicates that the long-term heat tolerant/bleached phenotype of cultured Symbiodinium and Breviolum spp. is not correlated with a capacity for different AEF across isolates during early onset of acute heat stress.