Sensitivity to oxidative stress is not a definite predictor of thermal sensitivity in symbiotic dinoflagellates

Abstract

Coral bleaching, the loss of symbiotic dinoflagellate algae (genus Symbiodinium) and/or photosynthetic algal pigments from corals, is thought to be primarily triggered by the thermal dysfunction of photosynthesis and a consequent build-up of reactive oxygen species. However, different corals exhibit differential bleaching susceptibilities, perhaps resulting from dissimilar abilities to deal with oxidative stress. We therefore tested whether thermal sensitivity in Symbiodinium is correlated with the capacity to deal with oxidative stress, by comparing the effects of increased temperature and hydrogen peroxide (H2O2) concentration on the photosynthetic performance of four cultured Symbiodinium ITS2 types (A1, B2, E, and F1) and two freshly isolated Symbiodinium ITS2 types (a temperate A-type and B1). Generally, the maximum quantum yield of photosystem II (Fv/Fm) declined in Symbiodinium with both increasing thermal and oxidative stress. However, not all types followed this pattern. Cultured Symbiodinium A1 showed a strong negative response to elevated temperature but little response to the addition of H2O2, while cultured F1 showed an increase in Fv/Fm at elevated temperature but a decline in this parameter to almost zero at high H2O2 concentrations. Furthermore, both freshly isolated Symbiodinium types appeared to be relatively stress tolerant, with the temperate A-type showing an especially high resistance to both stressors. In conclusion, a range of Symbiodinium types were shown to differ in their susceptibilities to both thermal and oxidative stress, though in contrast to our original hypothesis, sensitivity to oxidative stress did not necessarily predict thermal sensitivity (or vice versa).