Species traits dictate seasonal-dependent responses of octocoral–algal symbioses to elevated temperature and ultraviolet radiation

Abstract

Ultraviolet radiation (UVR) can exacerbate the effects of elevated seawater temperatures concomitant with climate change. Such stressors can collapse the mutualism of scleractinian corals with their endosymbiotic unicellular algae, Symbiodinium spp. Conversely, we found that the Caribbean octocorals Pseudoplexaura crucis and Eunicea tourneforti coped with thermal and UVR stress. But, the responses differed and depended on the species’ biochemical properties. Furthermore, the effects of the stressors varied seasonally. For 7 d, during summer and winter, we exposed the octocorals to either 3 °C above ambient temperature, UVR, or both. In either season, the stressors did not affect protein or carbohydrate content nor the Symbiodinium genotype. At the elevated temperature in the summer, the lipid content in P. crucis fell alongside a 35% decline in Symbiodinium. Conversely, in E. tourneforti, algal density dropped 26% with no lipid loss. In both species, the remaining Symbiodinium retained their chlorophyll and carotenoid content. In the winter, + 3 °C and UVR led to increased algal density, mitotic index, and chlorophyll and carotenoid contents. Temperature affected more parameters than UVR, but additive, synergistic and antagonistic interactions occurred. These findings show the resilience of Caribbean gorgonian corals against stressful environmental conditions and highlight the importance of including organismal biochemical differences and seasonal responses into interpreting the effects of multiple stressors associated with global climate change.