Photochemical efficiency and antioxidant capacity in relation to Symbiodinium genotype and host phenotype in a symbiotic cnidarian

This study analysed the effects of elevated temperature on chlorophyll fluorescence parameters and superoxide dismutase (SOD) activities in relation to Symbiodinium genotype, and host pigmentation in 2 distinct colour phenotypes (green and pink) of the sea anemone Entacmaea quadricolor. Overall, the phenotypes differed with respect to the relative content of Symbiodinium internal transcribed spacer 2 (ITS2) types C25 and C3.25 and their maximum quantum yield of PSII (F_v/F_m) during baseline conditions. However, different PSII photochemical efficiencies were not correlated with symbiont assemblage. Also, the responses to elevated temperatures were phenotype-specific. The PSII photochemical efficiencies had different critical thermal thresholds of <24.5° C in the green phenotype and >24.5° C in the pink phenotype. The highest temperature treatment (27.6° C) resulted in symbiont shuffling towards a higher relative content of C3.25 in the green but not the pink phenotype. However, the observed shuffling of Symbiodinium types could not be linked to enhanced algal SOD activity or PSII photochemical efficiency. These results suggest that different photobiological properties and thermal responses of Symbiodinium ITS2 consortia might be, at least in part, influenced by host-derived factors, possibly chromophore proteins that also determine host pigmentation. The differential ability to cope with elevated temperatures might have profound impacts on E. quadricolor phenotype abundance in response to changing climate.