Physiological diversity among sympatric, conspecific endosymbionts of coral (Cladocopium C1acro) from the Great Barrier Reef

Most of the scleractinian corals living in the photic zone form an obligate symbiosis with dinoflagellates in the family Symbiodiniaceae that promotes reef accretion and niche diversification. However, sea surface temperature surpassing the normal summer average disrupts the symbioses, resulting in coral bleaching and mortality. Under climate warming, temperature anomalies and associated coral bleaching events will increase in frequency and severity. Therefore, it is imperative to better understand the variability in key phenotypic traits of the coral-Symbiodiniaceae association under such high temperature stress. Here, we describe the extent of genetically fixed differences in the in vitro acclimatory response of four conspecific strains of the common coral endosymbiont, Cladocopium C1(acro). (formerly Symbiodinium type C1); these strains were isolated from Acropora corals from inshore sites on the Great Barrier Reef. We characterised algal growth and thylakoid membrane stability under different thermal scenarios and demonstrate previously undocumented physiological diversity among strains of a single Symbiodiniaceae species. Our results have important implications in terms of the perceived accuracy by which environmental stress tolerance of the coral holobiont can be predicted, potentially explaining patchiness in a coral community during bleaching based on the dominant Symbiodiniaceae genotype harboured by the host.