Coral thermal tolerance is strongly influenced by the identity of obligate photosymbionts, which encompass numerous types belonging to the dinoflagellate genus. Symbiodinium(1). Physiological advantages achieved by partnering with functionally diverse symbionts(2-4) have been assumed to be available only to corals that can form associations with multiple Symbiodinium types. Functional variation among populations of the same type of Symbiodinium has been overlooked, despite local adaptation being feasible because of large population sizes(5,6), genetic isolation(7,8) and short asexual generation times(9). Here we demonstrate divergent thermal tolerance in a generalist Symbiodinium type from two different thermal environments. Symbiodinium from the warmer reef maintained greater photochemical performance and survivorship when exposed to an elevated temperature of 32 degrees C, both in symbiosis and in culture. Juvenile corals associated with Symbiodinium from the warmer reef grew rapidly when exposed to 32 degrees C, yet underwent bleaching and tissue death when associated with Symbiodinium from the cooler reef. These results demonstrate that Symbiodinium types can adapt to local differences in thermal climate and that this adaptation shapes the fitness of coral hosts. If Symbiodinium populations are able to further adapt to increases in temperature at the pace at which ocean climates warm, they may assist corals to increase their thermal tolerance and persist into the future.
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