Photophysiological and thermal tolerance of various genotypes of the coral endosymbiont Symbiodinium sp (Dinophyceae)

Dinoflagellates of the genus Symbiodinium, the endosymbiont of reef corals, play an important role in coral-bleaching events which can be induced by prolonged enhanced seawater temperature anomalies, and which are predicted to occur at increasing frequencies and severities due to global warming. The genus Symbiodinium exhibits high genetic diversity with physiological variation within and among species, leading to a range of thermal tolerance. Although these variations have been examined in individual species, comprehensive comparative experimental data across several species are still rare. Therefore, in the present study, the photophysiological and thermal response patterns of six genetically characterized Symbiodinium genotypes were comparatively investigated. The six Symbiodinium genotypes were isolated from four different host species from the following biogeographic locations: Hawaii (Central Pacific), Panama (Caribbean), Florida (Caribbean), and Palau (West Pacific). Photosynthesis-irradiance curve and growth measurements at temperatures ranging from 20 to 33 A degrees C were carried out. All physiological data clearly indicate significant differences in the response patterns of the six Symbiodinium genotypes. While some types photosynthesized, respired, and grew at 33 A degrees C, others showed a partial or complete inhibition. The genotype-specific response over the experimental temperature reveals the potential range response of a given symbiont and variation between strains that adaptation might act upon.