Loss of Symbiodinium from bleached Australian scleractinian corals (Acropora hyacinthus, Favites complanata and Porites solida)

Abstract

Coral bleaching results from the dissociation of Symbiodinium and is primarily related to sea surface temperatures above mean yearly maximums. The numbers of live, dead, and mitotic Symbiodinium cells lost from three scleractinian corals from three different families (Acropora hyacinthus, Favites complanata, and Porites solida), which have not been studied previously in central Queensland (Australia), were compared at 28, 30, 32, and 34°C. Specific expulsion rates, growth rates, and mitotic indices were compared for each host at each temperature. Porites solida was the most robust coral, A. hyacinthus bleached more readily at low temperatures and F. complanata showed levels of intermediate bleaching tolerance to elevated temperatures. However, the timing of Symbiodinium cell loss was similar between all corals tested. Mitotic indices and specific growth rates were found to be positively associated with increasing temperature; thus, symbiont reproduction increased despite elevated losses of Symbiodinium from the host. Because all corals in the present study were symbiotic with Symbiodinium from clade C, different levels of stress tolerance to temperature suggests that bleaching resistance is an attribute associated with the coral host and, to a lesser degree, the symbiont.