Elucidating Host-Symbiont Interactions in Coral Bleaching by Symbiont Transfer

Coral bleaching, a current major ecological threat, is caused by the disruption of the endosymbiotic relationship between the host coral and single-celled algal symbionts known as Symbiodinium. Symbiodinium are divided into clades, with each clade having similar characteristics. It has been found that corals containing certain clades tend to fare better following bleaching incidents. In order to further elucidate the relationship between the Symbiodinium and the coral, symbiont transfer experiments are thus required. These experiments require first obtaining symbionts by stressing a coral, then culturing the symbionts ex hospite, and finally transferring them to a bleached host of another symbiont clade. For this purpose, we cultured clade C Symbiodinium by thermally stressing a Sympodium colony overnight. The resulting symbionts were lacking in photosynthetic pigments and required some time for recovery. After 2 weeks of culture in f/2 media in seawater (27 C and a 12hour light cycle at 55 µmol photons m-2s-1), single symbionts proliferated into a large mass of cells resembling a tumor. One week later, dense growth of competing algae required transfer to a new culture. Symbionts were maintained in this fashion for several months and a number of assays and experiments were performed. The ultimate goal is transferring these symbionts to a bleached coral host (Sarcothelia sp.) which normally contains clade D symbionts.