Symbiosis and stress in a thermally tolerant coral

Background/Question/Methods

Caribbean corals face increasing stress related to climate change, sedimentation resulting from changes in land use, and other changes to their physical environment. Two common species in the genus Siderastrea (S. siderea and S. radians), have previously been shown to have unusually high tolerance to heat, sedimentation, and wide swings in salinity. These species are found across a wide array of habitats, both on reefs and in shallow seagrass beds, bays, etc. This study aims to isolate the contribution of coral-algal symbiosis to thermal stress tolerance in Siderastrea species. 

To address this question, we tracked colonies of S. siderea and S. radians from 2008-present, and characterized the symbiont communities, and calculated total biomass per unit area, and the density of zooxanthellae within the corals quarterly. To isolate the contribution of endosymbiotic Symbiodinium, small colonies of S. radians were collected from Buttonwood Sound in Key Largo, Florida. Their initial endosymbiont community was characterized using PCR/DGGE genotyping, then decimated by treatment with DCMU, an herbicide. Colonies were then reseeded with either homologous or heterologous cultured symbiont strains. Once the polyps had been recolonized, the holobionts were subjected to thermal stress designed to induce a bleaching event, then mortality, symbiont community composition, and colony biomass were followed through a recovery period.

Results/Conclusions

Analyses of quarterly tissue sampling from 2008-2011 showed all monitored colonies retained their dominant symbiont type at all times, including through the winter 2010 cold weather event which killed or damaged many of the colonies of other species in the long-term monitoring project. Most colonies had a singular relationship with clade B5a, while two showed persistent dominance of clade C3. These results are consistent with previous data, showing dominant symbiont type within these colonies unchanged throughout multiple El Niño and bleaching events since monitoring began in the late 1990's.  In May 2012, colonies in the 2-4 cm size class were collected from shallow water in Buttonwood Bay and randomly assigned them to 3 treatment groups: control, repopulated with homologous symbiont, repopulated with heterologous symbiont. Half of each group was then exposed to elevated temperatures of 30 C, while the others were kept at a constant 26 C. All colonies had Symbiodinium clade B5a or C3 as the dominant zooxantheallae population at the time of collection. Monitoring of colony recovery in captivity is ongoing.