Date of Award
Ph.D. in Biological Science
Tamar L. Goulet
Stephen J. Brewer
A Caribbean octocoral colony is an assemblage of the octocoral host, dinoflagellate algae (family Symbiodiniaceae), bacteria and other microbes, together functioning as meta-organism (holobiont). Even though octocorals dominate many Caribbean reefs, compared to scleractinian corals, little is known about their biology under ambient or potentially stressful conditions. Of the nine species of octocorals (gorgonian corals, Order: Alcyonacea) studied, each had significantly different polyp behaviors, biochemical compositions, Symbiodiniaceae parameters, isotopic signatures, in addition to hosting distinct microbial assemblages. We identified multiple Symbiodiniaceae species (Breviolum minutum, Breviolum B41, Breviolum B1a and Breviolum B1i), inhabiting the gorgonians. These symbioses were temporally and spatially stable, remaining unchanged even when the holobionts were exposed to stressful environmental conditions. Furthermore, the gorgonian species maintained mixotrophic diets, with various reliance on autotrophy and heterotrophy. Particulate and suspended organic matter constituted the bulk of their heterotrophic quotient, with zooplankton contributing far less, explaining why the majority of gorgonians expanded their polyps and tentacles throughout the day but retracted them at night. Gorgonian species reacted to stressors in a species specific manner, with two species employing different techniques to cope with short term temperature and ultraviolet stress. Further, there were seasonal differences to these responses. Elevated temperature impacted multiple holobiont parameters, whereas ultraviolet radiation affected only a few, yet their combination led to additive, synergistic, and antagonistic interactions. Gorgonian colonies within the lagoon were nitrogen enriched, likely due to sewage released from nearby hotels. Experimental short term ammonium and phosphate enrichments did not affect the gorgonian host, but Symbiodiniaceae exhibited concentration dependent benefits. The bacterial phyla Proteobacteria and Tenericutes dominated the gorgonian bacterial microbiome, although each gorgonian species hosted a unique microbial assemblage. This assemblage varied temporally and spatially with changes in abundance of bacterial groups driving the majority of the differences. Under stressful conditions, shifts in bacterial composition may help maintain holobiont functional stability. Overall, I observed modifications to the host, algal and microbial parameters, enabling the octocoral holobiont to cope with changing environmental conditions. Such versatility in responding to environmental perturbations may help explain why gorgonian corals thrive in the Caribbean and may continue to do so.
McCauley, Mark, "Environnmental Influences on Caribbean Octocoral-Symbiodiniaceae Symbioses" (2019). Electronic Theses and Dissertations. 1671.