Award: OCE-1552949

Within the marine environment, microorganisms form one of the most important marine symbioses in the world: the symbiosis between corals and photosynthetic single-celled algal symbionts within the genus Symbiodinium. In nutrient poor waters of the tropics, this symbiosis maintains the coral?s high productivity, allowing corals to flourish and providing the foundation of the reef ecosystem. However, corals and associated ecosystems are under increasing stress due to climate change and other anthropogenic-induced perturbations. In fact, hard corals and their associated biodiversity on reefs are being lost at an alarming rate. In the Caribbean coral cover has declined by 80% over the last thirty years. Much of this decline has been attributed to coral bleaching, a loss of these algal symbionts in response to increase ocean temperatures. In contrast, octocorals (e.g., soft coral like sea fans, sea whips, sea feathers) do not show this decline and are increasing in relative abundance and importance in the Caribbean. Part of this has been attributed to the fact that bleaching is rarer among octocorals. Their growing importance on Caribbean reefs and the lack of knowledge of their response to bleaching events creates an urgency to understand the dynamics of these algal symbiont populations within octocorals during periods of bleaching. Bleaching susceptibility varies among both hard coral and octocoral species. This has been attributed in part to the type of algal symbiont that they contain. Although little is known about the diversity of algal symbiont species that dominant Caribbean octocorals, in at least some instances octocoral harbor symbionts that differ from those harbored by scleractinians. This may explain the observation that many octocoral species appear to be more resistant to disturbances such as bleaching induced by elevated ocean temperatures. This project focused on identifying symbiont variation within octocoral species before, during and after the 2015 bleaching event. Between May 2015 and August 2017, we monitored survival, symbiont type and density in four octocorals species on two shallow water patch reefs in the Florida Keys. These data were used to determine if bleaching susceptibility is correlated to symbiont type. Cell density were significantly lower in September 2015 in all octocoral species in the study, coinciding with abnormally high water temperatures during September 2015. At this time visible bleaching was observed in two of the species (Muricea atlantica and Plexaurella dichotoma). Normal, smaller seasonal decreases in cell density were seen the following fall (September 2016) although colonies did not visibly appear bleached. Three of the studied octocoral species harbored a single symbiont type/species within the B1 lineage which differed between host species. The fourth octocoral species (P. dichotoma) appeared to harbor two symbiont types at one time. One of these types (a species within the B1 lineage) was present throughout the study while the second type (a symbiont type within the B19 lineages) decreased during the bleaching event. Most surprisingly, the two most related octocorals in the study, M. atlantica and M. elongata, exhibited substantial differences in bleaching severity. Although sequence data did not clearly distinguish the symbionts within Muricea, the two host species harbored unique symbiont genotypes which suggests that the symbiont types associated with each host species represent distinct symbiont species that may respond to increased temperature differently. Based on the phylogenic analysis, symbionts of another host species, P. dichotoma, form a genetically distinct group from the symbionts hosted by the two Muricea species. Interestingly, although the symbionts of M. atlantica and P. dichotoma differed, both host species severely bleached and had similar symbiont densities throughout the study. This suggests either that these symbiont species are more temperature sensitive than the symbionts harbored by M. elongata and Eunicea flexuosa, that these host species are more temperature sensitive or that both parties play a role in bleaching resistance. Further research will be needed to resolve this. The change observed in P. dichotoma symbionts suggests that some of the large decrease in symbiont density is due to the loss of the B19 phylotype. This too will require additional study. This project has added to our understanding of cnidarian-algal symbioses that form the foundation of the coral reef ecosystem. Octocorals dominate many Caribbean reefs and serve as structure and habitat for numerous fish and invertebrates. These data contribute to our understanding of how these symbioses function and allow for a comparative study with bleaching among other cnidarians. Throughout this project, we have collaborated with the Aquarium of Niagara on various projects, including articles for the quarterly newsletter and docent training. This collaboration is ongoing. In addition we provided tours of our laboratory for various groups and made presentations to numerous schools and educators on coral reefs and bleaching. This project trained one graduate student and three female undergraduate students. Data from this work will be deposited in BCO-DMO and data deposition will continue as manuscripts are published. Last Modified: 10/11/2017 Submitted by: Mary Alice Coffroth