The goals of this funded project were to investigate the symbiosis ecology and physiological abilities of various algae that live in mutualism with reef building corals. These symbiotic algae are necessary for the health and growth of reef corals and are the reason for the existence of these ecosystems around the world. Our work sought to determine the relative ecological importance of thermally tolerant symbiont species. Given that the coral’s symbiont has a significant effect on colony physiology, large-scale changes in host-symbiont pairings may constitute the most significant response to ocean warming. One species in particular, Symbiodinium trenchii, may be critical to the future health and growth of reef-building corals in the Greater Caribbean and the Indo-Pacific Oceans. Some of our experiments were designed to examine the functional trade-offs that might exist among corals hosting thermally tolerant vs. thermally sensitive species of symbiont. We also sought to better understand how competition for host habitat influences the dominance and stability of physiologically different Symbiodinium species in colonies of coral. Do corals with thermally tolerant symbionts grow at reduced rates because these symbionts may not supply the necessary nutrients for maximal growth? How would this then affect the reef building function of corals in the decades to come, assuming that these tolerant symbionts spread to more and more corals as oceans continue to warm?

The project was based, in part, on the discovery that Symbiodinium trenchii recently invaded the Caribbean Sea, likely facilitated by increasing environmental degradation and probably transported by human activities (e.g. shipping commerce). This widespread symbiont has developed associations with several key Atlantic reef-building corals that appear to be poorly optimized, or mal-adapted, relative to the symbioses this symbiont naturally maintains with Indo-Pacific fauna. The experiments we have conducted were designed to address questions about the “health” costs vs. benefits of S. trenchii spreading to other coral communities around the world including its possible expansion throughout the Indo-Pacific as oceans continue to warm and/or as normal seasonal temperatures de-stabilize with changes in the direction and intensity of ocean currents.

Our analysis of population genetic data gathered prior to and during this project indicated that S. trenchii populations are genetically homogeneous over thousands of kilometers and more and hence can travel/disperse long distances. Thus, this Symbiodinium, and perhaps others like it, have the potential to undergo rapid geographic expansion in timeframes equivalent to the current pace of climate change (The rapid spread of S. trenchii across the Caribbean is a good indication of the potential for these kinds of symbionts to take over in a very short period of time-decades or years). We verified that the presence of this symbiont significantly raises the tolerance of a coral colony to thermal stress, although the magnitude may vary from species to species of coral. Moreover, we found that as symbionts in Pacific corals, S. trenchii does not measurably compromise the host’s calcification and growth rate. Our findings are important especially when placed in the context of how thermally tolerant symbionts like Symbiodinium trenchii can contribute to the rapid ecological response of reef corals to the current pace of climate change.

Our findings from this project raised additional questions, however. For example, we do not know whether temperature, light, pH, and/or nutrient availability and/or a combination of these factors affect the stability of thermally tolerant symbionts in coral colonies over time frames of months, years and decades. During our research we found that many corals with S. trenchii were also obtaining nutrients from feeding and that this additional source of energy could also be contributing to their heat tolerance. Lastly, corals obtained from warm water bays could have distinctive genotypes that enable them to endure warmer temperatures than other members of their species.

The broader impacts of this project included numerous presentations to the scientific community and general public. A number of educational outreach programs were initiated and conducted at local, national, and international venues. Visits to elementary schools to talk/teach about animal invertebrate diversity and biology with hands-on demonstrations challenged young minds about what defines an animal. “The Evolving Project,” comprised of an iPad app, crowd sourcing activity, and website taught young and old people basic concepts in evolution and how slow gradual change can produce big differences given enough time. This project also involved the training of 3 Ph.D. graduate students while helping a post-doc co-PI to obtain a tenure-track position.

Last Modified: 12/15/2017
Modified by: Todd C Lajeunesse