On coral reefs, mutualisms with unicellular algae within the family Symbiodiniaceae and reef species literally and figuratively form the foundation of reef ecosystems. Coral reefs are among the most threatened ecosystems under a changing climate. The warmer ocean temperatures can lead to a breakdown in the associations between these algal symbionts and their hosts, termed coral bleaching, and to a decline of reef ecosystems.  The threat of increasing temperature may be lessened if the host, symbiont, or holobiont (host + symbiont) can adapt or acclimatize to increased temperatures. In this study, we attempted to select for symbiont strains within the genus Breviolum with increased thermal tolerance by growing symbiont cultures at either 26^o or 30^o C for over 2 years and then investigating variation in symbiont performance and response of the symbiont and the holobiont (host + symbiont) to increased temperature. Significant variation was found in measured traits (chlorophyll content, photosynthetic efficiency, carrying capacity and growth) among symbiont cultures maintained at the same temperature and between cultures maintained at different temperatures, showing that functional diversity exists not only within symbiont genera, but also within a single symbiont species.  This variation suggests the potential for these important partners to evolve in response to temperature and established that within a symbiont species there was significant variation in traits that could affect the mutualism. However, all strains grew well at both ambient and elevated temperatures, suggesting that, at least for some symbiont species within the genus Breviolum, prior culturing at the elevated temperature did not result in increased thermal tolerance and instead this symbiont species may have already evolved some level of thermal tolerance.

An extension of the above work demonstrated that B. antillogorgium grows well at temperatures as high as at least 32^oC, with symbiont strains differing in the magnitude of the response of growth rate and carrying capacity to elevated temperature. This supports the above conclusion that some Breviolum species are relatively thermal tolerant and do not increase temperature tolerance when reared at the elevated temperatures of this study.  These results help to explain why octocorals, such as Antillogorgia bipinnata (Fig. 1), which harbors B. antillogorgium or other Breviolum species, bleach less readily and are more resilient to bleaching events.

These same B. antillogorgium strains established symbioses with newly settled A. bipinnata octocoral polyps (Fig. 2) and although survivorship of polyps maintained at elevated temperatures (30^o C) was significantly lower than survivorship at ambient temperatures (26^o C), all treatments had surviving polyps for at least 3 months post-infection. These results suggest broad thermal tolerance in B. antillogorgium.  Given that most Caribbean octocorals harbor symbionts within the genus Breviolum, these symbionts may play a part in the increased resilience of Caribbean octocorals during heat stress events.

Few studies have looked at the symbionts in octocorals. With the ongoing demise of scleractinian corals, these soft corals are becoming dominant organisms on some reefs.  This work adds to an understanding of their symbionts and how they might respond to the increasing ocean temperatures. Given that diversity is already being lost on many reefs, considering how evolutionary and acclimatory changes in these symbionts affect reef species is crucial for predicting the responses of reefs to future climate change. These data provide a ray of hope for the future of corals and coral reefs as they suggest that it is possible for some symbiont strains to survive at higher temperatures and possibly convey resistance/resilience to the cnidarian host.

This grant provided training and professional development for 38 people, 24 who are women and three who are from minority groups. This includes training for a graduate student, professional development of three technicians and laboratory experience for 35 undergraduate students. Results were disseminated to communities of interest through multiple talks and invited seminars, including eight presentations by undergraduate students at local and national meetings and resulted in 8 publications to date, including several with undergraduates as lead or co-authors.  In addition, presentations on climate change and coral reefs provided outreach to high school and college students and, through the Aquarium of Niagara, to the general public.

Last Modified: 01/17/2023
Modified by: Mary Alice Coffroth