Collaborative Research: The physiology and ecology of widespread 'stress tolerant' coral endosymbionts: coral 'saviors' or opportunistic invaders?

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Ocean warming is affecting life on our planet in many ways. High temperature can disrupt the endosymbioses between dinoflagellate algae (Symbiodinium spp.) and reef-building corals (i.e. coral bleaching), thereby risking the global loss of a critical marine ecosystem. The physiological, ecological and evolutionary responses of coral-dinoflagellate symbioses to environmental stress brought on by global climate change are complex. The spread of certain types of symbiotic algae may increase the thermal stress tolerance among corals and help them persist in warmer oceans, but perhaps not without trade-offs to the health of the coral. The dinoflagellate tentatively named Symbiodinium trenchi has become increasingly more common in numerous corals throughout the Caribbean, but is often at low-abundance relative to other symbionts. While S. trenchi can increase in abundance during and after warming, it is often displaced by other symbionts following a return to normal conditions. Genetic evidence indicates that S. trenchi recently invaded and/or expanded in the Caribbean and has developed associations with many corals that seem to be poorly optimized, or mal-adapted, relative to the symbioses it maintains with corals in the Indo-Pacific. This project will investigate the symbiosis ecology and physiology of S. trenchi in corals from the Atlantic and Pacific Oceans. Bleaching experiments will examine the effects of increased temperature on transfer of carbon from the algae to the host coral (via stable isotopic tagging), as well as photosynthesis and growth among colonies harboring S. trenchi compared to colonies harboring other Symbiodinium spp. The potential for symbiont community shifts as well as altered long-term colony growth based on bleaching severity and recovery time will be investigated. A reciprocal transplant study will examine the competitive interaction and stability of symbionts among Pacific corals. These studies will test if the continued spread of S. trenchi will affect coral growth in the Caribbean and whether it might behave similarly in the Indo-Pacific if environmental conditions worsen. The results from this project have the potential to supply transformative information regarding how (or if) a widely distributed symbiotic algal species may influence the resilience of reef-building corals and their potential to survive projected increases in ocean warming due to climate change. In addition to training one postdoctoral scholar and several graduate students, this project will enhance scientific discovery and participation of underrepresented groups via several outreach efforts with the Palau National Aquarium, Palau International Coral Reef Center, and local schools. Educational units in marine symbioses and science will be developed with several local high school teachers and students, and unique research opportunities will be provided to students at the Palau Community College. Likewise, a new educational display addressing how global climate may impact coral reefs, and describing the current research to better understand the physiology of coral-algal symbioses, will be developed and presented at the University of Delaware open house "Coast Day." The display will be donated subsequently to the Palau Aquarium for future use.This award is co-funded by NSF's Office of International and Integrative Activities.