Diversity, Distribution and Stability of Symbiodinium in Reef Corals of the Eastern Tropical Pacific

Coral reefs of the eastern tropical Pacific (ETP) are unique in being the only reef region in the world that has experienced multiple episodes of mass coral bleaching while also being exposed to chronically depressed aragonite saturation states as a result of regional upwelling. These characteristics make them ideal case studies for the continued effects of climate change on coral reefs, and in particular for the responses of reef corals and their dinoflagellate algal symbionts (Symbiodinium spp.) to combined climate stressors. As a result, the diversity, distribution and stability of Symbiodinium in ETP corals have been studied since the mid-1990s, shortly after contemporary molecular methods to identify Symbiodinium were first developed. ETP reefs have been instrumental in the discovery that certain members of Symbiodinium in clade D impart bleaching resistance to their coral hosts. In the ETP, clade D is represented by a single symbiont type (D1, also referred to as S. glynni), which has been shown to be tolerant of both episodic El Nino-driven high temperature stress (e.g., 1997 in the Gulf of Chiriqui, Panama), and low temperature stress during an unusually cold winter (e.g., 2008 in Baja California, Mexico). Virtually all studies in the region have focused on corals in the genus Pocillopora, which is both the dominant reef-building genus and the most symbiotically diverse, being the only coral genus in the ETP that routinely hosts heat tolerant D1 symbionts at high abundance. There is debate over the mechanisms by which D1 becomes dominant on pocilloporid reefs, with evidence for both differential mortality of corals, and dynamic change in symbiont communities in response to thermal history and/or disturbance. The relative importance of these two mechanisms is likely to be critical in determining reef survival trajectories over the coming century, as is the degree to which non-pocilloporid corals in the region can associate with, or become dominated by, D1. Dynamic change in symbiont communities may allow corals to survive recurrent bleaching events if stepwise increases in the abundance of D1 following each bleaching event allow these thermotolerant symbionts to accumulate. However, controlled experiments and continued monitoring are required to resolve the debate over symbiont stability versus lability, and there may be significant differences in coral response across the region. Finally, understanding whether different Symbiodinium can alter coral response to high CO2 or depressed aragonite saturation state (Ωarag) remains a priority research area for the region, especially given the potential for strong interactions between Ωarag, coral growth, and symbiont community structure.