Improving Thermal Tolerance in Reef-Building Corals
as ocean temperatures rise due to climate change, coral bleaching events are increasing in frequency and severity.
When a coral "bleaches", it expels the tiny, symbiotic algae (family Symbiodiniaceae) that live in its tissues. Because these algae photosynthesize to provide corals with nutrition and aid with calcification, it is harmful and sometimes fatal for corals to lose them. However, corals can recover their symbionts within a period of weeks to months if thermal stress ceases (Jones and Yellowlees 1997; Cunning et al. 2016).
By expelling and regaining symbionts, bleaching and recovery may enable shifts in symbiont community composition (Buddemeier and Fautin 1993). Referred to as “symbiont shuffling” (Baker 2003), this process may modify the relative abundance of different, functionally diverse symbiont types within the host, which may also alter the holobiont phenotype. For example, studies have found that many bleached corals recover with Durusdinium (formerly known as Symbiodinium clade D) as the dominant symbiont. Durusdinium has shown more thermal tolerance than other symbiont types, making the coral host more resistant to future heat stress and increasing its bleaching threshold by ~1-2°C (Berkelmans and van Oppen 2006; Silverstein et al. 2015; Cunning et al. 2015).
Symbiont shuffling, and in particular shifting symbiont communities towards dominance of Durusdinium, may allow corals to rapidly acclimatize when faced with environmental stress and help them become more resilient under climate change. The Coral Reef Futures Lab has achieved this "stress-hardening" technique in the lab using adults of many key Caribbean coral species, and lab members are currently investigating methods for in situ holobiont manipulation. In addition, one of the primary focuses of my research is encouraging coral recruits to take up Durusdinium from the very beginning of their lives. Our hope is that these methods will soon allow restoration practitioners to prime corals with thermotolerant symbionts to increase bleaching thresholds and protect against future heat stress.