Abstract
The persistence of coral reef ecosystems, valued in the hundreds of billions of dollars annually, relies on the survival and growth of the coral colonies that build the habitat. However, these ecosystem engineers are threatened by a host of stressors, including warming ocean temperatures due to global climate change. Anomalously high sea surface temperatures can result in coral bleaching, the breakdown in the obligate symbiosis between the coral host and its community of single-celled, dinoflagellate algae in the genus Symbiodinium. Bleaching stress that is too prolonged or too severe can result in coral mortality, and increasingly frequent and severe bleaching is one of the most pressing threats to coral reef systems today. However, corals can recover from bleaching, and this recovery can include associations with thermally tolerant symbiont types that can help corals resist future heat stress. Thus, studying the way corals recover from bleaching stress is critical to our understanding of the way reefs will respond to global climate change. This dissertation aims to elucidate the factors that influence the way symbiont type Symbiodinium D1a (S. trenchii), which has been shown to increase corals’ bleaching thresholds. By using highly sensitive, quantitative PCR assays to assess symbiont community structure, this work reveals dynamic community recovery following disturbance. First, I used the bleaching events in 2014 and 2015 as a natural experiment to examine the response of Orbicella faveolata colonies to back-to-back bleaching. By sampling colonies from two sites with differing thermal histories, I show that even small differences in thermal regime may drive dramatic differences in symbiont community structure. Further, examining the relationship between symbiont community structure and spawning revealed that the commonly accepted tradeoffs to hosting thermally tolerant clade D symbionts are context dependent. Next, I show that pCO2 elevated to end-of-century levels (900ppm) had no effect on the trajectories of recovering symbiont communities in Montastraea cavernosa corals. Then, I examined the effect of mild thermal stress and recovery temperature on symbiont communities in Acropora cervicornis. I show that even short-term thermal perturbations can result in dramatic dynamism in the symbiont to host cell ratio. Finally, I used repeated acute thermal stress and gradually warming baseline temperatures to experimentally explore a mechanism by which clade D symbionts may rise to dominate a coral’s algal community. I show that together, punctuated thermal stress and gradual warming acted to increase the level of clade D in a stepwise fashion, which affected the response of the coral to subsequent stress. These findings help to explain how even minor differences in thermal regime can lead to dramatically different symbiont community structures. In summary, this dissertation explored which factors significantly influence the reassembly of symbiont communities in bleached corals and presented an explanation for how clade D symbionts can become dominant. These findings illustrate the dynamic reassembly of coral symbiont communities and demonstrate that the costs and benefits of hosting thermally tolerant D1a are context dependent.