MECHANISMS AND CONSEQUENCES OF CROSS-GENERATIONAL ACCLIMATIZATION IN REEF-BUILDING CORALS

Abstract

Coral reefs are valued over trillions of dollars and are becoming increasingly degraded due to global climate change. Ocean warming disrupts the symbiotic relationship between corals and their algal symbiont, Symbiodinaceae, leading to severe energetic reduction and increased mortality. Recent studies have identified the potential for beneficial within-generational and cross-generational acclimatization to mitigate the impacts of future environmental change, however the mechanisms and outcomes remain understudied in tropical corals. Here, I present three manuscripts testing different mechanisms of within- and cross-generational acclimatization in corals to understand the long-term consequences of ocean warming. First, we tested the hypothesis that the survival in response to thermal stress is dependent on the metabolic capacity of the coral holobiont and associated microbial partners. We designed a 52-day thermal stress experiment on adult Porites astreoides colonies and examined physiological, microbiome, and metabolomic profiles. We determined that prior to thermal stress, colonies that exhibited higher metabolic capacity and more diverse bacterial communities survived prolonged bleaching with no tissue loss. During late-stage bleaching, colonies that primarily utilized carbohydrate storage showed signs of mortality, indicating that catabolizing multiple energy reserves, as opposed to just carbohydrates, may be an effective strategy in surviving bleaching events. To expand our understanding on the cross-generational consequences of coral bleaching, the next experiment tested the hypothesis that environmental memory plays a significant role in the physiological outcomes in response to thermal stress within and across coral generations. We designed a year-long experiment to manipulate eight different thermal histories in adult corals and monitored the physiological impacts on the adult colonies and subsequent offspring. Site of origin was the main predictor in determining the physiological responses at both adult and larval stages when assessing coral metabolic capacity and energetic reserves. Increased endosymbiont plasticity was observed in parental colonies and subsequent offspring who experienced a simulated bleaching event the year prior, outlining the long-term multigenerational impacts of thermal stress on corals. The final study tested the hypothesis that DNA methylation is a mechanism of environmental memory in corals and can be inherited across generations. Epigenetic mechanisms, such as DNA methylation, can be environmentally sensitive and heritable across generations, thus are a likely mechanism for the inheritance of phenotypic traits. We performed whole-genome bisulfite sequencing on adult colonies and their larvae from four different manipulated thermal histories to generate single-base pair resolution of DNA methylation data to test the effects of parental site of origin, a previous simulated bleaching event, and life stage. We determined parental site of origin was the main predictor of DNA methylation patterns at adult and larval life stages, with increased methylation on genes associated with cellular stress response, thermal stress, and innate immunity in samples originating from the more thermally challenging collection environment. Our results further support the role of epigenetic inheritance as a mechanism for environmental memory within and across generations. Together, the results from this dissertation identifies key drivers and mechanisms leading to the resilience of corals to ocean warming across generations, which is critical to forecast the future state of coral reefs.