Ocean acidification (OA) and rising sea surface temperatures will likely shape the structure and function of coral reefs in the future (Fig. 1). Understanding the sensitivity of corals to ongoing shifts in pCO2 and temperature is imperative as coral are the engineers of the coral reef ecosystem. Specifically, coral larvae may be a life history stage of corals that is particularly vulnerable to environmental stress. Shifts in physiological processes in response to environmental conditions may affect the success of larval dispersal and recruitment. The aim of this dissertation was to examine the physiological plasticity of coral larvae in response to two potentially interacting anthropogenic stressors - OA and warming. Pocillopora damicornis (Linnaeus, 1758) was an excellent study organism for this research given its ubiquitous Indo-Pacific distribution, reef-building role, and long dispersal potential. To accomplish the objectives of this dissertation, I conducted laboratory experiments in which P. damicornis larvae were exposed to seawater of different pCO2 and temperature conditions. P. damicornis larvae were collected from populations in Moorea, French Polynesia and southern Taiwan. To understand the consequences of OA and warming on the physiology of P. damicornis larvae, I employed a holistic approach, examining the response of a suite of physiological processes from the levels of gene expression up to the level of the whole organism. I used metabolic rate and lipid utilization as whole-organism techniques for assessing the effects of OA and warming on larval energy consumption. Planulae released after the peak of spawning experienced metabolic suppression under high-temperature, high-pCO2 conditions. There was evidence of biochemical limits of increasing oxidative capacity to satisfy elevated energy demands under future ocean conditions. Measurements of lipid utilization suggested that the metabolic costs of tolerating OA and warming will be greatest when both environmental stressors occur simultaneously. Larvae released at the peak of spawning will experience greatest changes in lipid content in response to OA and warming, with associated changes in buoyancy that affect their dispersal potential. The organism-level studies revealed physiological plasticity within coral larval energy consumption. Biochemical and transcriptomic techniques were used to better understand the underlying mechanisms. P. damicornis larvae increased their total antioxidant potential in response to oxidative stress under exposures to high pCO2 but not elevated temperature. Furthermore, OA-induced hypercapnia caused increased acid-base regulation, observed through elevated pNPPase activity of Na+ /K + -ATPase. As was observed at the whole-organism level, physiological traits varied between cohorts of larvae. Finally, comparisons of gene expression profiles revealed down-regulation of genes under single-stressor treatments but up-regulation of genes when high-pCO2 and high-temperature co-occurred. An important facet of my dissertation research was to provide environmental context for the results of my biological experiments. I deployed autonomous pH and temperature sensors on the fringing reefs where the experimental corals were collected. The time series of pH and temperature approximated the conditions of the water mass bathing the reef to which the study organisms were acclimatized and into which the larvae would have been released. These environmental data confirmed that control treatments in the laboratory experiments were within the range of conditions experienced on the reef. The elevated pCO2 treatment levels were not observed in the present-day time series. Additionally, pH and temperature regimes differed between reefs in Moorea and Taiwan. pH and temperature were on average lower in Taiwan and more variable in Moorea. Temperature was on average lower and more variable in Taiwan. These results highlight the importance of generating such an environmental context for study species. Environmental data informed metrics of biological performance of individuals at these sites as well as the potential heterogeneity of phenotypes across the biogeographic species range, products of local adaptation to regimes of pH and temperature. .
Thesis or Dissertation
Department of Ecology, Evolution and Marine Biology, UC Santa Barbara