Intraspecific variation in corals’ responses to environmental stressors

Abstract

Climate change threatens reef-building corals in various ways. Increasing temperatures disrupt coral–algal symbioses, acidification impacts calcification, and increasingly prevalent diseases cause tissue loss and mortality. This dissertation investigated coral responses to all of these stressors. First, I measured gene expression in response to natural disease in Acropora hyacinthus. I compared expression profiles of eight healthy colonies against eight colonies exhibiting symptoms commonly associated with white syndromes. Both visibly affected and apparently healthy tissues were collected from diseased colonies. Differences between healthy and diseased tissues indicated activation of innate immunity and tissue repair pathways accompanied by reduced calcification and metabolism of stored lipids. Expression profiles of unaffected tissues from diseased colonies were not significantly different from fully healthy samples, indicating weak systemic effects of white syndromes on A. hyacinthus. Next, I challenged eight A. millepora genotypes with a putative bacterial pathogen to assess intraspecific variation in disease susceptibility. Genotypes varied from zero to >90% mortality, with bacterial challenge increasing mortality rates 4–6 fold and shifting the microbiome in favor of stress-associated taxa. Immune and transcriptomic responses to the challenge were more prominent in high-mortality individuals, whereas low-mortality corals maintained expression signatures of a healthier condition. This study supports that intraspecific variation in disease susceptibility does exist; therefore, selection could promote disease resistance. My final dissertation project investigated the capacity for A. millepora to adapt to multiple environmental stressors: rising temperatures, ocean acidification, and infectious diseases. I measured growth rates, coral color (a proxy for algal symbiont density), survival, and a number of physiological estimates of coral and algal health in response to these three stressors and a combined treatment. Whereas treatments resulted in the predicted responses, I found no synergistic activity between stressors. A genetic variance–covariance matrix demonstrated within-trait variance and positive genetic covariances. Estimates for changes in trait means using the multivariate breeder’s equation showed that co-variances between these traits reinforce, rather than constrain, adaptation to environmental threats. These findings emphasize the importance of acknowledging adaptive capacity when predicting reef cover under future climate scenarios.