How Do Coral Reefs Respond to Climate Change? Investigating the Role of Symbiodiniaceae Community Composition on Coral Performance Under Long-Term Exposure to Warming and Acidification

Abstract

Coral reefs are one of the most biodiverse and ecologically important ecosystems on the planet, but they are increasingly threatened by ocean acidification and warming. Changes in environmental factors can cause the coral to expel their endosymbiotic community of single-celled dinoflagellates (family: Symbiodiniaceae), leaving them more vulnerable to disease and mortality. One proposed method through which coral can acclimatize to the fluctuations in ocean temperature is by shuffling their Symbiodiniaceae community compositions to increase the relative proportions of temperature-tolerant symbionts. However, the effects of ocean acidification on Symbiodiniaceae community compositions are still unknown. Here, I present data from a two-year mesocosm experiment investigating the effects of long-term exposure to ocean acidification and warming on Symbiodiniaceae community composition in eight common species of Hawaiian coral. Coral were collected from six geographically distinct locations around O’ahu and exposed to predicted end-of-century temperature and pH conditions for two years. Next Generation Sequencing (NGS) of the ribosomal internal transcribed spacer 2 (ITS2) region of the Symbiodiniaceae provided detailed insight into the distinct symbiont types and the changes in community composition resulting from environmental stressors. Our findings indicate that temperature is a more significant driver of changes to the Symbiodiniaceae community compositions than pH in Hawaiian corals. We additionally demonstrate that the changes in symbiont communities in response to experimental temperature and pCO2 conditions arise from the shuffling of current symbionts and the incorporation of novel symbionts from the environment, which has implications for future coral resilience against climate change.