Host and Symbionts in Pocillopora damicornis Larvae Display Different Transcriptomic Responses to Ocean Acidification and Warming

Emily B Rivest,Melissa B Debiasse,Morgan W Kelly,Gretchen E Hofmann

doi:10.3389/fmars.2018.00186

Abstract

As global ocean change progresses, reef-building corals and their early life history stages will rely on physiological plasticity to tolerate new environmental conditions. Larvae from brooding coral species contain algal symbionts upon release, which assist with the energy requirements of dispersal and metamorphosis. Global ocean change threatens the success of larval dispersal and settlement by challenging the performance of the larvae and of the symbiosis. In this study, larvae of the reef-building coral Pocillopora damicornis were exposed to elevated pCO2 and temperature to examine the performance of the coral and its symbionts in situ and better understand the mechanisms of physiological plasticity and stress tolerance in response to multiple stressors. We generated a de novo holobiont transcriptome containing coral host and algal symbiont transcripts and bioinformatically filtered the assembly into host and symbiont components for downstream analyses. Seventeen coral genes were differentially expressed in response to the combined effects of pCO2 and temperature. In the symbiont, 89 genes were differentially expressed in response to pCO2. Our results indicate that many of the whole-organism (holobiont) responses previously observed for P. damicornis larvae in scenarios of ocean acidification and warming may reflect the physiological capacity of larvae to cope with the environmental changes without expressing additional protective mechanisms. At the holobiont level, the results suggest that the responses of symbionts to future ocean conditions could play a large role in shaping success of coral larval stages.

Highlights

Understanding the sensitivities of reef-building corals to the ongoing simultaneous shifts in temperature and pCO2 is imperative, as corals are the engineers of the coral reef ecosystem that provides habitat for an incredible diversity of species, as well as food, income, and coastline security to millions of humans
The persistence of coral reefs is partially dependent on the successful dispersal and recruitment of early life history stages of coral, which may be affected by ocean acidification (OA) and warming
Previous work examining effects of climate change stressors on gene expression in corals has tended to focus on the coral host

Summary

Introduction

Understanding the sensitivities of reef-building corals to the ongoing simultaneous shifts in temperature and pCO2 is imperative, as corals are the engineers of the coral reef ecosystem that provides habitat for an incredible diversity of species, as well as food, income, and coastline security to millions of humans. OA and warming cause stress through different mechanisms, OA and warming may have synergistic effects for reef-building corals: increased physiological sensitivity to increased temperature and/or broader fitness-related consequences (Metzger et al, 2007; Walther et al, 2009; Pörtner et al, 2017; Prada et al, 2017). The persistence of coral reefs is partially dependent on the successful dispersal and recruitment of early life history stages of coral, which may be affected by OA and warming. While OA and warming are known to decrease calcification and cause bleaching in adult corals (Chan and Connolly, 2013; Hughes et al, 2017), their interactive effects on coral larvae are less understood. Larvae are especially vulnerable to changing environmental conditions (Pechenik, 1999; Kurihara, 2008; Byrne, 2011)

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