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

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 pCO 2 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 pCO 2 and temperature. In the symbiont, 89 genes were differentially expressed in response to pCO 2 . 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.

Ocean acidification promotes broad transcriptomic responses in marine metazoans: a literature survey

“…This observation does not seem to hold for all early life-history stages of corals. In Pocillopora damicornis larvae from French Polynesia, there was no observable change in expression of metabolism-related genes after an acute 24 h exposure to high pCO 2 (~1030 μatm) [33]. Overall, reef-building corals vary in how they regulate their metabolism in response to elevated pCO 2 even during early life-history stages, providing evidence that early stages might be more robust to environmental variability than previously predicted.…”

“…For example, there were greater changes in gene expression of adult P. damicornis under high pCO 2 , particularly of genes related to calcification processes [32], than of larval P. damicornis subjected to acute high pCO 2 exposures [33]. Specifically, Rivest et al [33] found a stronger transcriptomic response in the algal symbionts of P. damicornis than in the larvae themselves. In other reef-building coral species, symbiosis is established horizontally, typically after metamorphosis and during early skeletal formation.…”

“…This may be because coral larvae do not actively undergo calcification processes; the expression of genes related to biomineralization increases upon settlement [161]. For example, there were greater changes in gene expression of adult P. damicornis under high pCO 2 , particularly of genes related to calcification processes [32], than of larval P. damicornis subjected to acute high pCO 2 exposures [33]. Specifically, Rivest et al [33] found a stronger transcriptomic response in the algal symbionts of P. damicornis than in the larvae themselves.…”

“…Other studies examined the transcriptomic response of organisms exposed to low pH and temperature stress at the same time. Simultaneous exposure to elevated pCO 2 levels and temperatures has been shown to induce a transcriptomic stress response in coral and their symbionts [27,29,33], oysters [50][51][52]56], crabs [71], sea urchins [82], and fishes [91,94,97]. These studies have examined temperature in the context of ocean warming, in which temperatures are elevated relative to average, ambient temperatures.…”

The past, present, and future of coral heat stress studies

“…Environmental history can significantly impact coral's response to elevated temperatures and their overall tolerance to extreme events (Hawkins & Warner, 2017;Krueger et al, 2017;Rivest, Kelly, DeBiasse, & Hofmann, 2018). The hypothesis that prior heat exposure could improve a coral's response to follow-up stress events was proposed early on (Coles & Jokiel, 1978;Jokiel & Coles, 1977;Middlebrook, Anthony, Hoegh-Guldberg, & Dove, 2010;Middlebrook et al, 2008).…”