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Abstract The symbiosis between corals and dinoflagellates of the family Symbiodiniaceae is sensitive to environmental stress. The oxidative bleaching hypothesis posits that extreme temperatures lead to accumulation of photobiont-derived reactive oxygen species ROS, which exacerbates the coral environmental stress response (ESR). To understand how photosymbiosis modulates coral ESRs, these responses must be explored in hosts in and out of symbiosis. We leveraged the facultatively symbiotic coralAstrangia poculata, which offers an opportunity to uncouple the ESR across its two symbiotic phenotypes (brown, white). Colonies of both symbiotic phenotypes were exposed to three temperature treatments for 15 days: (i) control (static 18 °C), (ii) heat challenge (increasing from 18 to 30 °C), and (iii) cold challenge (decreasing from 18 to 4 °C) after which host gene expression was profiled. Cold challenged corals elicited widespread differential expression, however, there were no differences between symbiotic phenotypes. In contrast, brown colonies exhibited greater gene expression plasticity under heat challenge, including enrichment of cell cycle pathways involved in controlling photobiont growth. While this plasticity was greater, the genes driving this plasticity were not associated with an amplified environmental stress response (ESR) and instead showed patterns of a dampened ESR under heat challenge. This provides nuance to the oxidative bleaching hypothesis and suggests that, at least during the early onset of bleaching, photobionts reduce the host’s ESR under elevated temperatures inA. poculata.
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Heat challenge elicits stronger physiological and gene expression responses than starvation in symbiotic Oculina arbuscula
Heterotrophy has been shown to mitigate coral–algal dysbiosis (coral bleaching) under heat challenge, but the molecular mechanisms underlying this phenomenon remain largely unexplored. Here, we quantified coral physiology and gene expression of fragments from 13 genotypes of symbiotic Oculina arbuscula after a 28-d feeding experiment under (1) fed, ambient (24 °C); (2) unfed, ambient; (3) fed, heated (ramp to 33 °C); and (4) unfed, heated treatments. We monitored algal photosynthetic efficiency throughout the experiment, and after 28 d, profiled coral and algal carbohydrate and protein reserves, coral gene expression, algal cell densities, and chlorophyll-a and chlorophyll-c2 pigments. Contrary to previous findings, heterotrophy did little to mitigate the impacts of temperature, and we observed few significant differences in physiology between fed and unfed corals under heat challenge. Our results suggest the duration and intensity of starvation and thermal challenge play meaningful roles in coral energetics and stress response; future work exploring these thresholds and how they may impact coral responses under changing climate is urgently needed. Gene expression patterns under heat challenge in fed and unfed corals showed gene ontology enrichment patterns consistent with classic signatures of the environmental stress response. While gene expression differences between fed and unfed corals under heat challenge were subtle: Unfed, heated corals uniquely upregulated genes associated with cell cycle functions, an indication that starvation may induce the previously described, milder “type B” coral stress response. Future studies interested in disentangling the influence of heterotrophy on coral bleaching would benefit from leveraging the facultative species studied here, but using the coral in its symbiotic and aposymbiotic states.
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- Award ID(s):
- 1937650
- PAR ID:
- 10493253
- Editor(s):
- Zamudio, Kelly
- Publisher / Repository:
- Oxford University Press
- Date Published:
- Journal Name:
- Journal of Heredity
- Volume:
- 114
- Issue:
- 4
- ISSN:
- 0022-1503
- Page Range / eLocation ID:
- 312 to 325
- Subject(s) / Keyword(s):
- Heat challenge gene expression responses starvation symbiosis cnidarian Oculina arbuscula.
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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- Free Publicly Accessible Full Text
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Accepted Manuscript1.0
- Journal Article:
- https://doi.org/10.1093/jhered/esac068
