Photosymbiosis reduces the environmental stress response under a heat challenge in a facultatively symbiotic coral

The symbiosis between corals of the order Scleractinia 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 coral Astrangia poculata, which offers an opportunity to uncouple the ESR across its two symbiotic states (symbiotic, aposymbiotic). Colonies of both symbiotic states were exposed to three temperature treatments for 15 days: i) control (static 18{degrees}C), ii) heat challenge (increasing from 18 to 32{degrees}C), and iii) cold challenge (decreasing from 18 to 6{degrees}C) after which host gene expression was profiled. Cold challenged corals elicited widespread differential expression, however, there were no differences between symbiotic states. In contrast, symbiotic colonies exhibited greater gene expression plasticity under heat challenge, including enrichment of cell cycle pathways involved in controlling photobiont growth. Counter to the oxidative bleaching hypothesis, this plasticity did not include signatures of stress, and rather a dampened ESR under heat challenge was observed, suggesting that photobionts reduce the hosts ESR under elevated temperatures in A. poculata.