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

The symbiosis between corals of the order Scleractinia and dinoflagellates of the family Symbiodiniaceae is sensitive to environmental stress. Warmer temperatures can lead to an accumulation of photobiont-derived reactive oxygen species (ROS), ultimately exacerbating the coral environmental stress response (ESR). To understand how photosymbiosis modulates the coral's ESR, these responses must be explored in hosts in and out of symbiosis. We therefore leveraged the facultatively symbiotic coral Astrangia poculata, which offers an opportunity to uncouple the ESR across its two symbiotic states (symbiotic, aposymbiotic). Each symbiotic state was exposed to three temperature treatments: i) control (static 18C), ii) heat challenge (increasing from 18 to 32C over 15 days), and iii) cold challenge (decreasing from 18 to 6C over 15 days) after which coral 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 plasticity of gene expression when heat challenged, which included upregulation of cell cycle pathways involved in controlling photobiont growth. This plasticity did not include signatures of amplified stress, and rather exhibited a dampened ESR in heat challenge, suggesting that photobionts lessen the host's ESR under elevated temperatures in A. poculata.