Symbiodinium functional diversity and clade specificity under global change stressors

Coral bleaching episodes are increasing in frequency, demanding examination of the physiological and molecular responses of corals and their Symbiodinium to climate change. Here we quantify bleaching and Symbiodinium photosynthetic performance of Siderastrea siderea from two reef zones after long-term exposure to thermal and CO2-acidification stress. Molecular response of in hospite Symbiodinium to these stressors was interrogated with RNAseq. Elevated temperatures reduced photosynthetic efficiency, which was highly correlated with bleaching status. However, photosynthetic efficiencies of forereef symbionts were more negatively affected by thermal stress than nearshore symbionts, indicating greater thermal tolerance in nearshore corals. At control temperatures, CO2-acidification had little effect on symbiont physiology, although forereef symbionts exhibited greater photosynthetic efficiencies than nearshore symbionts. Transcriptome profiling revealed that S. siderea were dominated by clade C Symbiodinium, except under thermal stress, which caused shifts to thermotolerant clade D. Comparative transcriptomics of conserved genes across symbiotic partners revealed few differentially expressed Symbiodinium genes when compared to corals. Instead of responding to stress, clade C transcriptomes varied by reef zone, with forereef Symbiodinium exhibiting enrichment of genes associated with photosynthesis. Our findings suggest that functional variation in photosynthetic architecture exists between forereef and nearshore Symbiodinium populations.