A Single-cell Atlas of Coral Bleaching

The symbiosis between corals and dinoflagellate algae is disrupted by heat stress, leading to bleaching. Recurring bleaching events, driven by the climate crisis, are causing massive coral mortality and threatening reefs worldwide. Despite its planetary scale impact, bleaching occurs at the cellular level. While much is known about the physiological and genomic responses of corals to bleaching, our understanding of it at the cellular level remains limited. Using single-cell transcriptomics, we show that individual coral and symbiont cells respond differently to thermal stress. In heat stress experiments with the coral Orbicella faveolata, hosting two co-dominant algal symbionts, we found that calicoblastic cells and cnidocytes were particularly susceptible, with nitrogen metabolism disrupted across all cell types. We also provide the first evidence of differing transcriptomic responses between symbionts with varying heat tolerances within the same coral host. Furthermore, we discovered that symbionts suppress the expression of heat stress related genes of their host cells. These findings reveal that coral cellular responses to bleaching are more complex and varied than previously thought. This study marks a starting point toward understanding the cellular dynamics of coral holobionts, shedding light on the mechanisms behind symbiosis breakdown, coral mortality, and ultimately, reef decline.