Unraveling the impact of marine heatwaves on the Eukaryome of the emblematic Mediterranean red coral Corallium rubrum.

Global warming is intensifying heatwaves worldwide, leading to more frequent and severe temperature extremes. This study investigates the impact of the unprecedented 2022 Mediterranean heatwaves on the coral eukaryome, which has received little attention despite its known importance to coral holobiont functioning. Fifty-six colonies of the iconic red coral Corallium rubrum from the Mediterranean Sea were collected at different sites, depths, and health states. The microeukaryotic communities were analyzed using an 18S rRNA gene metabarcoding approach. Primers were designed to reduce amplification of the 18S rRNA gene sequences of the red coral while being universal for amplification of microeukaryotes. Our results showed that the red coral eukaryome was dominated by Dino-Group I, Licnophoridae, and Labyrinthulomycetes in the control sites that were not affected by the heat waves. In the heat-affected colonies, the composition of the coral eukaryome changed, with the relative abundances of Ephelotidae, Exobasidiomycetes, Corallicolidae, Labyrinthulomycetes, and/or the epibionts Phaeophyceae increasing depending on the intensity of heat stress experienced by the colonies. It was thus possible to link colony health to changes in the eukaryome. Finally, we illustrated putative interactions (competition, predator-prey relationship, and parasitism) occurring within C. rubrum eukaryome that could explain the compositional changes observed in the microeukaryotic communities under heat stress. Our findings improve our understanding of the ecological effects of heatwaves on marine ecosystems.