Apicomplexans predict thermal stress mortality in the Mediterranean coral Paramuricea clavata

Paramuricea clavata is an azooxanthellate octocoral currently threatened by episodic mass mortality events related to global warming. The microbiome may play an active role in the thermal stress susceptibility of corals, potentially holding the answer as to why corals show differential sensitivity to heat-stress. The microbiome of P. clavata collected from around the Mediterranean was characterized right before experimental heat-stress using the 16S and 18S small subunits (SSU) of the rRNA gene to determine if it influences the thermal response of the holobiont. P. clavata’s microeukaryotic community was significantly correlated with thermal stress sensitivity, while bacterial assemblages were not. Syndiniales dinoflagellates Group I Clade 1 were significantly enriched in thermally resistant corals, while the apicomplexan Corallicolida were significantly enriched in thermally susceptible corals. Our results show the importance of the microeukaryotic communities to understanding thermal stress response in corals and provides a useful tool to guide conservation efforts.