Assessing the contribution of bacteria to the heat tolerance of experimentally evolved coral photosymbionts

Abstract Coral reefs are extremely vulnerable to ocean warming, which triggers coral bleaching—the loss of endosymbiotic microalgae (Symbiodiniaceae) from coral tissues, often leading to death. To enhance coral climate resilience, the symbiont, Cladocopium proliferum was experimentally evolved for >10 years under elevated temperatures resulting in increased heat tolerance. Bacterial 16S rRNA gene metabarcoding showed the composition of intra‐ and extracellular bacterial communities of heat‐evolved strains was significantly different from that of wild‐type strains, suggesting bacteria responded to elevated temperatures, and may even play a role in C. proliferum thermal tolerance. To assess whether microbiome transplantation could enhance heat tolerance of the sensitive wild‐type C. proliferum , we transplanted bacterial communities from heat‐evolved to the wild‐type strain and subjected it to acute heat stress. Microbiome transplantation resulted in the incorporation of only 30 low‐abundance strains into the microbiome of wild‐type cultures, while the relative abundance of 14 pre‐existing strains doubled in inoculated versus uninoculated samples. Inoculation with either wild‐type or heat‐evolved bacterial communities boosted C. proliferum growth, although no difference in heat tolerance was observed between the two inoculation treatments. This study provides evidence that Symbiodiniaceae‐associated bacterial communities respond to heat selection and may contribute to coral adaptation to climate change.