Anaerobic protists are diverse, ecologically important members of anoxic microbial communities, acting as grazers, nutrient cyclers, and partners in multi-domain associations, yet remain understudied ...

Anaerobic protists are diverse, ecologically important members of anoxic microbial communities, acting as grazers, nutrient cyclers, and partners in multi-domain associations, yet remain understudied ...

Background Corals are known for their symbiotic relationships, yet there is limited evidence of chemoautotrophic associations. This is despite some corals occurring near cold seeps where chemosymbiotic fauna abound including mussels that host sulfur-oxidizing chemoautotrophs from the SUP05 cluster (family Ca. Thioglobaceae). We investigated whether corals near cold seeps associate with related bacteria and report here that these associations are widespread. Results We screened corals, water, and sediment for Thioglobaceae using 16S metabarcoding and found ASVs associated with corals at high relative abundance (10 – 91%). These ASVs were specific to coral hosts, absent in water samples, and rare or absent in sediment samples. Using metagenomics and transcriptomics, we assembled the genome of one phylotype associated with Paramuricea sp. B3 (ASV 4) which contained the genetic potential to oxidize sulfur and fix carbon, and confirmed that these pathways were transcriptionally active. Furthermore, its relative abundance was negatively correlated with the stable isotopic composition of its host coral’s tissue suggesting some contribution of chemoautotrophy to the coral holobiont. Conclusions We propose that some lineages of Thioglobaceae may facultatively supplement the diet of their host corals through chemoautotrophy at seeps or may provide essential amino acids or vitamins. This is the first documented association between chemoautotrophic symbionts and corals at seeps and suggests that the footprint of chemosynthetic environments is wider than currently understood.