# Deep-sea corals near cold seeps associate with sulfur-oxidizing chemoautotrophs in the family Ca. Thioglobaceae

## TL;DR

Some deep-sea corals near cold seeps associate with sulfur-oxidizing bacteria, suggesting a new type of symbiotic relationship.

## Contribution

First evidence of chemoautotrophic Thioglobaceae bacteria associated with deep-sea corals near cold seeps.

## Key findings

- Thioglobaceae ASVs were found at high abundance in corals but not in surrounding water or sediment.
- A coral-associated Thioglobaceae genome showed genetic potential for sulfur oxidation and carbon fixation.
- The bacteria's abundance was negatively correlated with coral tissue isotopic composition, suggesting nutritional contribution.

## Abstract

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.

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.

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.

The online version contains supplementary material available at 10.1186/s40168-025-02254-z.