Metagenomic analysis reveals a green sulfur bacterium as a potential coral symbiont

Coral reefs are ecologically significant habitats. Coral-algal symbiosis confers ecological success on coral reefs and coral-microbial symbiosis is also vital to coral reefs. However, current understanding of coral-microbial symbiosis on a genomic scale is largely unknown. Here we report a potential microbial symbiont in corals revealed by metagenomics-based genomic study. Microbial cells in coral were enriched for metagenomic analysis and a high-quality draft genome of “ Candidatus Prosthecochloris korallensis” was recovered by metagenome assembly and genome binning. Phylogenetic analysis shows “ Ca . P. korallensis” belongs to the Prosthecochloris clade and is clustered with two Prosthecochloris clones derived from Caribbean corals. Genomic analysis reveals “ Ca . P. korallensis” has potentially important ecological functions including anoxygenic photosynthesis, carbon fixation via the reductive tricarboxylic acid (rTCA) cycle, nitrogen fixation, and sulfur oxidization. Core metabolic pathway analysis suggests “ Ca . P. korallensis” is a green sulfur bacterium capable of photoautotrophy or mixotrophy. Potential host-microbial interaction reveals a symbiotic relationship: “ Ca . P. korallensis” might provide organic and nitrogenous nutrients to its host and detoxify sulfide for the host; the host might provide “ Ca . P. korallensis” with an anaerobic environment for survival, carbon dioxide and acetate for growth, and hydrogen sulfide as an electron donor for photosynthesis.