Nitrogen and sulfur cycling driven by Campylobacterota in the sediment-water interface of deep-sea cold seep: a case in the South China Sea.

Chemoautotrophs within Campylobacterota, especially and , are abundant in the seawater-sediment interface of the Formosa cold seep in the South China Sea. However, the activity and function of Campylobacterota are unknown. In this study, the geochemical role of Campylobacterota in the Formosa cold seep was investigated with multiple means. Two members of and were isolated for the first time from deep-sea cold seep. These isolates are new chemoautotrophic species that can use molecular hydrogen as an energy source and CO as a sole carbon source. Comparative genomics identified an important hydrogen-oxidizing cluster in and . Metatranscriptomic analysis detected high expression of hydrogen-oxidizing gene in the RS, suggesting that H was likely an energy source in the cold seep. Genomic analysis indicated that the and isolates possess a truncated sulfur-oxidizing system, and metatranscriptomic analysis revealed that and with this genotype were active in the surface of RS and likely contributed to thiosulfate production. Furthermore, geochemical and analyses revealed sharply decreased nitrate concentration in the sediment-water interface due to microbial consumption. Consistently, the denitrification genes of and were highly expressed, suggesting an important contribution of these bacteria to nitrogen cycling. Overall, this study demonstrated that Campylobacterota played a significant role in the cycling of nitrogen and sulfur in a deep-sea cold seep. IMPORTANCE Chemoautotrophs within Campylobacterota, in particular and , are ubiquitous in deep-sea cold seeps and hydrothermal vents. However, to date, no or has been isolated from cold seeps, and the ecological roles of these bacteria in cold seeps remain to be investigated. In this study, we obtained two isolates of and from Formosa cold seep, South China Sea. Comparative genomics, metatranscriptomics, geochemical analysis, and experimental study indicated collectively that Campylobacterota played a significant part in nitrogen and sulfur cycling in cold seep and was the cause of thiosulfate accumulation and sharp reduction of nitrate level in the sediment-water interface. The findings of this study promoted our understanding of the function and ecological role of deep-sea Campylobacterota.