A new abundant nitrite-oxidizing phylum in oligotrophic marine sediments

Nitrite-oxidizing bacteria (NOB) are important nitrifiers whose activity regulates the availability of nitrite and links reduced ammonium and oxidized nitrate in ecosystems. In oxic marine sediments, ammonia-oxidizing archaea (AOA) and NOB together catalyze the oxidation of ammonium to nitrate, but the observed abundance ratios of AOA to canonical NOB are significantly higher than the theoretical ratio predicted from microbial physiology, indicating that many novel NOBs are yet to be discovered. Here we report a new bacterial phylum Candidatus Nitrosediminicolota, members of which are more abundant than canonical NOBs and are widespread across global oligotrophic sediments. Ca. Nitrosediminicolota members have the functional potential to oxidize nitrite, in addition to other accessory functions such as urea hydrolysis and thiosulfate reduction. While one recovered species (Ca. Nitrosediminicola aerophilis) is generally confined within the oxic zone, another (Ca. Nitrosediminicola anaerotolerans) can additionally thrive in anoxic sediments. Counting Ca. Nitrosediminicolota as a nitrite-oxidizer resolves the apparent abundance imbalance between AOA and NOB in oxic marine sediments, and thus its activity may exert a critical control on the nitrite budget.