Molecular biological and isotopic biogeochemical prognoses of the nitrification-driven dynamic microbial nitrogen cycle in hadopelagic sediments

Summary There has been much progress in understanding the nitrogen cycle in oceanic waters including the recent identification of ammonia‐oxidizing archaea and anaerobic ammonia oxidizing (anammox) bacteria, and in the comprehensive estimation in abundance and activity of these microbial populations. However, compared with the nitrogen cycle in oceanic waters, there are fewer studies concerning the oceanic benthic nitrogen cycle. To further elucidate the dynamic nitrogen cycle in deep‐sea sediments, a sediment core obtained from the Ogasawara Trench at a water depth of 9760 m was analysed in this study. The profiles obtained for the pore‐water chemistry, and nitrogen and oxygen stable isotopic compositions of pore‐water nitrate in the hadopelagic sediments could not be explained by the depth segregation of nitrifiers and nitrate reducers, suggesting the co‐occurrence of nitrification and nitrate reduction in the shallowest nitrate reduction zone. The abundance of SSU rRNA and functional genes related to nitrification and denitrification are consistent with the co‐occurrence of nitrification and nitrate reduction observed in the geochemical analyses. This study presents the first example of cooperation between aerobic and anaerobic nitrogen metabolism in the deep‐sea sedimentary environments.