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Edited by Edward F. DeLong of the University of Hawaii at Manoa, Honolulu, Hawaii, approved on October 11, 2021 (review received on June 11, 2021)

The Hadar Trench represents the deepest part of the ocean and clearly provides a disadvantage for life because of their high hydrostatic pressure and extensive separation from productive surface water. However, the concentration of reactive organic matter in ditch and axis sediments may promote complex but unexplored microbial communities and related biogeochemical processes. We proved that the abyssal sediments with a depth of 10 kilometers are hotbeds for anaerobic nitrogen conversion, and are dominated by anammox processes. The known depth range of this process has been extended by approximately 6 kilometers. The diversity and phylogeny of the identified hadal anammox bacteria indicate that high hydrostatic pressure is not the main evolutionary driving factor. Our findings provide insights into the biogeochemistry of the seafloor and the ability of microorganisms to adapt to hydrostatic pressure.

Benthic N2 produced by microbial denitrification and anammox is the largest fixed nitrogen sink in the ocean. Most N2 production occurs on the continental shelf, where high fluxes of active organic matter consume nitrates near the surface of sediments. In contrast, due to the low input of active organic matter, the formation rate of N2 in deep-sea sediments is low, and the nitrogen conversion is mainly caused by aerobic nitrification and the release of nitrate into the bottom water. Here, we prove that this trend is reversed in the deepest part of the ocean, the trench, where the accumulation of active organic matter enhances the activity of benthic microorganisms. Therefore, at a depth of about 8 kilometers in the Atacama Trench, the underlying productive surface water, nitrates are depleted within a few centimeters of the sediment surface, and the N2 production rate reaches the rate reported by some continental marginal sites, and the loss of fixed nitrogen is mainly through Anammox bacteria. These bacteria are closely related to the known bacteria in the waters of the lowest shallow oxygen zone, and the comparison of laboratory and in-situ measurements shows that they are pressure resistant. Even the Kermadec Trench, which is located below the oligotrophic surface water, exhibits a large amount of fixed N removal. Our research results emphasize the role of deep-sea sediments as hot spots for deep-sea biological activities, reveal a fully functional benthic nitrogen cycle under high hydrostatic pressure, and point out that deep-sea sediments are previously undeveloped anaerobic microbial ecology and diagenetic niches. .

↵1 Current address: Department of Biology, University of Southern Denmark, 5230 Odense M, Denmark.

Author contributions: B. Thamdrup, ML, LM, SA-H., FW and RNG design research; B. Thamdrup, CS, ML, B. Trouche, FW and RNG conducted research; B. Thamdrup, CS, ML, B Trouche, LM, SA-H. and RNG analyzed the data; and B. Thamdrup wrote this paper based on the opinions of CS, ML, B. Trouche, LM, SA-H., FW and RNG

The author declares no competing interests.

This article is directly contributed by PNAS.

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