Hydrogen-dependent dissimilatory nitrate reduction to ammonium enables growth ofCampylobacterotaisolates

ABSTRACT Dissimilatory nitrate reduction to ammonium (DNRA) is a key process in global nitrogen cycling, supporting the energy conservation of diverse microbes. For a long time, DNRA has been thought to primarily depend on organic electron donors, and thus to be governed by carbon-to-nitrogen (C:N) ratios. However, recent studies suggest that inorganic electron donors, such as sulfur compounds and iron, may also facilitate DNRA. The coupling of DNRA with molecular hydrogen (H 2 ) oxidation is theoretically feasible, but largely unexplored microbial process. Here, we report the isolation of two Campylobacterota strains, Aliarcobacter butzleri hDNRA1 and Sulfurospirillum sp. hDNRA2, that grow by using H 2 as an electron donor for DNRA. In both batch and continuous cultures, DNRA sensu stricto , i.e., NO 2− -to-NH + reduction, depended on the presence of H 2 and was stoichiometric with H 2 oxidation. The electrons for NO − reduction were clearly derived from H, and hydrogenotrophic DNRA was largely unaffected by the ratio of either carbon or electron donor to NO − /NO − . Genomic and transcriptomic analyses indicate that group 1b [NiFe]-hydrogenase and cytochrome c 552 nitrite reductase are the key enzymes catalyzing hydrogenotrophic DNRA. These findings reveal novel physiological mechanisms enabling anaerobic bacterial growth, challenge the traditional C:N ratio paradigm, and uncover new biogeochemical processes and mediators controlling the global nitrogen and hydrogen cycles.