Unexpected Diversity and High Abundance of Putative Nitric Oxide Dismutase (Nod) Genes in Contaminated Aquifers and Wastewater Treatment Systems

The oxygenic dismutation of NO into N 2 and O 2 has recently been suggested for the anaerobic methanotrophic Candidatus Methylomirabilis oxyfera and the alkane-oxidizing gammaproteobacterium HdN1. It represents a new pathway in microbial nitrogen cycling and is catalyzed by a putative NO dismutase (Nod). The formed O 2 enables microbes to employ aerobic catabolic pathways in anoxic habitats, suggesting an ecophysiological niche space of substantial appeal for bioremediation and water treatment. However, it is still unknown whether this physiology is limited to M. oxyfera and HdN1, and whether it can be coupled to the oxidation of electron donors other than alkanes. Here, we report first insights into an unexpected diversity and remarkable abundance of nod genes in natural and engineered water systems. Phylogenetically diverse nod genes were recovered from a range of contaminated aquifers and N-removing wastewater treatment systems. Together with nod genes from M. oxyfera and HdN1, the novel environmental nod sequences formed no less than 6 well-supported phylogenetic clusters, clearly distinct from canonical NO-reductase (qNor and cNor) genes. The abundance of nod genes in the investigated samples ranged from 1.6 * 10 7 to 5.2 * 10 10 copies g -1 wet sediment or sludge biomass, accounting for up to 10% of total bacterial 16S rRNA gene counts. In essence, NO dismutation could be a much more widespread physiology than currently perceived. Understanding the controls of this emergent microbial capacity could offer new routes for nitrogen elimination or pollutant remediation in natural and engineered water systems. Importance statement NO dismutation into N 2 and O 2 is a novel process, catalyzed by putative NO dismutase (Nod). To date only two bacteria, the anaerobic methane oxidizing bacterium Methylomirabilis oxyfera and the alkane-oxidizing gammaproteobacterium HdN1, are known to harbor nod genes. In this study, we report the first efficient molecular tools that can detect and quantify a wide diversity of nod genes in environmental samples. A surprising high diversity and abundance of nod genes was found in contaminated aquifers as well as wastewater treatment systems. This first evidence indicates that NO dismutation may be a much more widespread physiology in natural and man-made environments than currently perceived. The molecular tools presented here will facilitate further studies on these enigmatic microbes in the future.