Phylogeny of nitrous oxide reducers in wastewater treatment
Nitrous oxide (N2O) is the third most important greenhouse gas, with the only known biological sink being the last step in the denitrification process with the enzyme-catalysed reduction by nitrous-oxide reductase (nosZ). However, the phylogeny of nosZ and the physiology of the organisms harbouring this gene are largely undescribed, and large novelty still remains. In this study, 1083 published high-quality metagenome-assembled genomes (HQ MAGs) from Danish wastewater treatment plants (WWTP) were characterised with regard to the nosZ-containing populations. A significant part of the community harboured a nosZ gene (48.7%), and the clade specificity of said organisms revealed that the primary nosZ clade I-containing organisms were affiliated with Proteobacteria, Chloroflexi, Latescibacterota, and Myxococcota, while the clade II-containing organisms were predominantly Bacteroidota and other less frequently occurring phyla such as Gemmatimonadota, Acidobacteriota, and Spirochaetota. Within both clades, specialised non-denitrifying N2O-reducers were identified, and their genetic repertoire highlights a co-tendency of these specialised organisms also to contain DNRA genes in addition to the nosZ gene. This study provides an insight into and knowledge of the organisms involved in the reduction of N2O and their potential role as N2O sinks in full scale WWTPs.