Identification of nosZ-expressing microorganisms consuming trace N 2 O in microaerobic chemostat consortia dominated by an uncultured Burkholderiales.

Microorganisms possessing N 2 O reductases (NosZ) are the only known environmental sink of N 2 O. While oxygen inhibition of NosZ activity is widely known, environments where N 2 O reduction occurs are often not devoid of O 2 . However, little is known regarding N 2 O reduction in microoxic systems. Here, 1.6-L chemostat cultures inoculated with activated sludge samples were sustained for ca. 100 days with low concentration (<2 ppmv) and feed rate (<1.44 µmoles h -1 ) of N 2 O, and the resulting microbial consortia were analyzed via quantitative PCR (qPCR) and metagenomic/metatranscriptomic analyses. Unintended but quantified intrusion of O 2 sustained dissolved oxygen concentration above 4 µM; however, complete N 2 O reduction of influent N 2 O persisted throughout incubation. Metagenomic investigations indicated that the microbiomes were dominated by an uncultured taxon affiliated to Burkholderiales, and, along with the qPCR results, suggested coexistence of clade I and II N 2 O reducers. Contrastingly, metatranscriptomic nosZ pools were dominated by the Dechloromonas-like nosZ subclade, suggesting the importance of the microorganisms possessing this nosZ subclade in reduction of trace N 2 O. Further, co-expression of nosZ and ccoNO/cydAB genes found in the metagenome-assembled genomes representing these putative N 2 O-reducers implies a survival strategy to maximize utilization of scarcely available electron acceptors in microoxic environmental niches.