Abstract
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.