Identification of nosZ-expressing microorganisms consuming trace N2O in microaerobic chemostat consortia dominated by an uncultured Burkholderiales

Abstract

Microorganisms possessing N2O reductases (NosZ) are the only known environmental sink of N2O. While oxygen inhibition of NosZ activity is widely known, environments where N2O reduction occurs are often not devoid of O2. However, little is known regarding N2O 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 N2O, and the resulting microbial consortia were analyzed via quantitative PCR (qPCR) and metagenomic/metatranscriptomic analyses. Unintended but quantified intrusion of O2 sustained dissolved oxygen concentration above 4 µM; however, complete N2O reduction of influent N2O 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 N2O 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 N2O. Further, co-expression of nosZ and ccoNO/cydAB genes found in the metagenome-assembled genomes representing these putative N2O-reducers implies a survival strategy to maximize utilization of scarcely available electron acceptors in microoxic environmental niches.