Nitrous Oxide Reduction Kinetics Distinguish Bacteria Harboring Clade I NosZ from Those Harboring Clade II NosZ

Abstract

ABSTRACT Bacteria capable of reduction of nitrous oxide (N 2 O) to N 2 separate into clade I and clade II organisms on the basis of nos operon structures and nosZ sequence features. To explore the possible ecological consequences of distinct nos clusters, the growth of bacterial isolates with either clade I ( Pseudomonas stutzeri strain DCP-Ps1, Shewanella loihica strain PV-4) or clade II ( Dechloromonas aromatica strain RCB, Anaeromyxobacter dehalogenans strain 2CP-C) nosZ with N 2 O was examined. Growth curves did not reveal trends distinguishing the clade I and clade II organisms tested; however, the growth yields of clade II organisms exceeded those of clade I organisms by 1.5- to 1.8-fold. Further, whole-cell half-saturation constants ( K s s) for N 2 O distinguished clade I from clade II organisms. The apparent K s values of 0.324 ± 0.078 μM for D. aromatica and 1.34 ± 0.35 μM for A. dehalogenans were significantly lower than the values measured for P. stutzeri (35.5 ± 9.3 μM) and S. loihica (7.07 ± 1.13 μM). Genome sequencing demonstrated that Dechloromonas denitrificans possessed a clade II nosZ gene, and a measured K s of 1.01 ± 0.18 μM for N 2 O was consistent with the values determined for the other clade II organisms tested. These observations provide a plausible mechanistic basis for why the relative activity of bacteria with clade I nos operons compared to that of bacteria with clade II nos operons may control N 2 O emissions and determine a soil's N 2 O sink capacity. IMPORTANCE Anthropogenic activities, in particular fertilizer application for agricultural production, increase N 2 O emissions to the atmosphere. N 2 O is a strong greenhouse gas with ozone destruction potential, and there is concern that nitrogen may become the major driver of climate change. Microbial N 2 O reductase (NosZ) catalyzes N 2 O reduction to environmentally benign dinitrogen gas and represents the major N 2 O sink process. The observation that bacterial groups with clade I nosZ versus those with clade II nosZ exhibit distinct affinities to N 2 O has implications for N 2 O flux models, and these distinct characteristics may provide opportunities to curb N 2 O emissions from relevant soil ecosystems.