Abstract
Reduction of the greenhouse gas N(2)O to N(2) is a trait among denitrifying and non-denitrifying microorganisms having an N(2)O reductase, encoded by nosZ. The nosZ phylogeny has two major clades, I and II, and physiological differences among organisms within the clades may affect N(2)O emissions from ecosystems. To increase our understanding of the ecophysiology of N(2)O reducers, we determined the thermodynamic growth efficiency of N(2)O reduction and the selection of N(2)O reducers under N(2)O- or acetate-limiting conditions in a continuous culture enriched from a natural community with N(2)O as electron acceptor and acetate as electron donor. The biomass yields were higher during N(2)O limitation, irrespective of dilution rate and community composition. The former was corroborated in a continuous culture of Pseudomonas stutzeri and was potentially due to cytotoxic effects of surplus N(2)O. Denitrifiers were favored over non-denitrifying N(2)O reducers under all conditions and Proteobacteria harboring clade I nosZ dominated. The abundance of nosZ clade II increased when allowing for lower growth rates, but bacteria with nosZ clade I had a higher affinity for N(2)O, as defined by μ(max)/K(s). Thus, the specific growth rate is likely a key factor determining the composition of communities living on N(2)O respiration under growth-limited conditions.