Life on N₂O: deciphering the ecophysiology of N₂O respiring bacterial communities in a continuous culture

Reduction of the greenhouse gas N₂O to N₂ is a trait among denitrifying and non-denitrifying microorganisms having an N₂O reductase, encoded by nosZ. The nosZ phylogeny has two major clades, I and II, and physiological differences among organisms within the clades may affect N₂O emissions from ecosystems. To increase our understanding of the ecophysiology of N₂O reducers, we determined the thermodynamic growth efficiency of N₂O reduction and the selection of N₂O reducers under N₂O- or acetate-limiting conditions in a continuous culture enriched from a natural community with N₂O as electron acceptor and acetate as electron donor. The biomass yields were higher during N₂O 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 N₂O. Denitrifiers were favored over non-denitrifying N₂O 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 N₂O, as defined by μ_max/K_s. Thus, the specific growth rate is likely a key factor determining the composition of communities living on N₂O respiration under growth-limited conditions.