Life on NO: deciphering the ecophysiology of NO respiring bacterial communities in a continuous culture.

Reduction of the greenhouse gas NO to N is a trait among denitrifying and non-denitrifying microorganisms having an NO reductase, encoded by nosZ. The nosZ phylogeny has two major clades, I and II, and physiological differences among organisms within the clades may affect NO emissions from ecosystems. To increase our understanding of the ecophysiology of NO reducers, we determined the thermodynamic growth efficiency of NO reduction and the selection of NO reducers under NO- or acetate-limiting conditions in a continuous culture enriched from a natural community with NO as electron acceptor and acetate as electron donor. The biomass yields were higher during NO 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 NO. Denitrifiers were favored over non-denitrifying NO 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 NO, as defined by μ/K. Thus, the specific growth rate is likely a key factor determining the composition of communities living on NO respiration under growth-limited conditions.