Cell density dependent restriction of N2O reductase in denitrifying Pseudomonas spp., a potential driver of N2O emissions in natural systems.

Denitrification is a major biological source and sink for the ozone-depleting greenhouse gas N2. Thus, the respiratory physiology of denitrifiers and the mechanisms determining their propensity for accumulation of N-oxides are of fundamental interest. Here, we report a pervasive positive correlation between cell density and N2O accumulation in Pseudomonas aeruginosa and P. fluorescens F113. We show that this was a result of quorum sensing by comparing the P. aeruginosa PAO1-UW wild type to a rhlI/lasI knockout mutant able to sense, but not synthesize the N-acyl-homoserine lactones (AHL) of the Rhl and Las circuits. Neither the transcription of nosZ (encoding N2O reductase, N2OR) nor the abundance of peptides of known relevance to denitrification could explain the restriction of N2O reduction in AHL-affected cultures. However, a protein shown to be involved in synthesis and repair of iron-sulphur (Fe-S) centers under NO stress, CyaY, was significantly downregulated in the AHL producing wild type. This hints to a possible route of N2OR-suppression via compromised Fe-S centers in the ancillary protein NosR. While the exact mechanism remains obscure, it appears that quorum sensing driven restriction of N2OR activity is common. Thus, given its ubiquity among prokaryotes, and the potential for cross-species and -strain effects, quorum sensing is plausibly a driver of N2O emissions in a range of systems.