Influence of copper on expression of nirS, noRrB and nosZ and the transcription and activity of NIR, NOR and N₂O in the denitrifying soil bacteria Pseudomonas stutzeri

Abstract

Reduction of the potent greenhouse gas nitrous oxide (N₂O) occurs in soil environments by the action of denitrifying bacteria possessing nitrous oxide reductase (N₂OR), a dimeric copper (Cu)-dependent enzyme producing environmentally benign dinitrogen (N₂). We examined the effects of increasing Cu concentrations on the transcription and activity of nitrite reductase (NIR), nitric oxide reductase (NOR) and N₂OR in Pseudomonas stutzeri grown anaerobically in solution over a 10-day period. Gas samples were taken on a daily basis and after 6 days, bacterial RNA was recovered to determine the expression of nirS, norB and nosZ encoding NIR, NOR and N₂OR respectively. Results revealed that 0.05 mM Cu caused maximum conversion of N₂O to N₂ via bacterial reduction of N₂O. As soluble Cu generally makes up less than 0.001% of total soil Cu, extrapolation of 0.05 mg l-l soluble Cu would require soils to have a total concentration of Cu in the range of, 150-200 μg g-1 to maximize the proportion of N₂O reduced to N₂. Given that many intensively farmed agricultural soils are deficient in Cu in terms of plant nutrition, providing a sufficient concentration of biologically accessible Cu could provide a potentially useful microbial-based strategy of reducing agricultural N₂O emissions.