The impact of copper, nitrate and carbon status on the emission of nitrous oxide by two species of bacteria with biochemically distinct denitrification pathways

Denitrifying bacteria convert nitrate (NO3-) to dinitrogen (N2) gas through an anaerobic respiratory process in which the potent greenhouse gas nitrous oxide (N2O) is a free intermediate. These bacteria can be grouped into classes that synthesize a nitrite (NO2-) reductase (Nir) that is solely dependent on haem-iron as a cofactor (e.g. Paracoccus denitrificans) or a Nir that is solely dependent on copper (Cu) as a cofactor (e.g. Achromobacter xylosoxidans). Regardless of which form of Nir these groups synthesize, they are both dependent on a Cu-containing nitrous oxide reductase (NosZ) for the conversion of N2O to N2. Agriculture makes a major contribution to N2O release and it is recognized that a number of agricultural lands are becoming Cu-limited but are N-rich because of fertilizer addition. Here we utilize continuous cultures to explore the denitrification phenotypes of P. denitrificans and A. xylosoxidans at a range of extracellular NO3-, organic carbon and Cu concentrations. Quite distinct phenotypes are observed between the two species. Notably, P. denitrificans emits approximately 40% of NO3- consumed as N2O under NO3--rich Cu-deficient conditions, while under the same conditions A. xylosoxidans releases approximately 40% of the NO3- consumed as NO2-. However, the denitrification phenotypes are very similar under NO3--limited conditions where denitrification intermediates do not accumulate significantly. The results have potential implications for understanding denitrification flux in a range of agricultural environments.