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

Abstract

Summary Denitrifying bacteria convert nitrate (NO 3 ‐ ) to dinitrogen (N 2 ) gas through an anaerobic respiratory process in which the potent greenhouse gas nitrous oxide (N 2 O) is a free intermediate. These bacteria can be grouped into classes that synthesize a nitrite (NO 2 ‐ ) 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 N 2 O to N 2 . Agriculture makes a major contribution to N 2 O 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 NO 3 ‐ , organic carbon and Cu concentrations. Quite distinct phenotypes are observed between the two species. Notably, P. denitrificans emits approximately 40% of NO 3 ‐ consumed as N 2 O under NO 3 ‐ ‐rich Cu‐deficient conditions, while under the same conditions A. xylosoxidans releases approximately 40% of the NO 3 ‐ consumed as NO 2 ‐ . However, the denitrification phenotypes are very similar under NO 3 ‐ ‐limited conditions where denitrification intermediates do not accumulate significantly. The results have potential implications for understanding denitrification flux in a range of agricultural environments.

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