How does the production of N₂O fit into the normal physiology and genetics of nitrifying bacteria?

Genetic investigation has revealed the presence of nitrite reductase (nirK) genes (Casciotti and Ward, 2001) and nitric oxide reductase (norB) genes (Casciotti and Ward, in prep) in AOB. The similarities of nitrifier nirK and norB genes to those found in denitrifying bacteria suggest that these bacteria share a common pathway for N₂O production. The similarity of nirK (and norB) among nitrifiers and denitrifiers also suggests shared ancestry for these enzymes. These findings are surprising given the significant metabolic differences between nitrifying and denitrifying bacteria, and they raise fundamental questions about the origin and the function of nirK and norB in nitrifying bacteria.

The existence of nirK and norB in nitrifiers also raises questions about how this classically anaerobic pathway fits into their physiology. In denitrifying bacteria, nitrite reductase offers an alternative means for energy generation in the absence of oxygen, and the nirK gene is induced in response to low oxygen supply (Korner and Zumft, 1989). Nitrifying bacteria have a wide range of oxygen tolerance but require at least low levels of O₂ for growth. Still, O₂ may be an important factor in directing the expression of genes in AOB and has been shown to dramatically affect the yield of N₂O in AOB. Investigating the regulation of this pathway in nitrifying bacteria (in cultures and in the field) will be crucial for elucidating controls on N₂O production by nitrifying bacteria and understanding how this source may respond to environmental change.

Independent findings support a second pathway for N₂O production in nitrifying bacteria, indicating that the primary mechanism for N₂O production by nitrifiers is still uncertain and requires further investigation. Outlining the involvement of different pathways is important for understanding how N₂O production fits into nitrifier metabolism and for interpreting the isotopic signatures of N₂O released from nitrifying bacteria. To accomplish this we need to identify genes and enzymes involved in N₂O production and design experiments to test the conditions under which these enzymes are induced. In addition to studying the expression of individual genes, one could use the full genome sequence of Nitrosomonas europaea (a terrestrial nitrifier) to examine the entire suite of enzymes whose expression change under changes in O₂. Completion of additional pending nitrifier genome sequences will enable the extension of these investigations to diverse nitrifier species and will allow targeted genetic investigation of these organisms' physiology and regulation of N₂O production.

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