Research Foci
Physiology and Genetics of Nitrifying Bacteria
Environmental Controls on N2O Production by Nitrifying Bacteria
Nitrifying Bacteria and d18O signatures of NO3- in the Ocean
Isotopic Signatures as Indicators of Nitrification & Denitrification
Sources of N2O in Past Ocean Ecosystems
Co-Evolution of Organisms and Environment

How is N2O production by nitrifying bacteria controlled in response to changes in the environment?

Both nitrifying and denitrifying bacteria appear to alter the amounts of N2O that they produce in response to environmental conditions, particularly oxygen concentrations. Enhanced production of N2O by nitrifying bacteria at low O2 concentrations has been well documented, but whether the enhanced production is due to an increase in the production by a single pathway or the induction of a second pathway has not yet been answered. With the identification of nitrite reductase (Casciotti and Ward, 2001) and nitric oxide reductase (Casciotti and Ward, in prep) genes in nitrifying bacteria, potential changes in the role of the "denitrification" pathway in N2O production by nitrifiers can be more readily studied in cultures and in field assemblages by detection of expression of these genes in relation to N2O production. Understanding the role of norB in N2O production by nitrifiers will also be important for developing a genetic tool for distinguishing nitrifier from denitrifier N2O production based on the distribution of norB genes.

Changes in the pathway or mechanism of N2O production may also be connected to changes in the isotopic signature of N2O produced by nitrifiers. The measurement of N2O isotopes from nitrifying bacteria grown under different conditions could thus be important in two ways: first, in assessing the relative importance of either pathway in N2O production, and second in gaining a better understanding of the N2O isotope signatures from nitrifying bacteria.

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