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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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