Ammonia-oxidizing bacteria are believed to be an
important source of the climatically important trace gas
nitrous oxide (N2O). The primary goals of this
dissertation were to characterize the genes for
enzymes involved in N2O production in nitrifying
bacteria and to establish the groundwork for using
stable isotopes to constrain their contribution to N2O
production in the environment.
The genes for nitrite reductase (nirK) and nitric oxide
reductase (norB) were identified in marine nitrifying
bacteria and were found to be homologous to those in
denitrifying bacteria. This observation suggests that
they shared a common evolutionary origin and may
function similarly in these metabolically distinct
organisms. The use of norB gene sequences as
genetic probes may also allow for distinction between
the roles of nitrifiers and denitrifiers in N2O production
in the environment.
Isotope effects for ammonia oxidation and nitrite
reduction in marine nitrifiers were found to be much
lower than the corresponding isotope effects for the
terrestrial nitrifier Nitrosomonas europaea.
Furthermore, concentration dependence of the isotope
effect for nitrite reduction suggests that differential
substrate availability is a possible explanation for
previously recognized isotopic discrepancies between
culture and field studies of N2O production. More
broadly, these results may be used to assess the effect
of nitrification processes on nitrogen isotopes of
dissolved inorganic nitrogen species (NH4+, NO3- and
N2O) in the ocean.
An intriguing correspondence was discovered between
nitrogen isotope effects for ammonia monooxygenase
and nitrite reductase and the amino acid sequences of
these enzymes. Such a relationship has not been
observed in other systems, and it raises the possibility
of combining genetic information with stable isotope
measurements to study the biogeochemical impact of
specific groups of ammonia-oxidizing bacteria.
Finally, the first method for the oxygen isotopic analysis
of nitrate in seawater was developed, standardized,
and tested as part of this dissertation. This method,
based on the isotopic analysis of N2O, has many
benefits over published freshwater methods, and it
opens up opportunities for new insight into the
interaction of nitrogen cycle processes in the modern
ocean.