Environmental Engineering Reference
In-Depth Information
fixation, including the
nif
-like genes of Archaea and the protochlorophyllide
reductases.
Within the Archaeal methanogens, the nitrogenase genes are homologues of
those found in Bacteria, and there are at least 6
nif
genes (H, D, K, E, N, X)
found in both the Archaea and the Eubacteria. Some Archaeal nitrogenase genes
are
nif
homologs of unknown function [76]. Most of the methanogen nitroge-
nases are Cluster III nitrogenases, and are presumed to be Mo-containing en-
zymes, although vanadium-containing nitrogenases may be present in selected
methanogens [76]. N
2
fixation by methanogens is regulated at the transcrip-
tional and post-transcriptional level by ammonium; however, the mechanism
of this repression appears different than regulation in Proteobacteria [76].
2.3 Regulation of Nitrogen Fixation
Nitrogenase is highly regulated at both transcriptional and post translational
levels. Regulation occurs through cascades of regulatory proteins that activate
transcription, and by post-translational mechanisms that inactivate nitrogenase
through modification of the Fe protein. The mechanisms of regulation differ
among microorganisms, and for many organisms the mechanisms are not well
understood. Nitrogenase regulatory systems are generally assumed to operate
analogously to model organisms such as
Klebsiella
.
Fixation of N
2
is energetically more expensive than utilization of ammonium
(NH
4
+
). N
2
fixation is also sensitive to oxygen (O
2
). As a result, nitrogenase
gene expression appears to be regulated by the presence of NH
4
+
and O
2
at
transcriptional and post-translational levels in many organisms. In
Klebsiella
,
oxygen presence results in binding of a protein (
nifL
) to the
nif
activator
nifA
,
preventing activation of transcription [52]. The mechanism in other bacteria,
such as the purple nonsulfur bacteria [89] is not as well known.
Oxygen and ammonium are two major variables in aquatic environments
[55], and are particularly relevant to the study of N
2
fixation in anoxic envi-
ronments since anoxic basins often accumulate relatively high concentrations
of ammonium. Two important questions are: 1) what are the mechanisms that
regulate N
2
fixation in aerobic waters? and 2) how and why do anaerobic
microorganisms utilize nitrogenase in anoxic waters that are typically charac-
terized by high concentrations of ammonium? The regulation of nitrogenase is
a complex topic [52] and only major factors of relevance to water column N
2
fixation will be highlighted here.
Nitrogenase is rapidly inactivated by oxygen
in vitro
[126], suggesting N
2
fixation is a strictly anaerobic process [37]. The sensitivity of nitrogenase to O
2
is particularly relevant in the oceans because O
2
levels are at or above saturating
concentrations throughout much of the sunlit portion of the upper oceans. As a
