Environmental Engineering Reference
In-Depth Information
result, O
2
may be an important factor limiting N
2
fixation in the modern, oxic
ocean.
Despite the apparent sensitivity of nitrogenase to O
2
, there is substantial
variability in oxygen sensitivity
in vivo
[37, 50]. A number of aerobic microor-
ganisms, including heterotrophic bacteria and O
2
evolving cyanobacteria are
capable of
in vivo
aerobic N
2
fixation. It appears that it is not oxygen itself that
is toxic to nitrogenase, and that oxygen inactivation does not occur at the FeS
centers [37, 126]. Thorneley and Ashby [126] demonstrated that Component
II of nitrogenase actually reduces oxygen without inactivation, and is only in-
activated by the products of oxygen reduction, such as superoxide or hydrogen
peroxide. Thus, Component II of nitrogenase may serve as an “autoprotection”
mechanism against inactivation by oxygen for nitrogenase, reducing oxygen to
water, as long as the concentration of Component II exceeds the concentration
of oxygen by about fourfold [126].
N
2
-fixing microorganisms have evolved several ways of avoiding oxygen in-
activation of nitrogenase. Aerobic heterotrophic bacteria, such as
Azotobacter,
utilize several mechanisms to help balance oxygen requirements with oxygen
sensitivity. Such mechanisms include the production of polysaccharides, main-
taining relatively high respiration rates and thus low oxygen concentrations,
and production of the Shethna or FeSII protein that appears to provide con-
formational protection to nitrogenase [50]. Some facultative microorganisms
only express nitrogenase under anaerobic or microaerophilic conditions, con-
trolled by a regulatory network involving
nifL
and
nifA
[52]. Most cyanobacteria
protect N
2
fixation by the oxygen-sensitive nitrogenase from oxygen evolved
through photosynthesis by temporal and spatial mechanisms [6, 7].
In at least some species of nonsulfur purple bacteria, including
Rhodospir-
illum
,N
2
fixation is regulated in response to light, oxygen and fixed nitrogen
availability. These microorganisms grow photoheterotrophically under anaero-
bic conditions. A reversible post-translational mechanism for regulating nitro-
genase activity has been well characterized in some microorganisms involving
enzymatic ADP-ribosylation of the Fe protein [107]. The modification and
demodification of nitrogenase are catalyzed by specific enzymes (DRAT and
DRAG) that respond to ammonium, oxygen, and light [77]. This mechanism is
present in other microorganisms as well. A shift in apparent molecular weight
of the Fe protein of nitrogenase (which is indicative of the ADP ribosyla-
tion) has been observed in cyanobacteria, [90, 137], but the ADP-ribosylation
mechanism itself has never been demonstrated in cyanobacteria [37].
The N
2
fixation proteins and genes of strict anaerobes, such as clostridia and
sulfate reducers, have been characterized to some extent, but little is known
about their regulation due to the lack of model genetic systems. N
2
fixation in
sulfate reducing bacteria also appears regulated similar to other microbes in
response to oxygen or ammonium [99]. In present anoxic environments (sedi-
