Environmental Engineering Reference
In-Depth Information
ments, microbial mats, and water columns), there are generally relatively high
concentrations of ammonium, potentially obviating the need for N 2 fixation.
However, N 2 fixation (or acetylene reduction) has been detected despite mi-
cromolar concentrations of porewater ammonium in salt marsh sediments [5,
97]. Despite the potential repression of nitrogenase by NH 4 + , studies of extant
microbial mats indicate that many microorganisms express nitrogenase genes
in anoxic regions of the mats where NH 4 + is present. Similarly, N 2 fixation has
been reported in sediments, where ammonium is usually found at high con-
centrations. The presence of N 2 fixation genes in microorganisms that thrive in
anaerobic environments, including sulfate reducers and methanogens suggests
that either anaerobic microorganisms benefit (or have benefited in the past)
from N 2 fixation, or that the N 2 fixation apparatus serves (or has served in the
past) another purpose.
Nitrogenase expression has been demonstrated in non sulfur purple bacteria
grown photoheterotrophically in the presence of ammonium when alternate
electron sinks are unavailable (the Calvin-Benson-Basham pathway or DMSO)
[127]. In this case, nitrogenase acts as an electron sink under specific physio-
logical conditions. It's unknown how widespread the use of nitrogenase as an
electron sink may be in other anaerobes, but it could provide clues as to the
original function of the protein. However, the conditions under which this has
been demonstrated are specific to photoheterotrophic growth under anaerobic
conditions.
In contrast to ammonium, the availability of nitrate does not appear to
completely repress nitrogenase synthesis or activity [143]. Nitrate assimilation
is energetically costly and some cyanobacteria do not appear to regulate N 2
fixation in the presence of nitrate [35]. For example, Nodularia appears to
continue to fix N 2 even when nitrate is present, although these cyanobacteria
may lack the ability to reduce nitrate [112]. Thus, under some circumstances,
N 2 fixation can proceed in the presence of nitrate [61].
Nitrogen fixation is dependent upon the availability of metals (Mo or V, and
Fe) for active enzyme synthesis. The availability of these metals partly depends
on oxygen and pH. The expression of the conventional and alternative nitroge-
nases appears regulated in response to the availability of Mo and V [10]. In the
modern day oxygenated ocean, Fe availability is low (typically in the picomolar
to nanomolar range) due to the low solubility of the Fe (Fe 3 + ) hydroxides. Mo
concentrations (in the form of MoO 4 ) are relatively high (typically nanomolar
to micromolar range) but Mo availability has been hypothesized to limit N 2
fixation due to competitive uptake of sulfate (an analogue of molybdate) [54].
Possible limitation of N 2 fixation due to competition between Mo and sulfate
uptake remains unresolved and evidence to the contrary has been provided
[96].
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