Environmental Engineering Reference
In-Depth Information
How the availability of fixed inorganic nitrogen, other nutrients, trace metals
and concentration of oxygen interact to regulate N
2
fixation in an ecosystem
context is beyond the scope of this review, and elaborately covered by others [38,
49, 55, 61, 94, 131]. One critical factor to consider is the interaction between Fe
and phosphorus [111]; the nitrogenase requirement for Fe has been suggested
to control phosphorus availability [73, 134]. Often the N:P ratio, rather than
the concentration of N or P individually, is believed to control N
2
fixation [48].
Temperature can determine the distribution of individual diazotrophic species
and has been hypothesized to explain the lack of heterocystous species in the
tropical and subtropical ocean [119].
The ecosystem regulation of N
2
fixation has been studied in aerobic wa-
ter columns overlying anoxic basins, but not within anoxic water columns
themselves. Anoxic basins can promote the solubilization of metals, but can
also promote precipitation, depending upon chemical composition and salinity.
Anoxic conditions can also promote P dissolution and solubilization, which
can stimulate N
2
fixation by decreasing the N:P ratio. Thus, there are complex
ways that anoxia interact with the availability of nutrients and metals needed
for N
2
fixation.
3. EVOLUTION OF NITROGENASE
The Earth's atmosphere has always been dominated by N
2
[39]. However,
ammonium may have been present in the early Earth's atmosphere, and this
has been argued to preclude the need for N
2
fixation [128]. Nonetheless, ni-
trogenase, or some form of pre-nitrogenase genes, could have initially served
a function other than for N
2
fixation (Fig. 2). In addition to cleaving the triple
N
2
bond, nitrogenase also reduces cyanide. This capability could have been
particularly advantageous during periods in the Earth's history when methane
was prevalent in the Earth's atmosphere and cyanide was formed through pho-
tooxidation of methane [62]. However, cyanide can be assimilated by other
pathways and can be used directly as a source of nitrogen. Nonetheless, it is
conceivable that nitrogenase may have initially served as detoxyases and later
used for making atmospheric N
2
biologically available [34, 62, 105].
Ammonium in the early Archean could have been rapidly depleted by the
evolution of living organisms [104]. The composition of the atmosphere was
changing throughout the Archean. The high atmospheric concentrations of
CO
2
and CH
4
declined, and coincident with these changes rates of abiotic
(photochemical and catalyzed by lightning) production of reduced nitrogen
compounds probably decreased [62, 83] (Fig. 2). In addition, autotrophic life
forms evolved that would have depleted the fixed nitrogen reserves [104].
The oceans are believed to have been anoxic, even subsequent to the de-
velopment of oxygenic photosynthesis [1, 19, 100]. Thus, N
2
fixation likely
