Environmental Engineering Reference
In-Depth Information
is a substantial supply of ammonium to the photic zone from the anoxic deeper
waters.
It appears that N
2
fixation may occur in anoxic zones even if fixed nitrogen is
present, although at low rates. It is possible that that this low level of N
2
fixation
is performed by a small group of microorganisms or is somehow involved in
minor redox reactions as an electron sink. N
2
fixation could occur in anoxic
water columns for similar reasons.
7. CONCLUDING REMARKS
The importance of N
2
fixation in providing fixed nitrogen for the world's
oceans is now clear, but the process is not well-understood: previously un-
recognized diazotrophs are still being discovered, and how the availability of
nutrients and trace metals control N
2
fixation and primary productivity is still
largely unknown.
N
2
fixation evolved
under
anoxic
conditions, although the evolution
of N
2
Anoxic marine environments represent conditions where evolutionarily ancient
biogeochemical processes and microbial communities are found. Since nitro-
genase evolved under anoxic conditions, it may also be expressed in anoxic
water columns. There simply is too little data on the presence and expression of
nitrogenase genes in anoxic water columns to know how important of a process
it is. It may, for example, play a role in localized regions at the interface between
oxic and anoxic regions.
Given the concentrations of fixed inorganic nitrogen in the form of ammo-
nium and organic nitrogen in extant anoxic water columns, it seems unlikely
that N
2
fixation plays an important role in the N budget except in aerobic wa-
ters overlying anoxic water columns. However, in the anoxic or microaerobic
environments examined so far, including microbial mats, sediments, estuaries
and Mono Lake, there is a high diversity of N
2
In Mono Lake, at least a few of these microorganisms are expressing the nitro-
genase genes. This raises intriguing questions: What is the role of N
2
fixation
in anoxic environments? Does nitrogenase serve another function for microor-
ganisms in these environments? Why is the diversity of nitrogenase-containing
microorganisms so high in these environments? What are the selection forces
and why do these microorganisms still retain the nitrogenase genes in their
genomes?
It is clear that N
2
fixation is important in the aerobic open ocean, and in
oxygenated waters of many anoxic basins such as the Baltic Sea. However,
more research is needed to determine if N
2
fixation occurs within the anoxic
water column. It is an intriguing question because of the parallel with the anoxic
fixation and the underlying genes is speculative and poorly understood.
-fixing microorganisms [142].
