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detected by molecular techniques in extant microbial mats [91, 122]. It is not
clear why these organisms fix N
2
in environments where concentrations of
ammonium can be high, but the chemical conditions and microbial survival
strategies are likely to be similar to those found in anoxic basins.
Nitrogen fixation has been studied extensively in salt marshes [49]. Sulfate
reducing bacteria have been implicated in N
2
fixation in a number of studies
[49]. Sulfate reducing bacteria and potentially other anaerobes have been shown
to express nitrogenase genes in salt marsh sediments [15]. The high sulfate
concentrations and anoxia are likely to be similar to anoxic water columns,
suggesting that similar activities may be present in anoxic water columns as
well.
6.2 Anoxic Water Columns
Anoxic basins can be extremely productive regions due to the interface
between anoxic and oxic regions, and associated juxtaposition of nutrients,
trace metals and microbial processes (Fig. 3). Examples of anoxic basins in
extant oceans include the Black Sea and the Cariaco Trench. High ammonia
concentrations in anoxic water columns in places such as the Black Sea [71],
certain Northern European fjords [101, 130], and the Baltic Sea [44] suggest
N
2
fixation to be unlikely; however, the presence of N
2
-fixers and rates of N
2
fixation have not been intensively studied in these systems. Delta proteobacteria
related to sulfate reducers, many of which are known to be N
2
fixers, were
reported at high abundances in anoxic and microaerophilic water columns in a
Danish fjord [101].
Nitrogen fixation varies extensively in time and space in these basins with
rates varying from the undetected to high rates associated with cyanobacterial
blooms. There is also considerable seasonal, annual and decadal variability
in N
2
fixation in these waters. N
2
fixation is controlled by the availability
of nutrients, and is selected for by the availability of nutrients relative to the
stoichiometric requirements for life. In waters overlying anoxic layers, the
availability of trace metals (Fe, V, Mo) can be affected by the chemistry of
the oxic and anoxic waters, and the availability of other nutrients (P). Anoxic
layers may control surface water cyanobacterial N
2
fixation by its effect on the
N:P ratio in the nutrient supply [47].
The Mediterranean Sea has low concentrations of nutrients, but N
2
fixation
has not been extensively studied. Cyanobacterial N
2
fixation is occasionally
present in the Mediterranean surface waters but is believed to be of minor
importance [8]. However, sediment stable isotopic record from the eastern
Mediterranean Sea suggests widespread N
2
fixation has been present in the
water column of the basin during anoxic deep water events that have reoccurred
over geological time scales [109, 124]. Records in these organic carbon rich
