Environmental Engineering Reference
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blooms [70, 86]. In addition, euphotic zone wintertime P concentrations may be
important proxies for predicting the intensity of the forthcoming cyanobacterial
blooms that occur in the subsequent summer [65].
In Baltic Sea deep waters, another microbially mediated process, denitrifi-
cation, counteracts the inputs of new nitrogen from N 2 fixation by removing
dissolved inorganic nitrogen from the system [72], thus enhancing N limitation
and potentially favoring N 2 fixation. Due to the processes of PO 4 3 release
and denitrification, Baltic Sea bottom waters typically have a low N:P ratio,
suggested to favor N 2 -fixing cyanobacterial blooms [86].
6. N 2 -FIXATION IN ANOXIC ENVIRONMENTS
Nitrogen fixation has received little attention in most anoxic basins. Many,
if not most, anoxic environments have high concentrations of NH 4 + due to de-
composition of organic matter and lack of photosynthetic uptake. Furthermore,
N 2 fixation enzymes require metals, Fe and Mo, whose availability are affected
by redox chemistry.
There are few reports of attempts to measure N 2 fixation in anoxic wa-
ter columns, presumably either because it is assumed not to be an important
process or because of lack of positive results with N 2 fixation assays. Although
N 2 fixation has not been measured in many anoxic water columns, there are
analogous habitats that have been studied. These include sediments, microbial
mats and salt marsh rhizospheres. The same gradients in oxygen, nutrients and
metals that are present in anoxic water columns are found in these habitats, but
over smaller vertical length scales.
6.1 Microbial Mats and Salt Marshes
Microbial mats are believed to have been important in the evolution of
life on Earth. Remnants of mats, in the form of stromatolites, are some of
our earliest records of life [25]. Early microbial communities were probably
important in the exchange and cycling of materials between microbes, including
genetic information. Modern day mats mimic these early communities, and
often contain layers of cyanobacteria that are important in carbon fixation as
well as N 2 fixation. The hydrogen produced by N 2
ancient mats has been speculated to have resulted in hydrogen loss to space,
facilitating oxygenation of the Earth's atmosphere [51]. Oxygen availability
within the mats closely follows the diurnal cycle in photosynthesis and light, and
anoxic environments are frequently found in mats [121]. Once photosynthesis
ceases, microaerophilic or anaerobic N 2 fixation can occur. The deeper layers of
mats are typically anoxic and dominated by sulfate reducing microorganisms as
well as other anaerobes, many of which contain nitrogenase genes. Expression
of nitrogenase by these anaerobic organisms (Cluster III nitrogenases) has been
-fixing cyanobacteria in
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