Environmental Engineering Reference
In-Depth Information
N
2
fixing unicellular cyanobacteria play important roles in the ecology and
biogeochemistry of diverse environments ranging from microbial mats to the
open ocean.
4.3 Chemolithotrophs
A variety of chemolithotrophic bacteria have the ability to fix N
2
. These
include the Fe oxidizers, such as
Thiobacillus
. Some species of the sulfur oxi-
dizing
Beggiatoa
are also diazotrophic. These microorganisms oxidize sulfide
to elemental sulfur aerobically for energy, and are found at interfaces between
sulfidic and oxic zones. The availability of reduced compounds in anoxic water
columns supports a variety of chemolithotrophic metabolisms. Anoxic envi-
ronments support diverse chemolithotrophic bacteria, and it seems likely that
members of these anaerobic prokaryote assemblages might fix N
2
.
5. MARINE N
2
-FIXATION
Nitrogen fixation in the oceans occurs in the water column and the benthos.
Highest rates of N
2
fixation are typically found in intertidal and shallow water
cyanobacterial mats, and in salt marshes [49]. N
2
fixation in sediments occurs
at low rates, presumably because of the high ammonium concentrations, anal-
ogous to anoxic water columns. Cyanobacteria are important in N
2
fixation in
both the water column and in microbial mats. Heterocystous species (such as
the symbiont of diatoms,
Richelia
) and unicellular species (e.g. the recently
reported
Crocosphaera
, [141]) play a role in N
2
fixation in the open ocean.
Blooms of free-living planktonic heterocystous species are found in certain
coastal and estuarine waters.
5.1 N
2
Fixation in Surface Oceans
The surface waters of large regions of the world's oceans have vanishingly
low concentrations of bioavailable-fixed inorganic nitrogen (NO
3
−
,NO
2
−
,
NH
4
+
). Nitrate + nitrite (NO
3
+NO
2
) concentrations in the surface waters
−
1
, and NH
4
+
of oligotrophic ocean ecosystems are typically 10-100 nmol L
1
. Rapid recycling of nitrogen in the upper
ocean appears to support > 90% of measured primary production in large areas
of the world's oceans. Inputs of allochthonous nitrogen to the upper ocean are
restricted to 1) diffusive fluxes across the thermocline, 2) convective overturn
and mixing of nitrogen from depth, 3) advection from frontal regions, mesoscale
eddies, and other hydrodynamic processes, 4) atmospheric deposition, and 5)
biologically-mediated N
2
ocean include assimilation and export by plankton (including sedimentation),
and denitrification. The contributions of these various sources and sinks of
fixed nitrogen are variable in both time and space; however, the balance among
−
concentrations range 10-50 nmol L
fixation. Losses of fixed nitrogen from the upper
