Environmental Engineering Reference
In-Depth Information
these processes ultimately constrains plankton productivity and fixed nitrogen
availability in the upper ocean.
Recent increases in anthropogenic CO
2
emissions have stimulated interest in
the role of the oceans in modulating global climate. As a result, there has been
considerable interest in quantifying the processes that limit N
2
fixation and the
role of N
2
fixing microorganisms on biogeochemical fluxes and inventories in
the oceans. Initial studies suggested that N
2
fixation supplied little fixed nitrogen
to plankton assemblages in the oceans, and that the oceans were inhabited by
relatively few species of diazotrophs. However, numerous lines of evidence
now suggest that in large regions of the open ocean, N
2
fixation plays a central
role in modifying elemental cycling in the sea, and might even play a prominent
role in regulating global climate [33, 43, 60, 79].
The most prominent oceanic diazotrophs are species of the non hetero-
cystous, filamentous cyanobacteria genus
Trichodesmium
. Filaments of
Tri-
chodesmium
often grow to be several millimeters in length, making them easily
identifiable, even from space, as the filaments accumulate to surface blooms.
As a result, estimates of oceanic N
2
fixation have relied heavily on rates of N
2
fixation by colonies of
Trichodesmium
. Extrapolation of measured N
2
fixation
rates to ocean basin scales suggests
Trichodesmium
may be responsible for
∼
20-100 µmolNm
−
2
d
−
1
[21, 60, 82]. These rates are comparable to diffusive
NO
3
fluxes to the upper ocean in subtropical and tropical ocean ecosystems.
However, analysis of
nifH
genes in oceanic plankton has indicated there may be
a much greater diversity of diazotrophic plankton than previously suspected. A
number of studies have identified uncultivated cyanobacterial, proteobacterial,
and Cluster III
nifH
genes from plankton samples in the North Pacific and
North Atlantic oceans [13, 32]. Nitrogenase genes of unicellular cyanobacteria
were found [140], that have also been found elsewhere in the oceans [32].
NifH
mRNA of unicellular cyanobacteria have also been detected [141]. These
unicellular cyanobacteria appear to actively fix N
2
at rates comparable to fil-
amentous cyanobacteria based on
15
N tracer studies (ranging
∼
2-500 µmol N
m
−
2
d
−
1
) [82].
Recent direct measurements of N
2
fixation support geochemical determi-
nations of oceanic N
2
fixation. Depth profiles of the elemental stoichiometry
of NO
3
−
and PO
4
3
−
have been utilized to quantify excess NO
3
−
relative to
PO
4
3
−
, referred to as the N* anomaly [26, 43, 79]. Depletions of upper ocean
CO
2
inventories in the absence of NO
3
−
in both the Atlantic and Pacific may be
supported by N
2
fixation, and can be used to quantify the amount of nitrogen
required to support CO
2
uptake in the upper ocean. In addition, measurements
of the isotopic signatures (
15
N) of particles exported from the upper ocean com-
bined with long-term analyses of nutrient stoichiometry all suggest N
2
fixation
plays an important role in supporting plankton production and carbon export in
the oceans [28, 60]. The use of these approaches to quantify N
2
fixation suggest
