or anoxic habitats with only a slight loss of efficiency
relative to O 2 -based metabolism. A majority of
marine bacteria have the ability for NO 3
À respiration
under the appropriate environmental conditions
(Table 1). Potential by-products of NO 3
À respiration
are NO 2
À , N 2 , and N 2 O; if a gas is formed (N 2 /N 2 O)
then the process is termed denitrification because the
net effect is to remove bioavailable nitrogen from the
local environment. The total rate of denitrification is
generally limited by the availability of NO 3
À , and a
continued supply of NO 3
À via nitrification is
dependent upon the availability of NH 4
þ and free
O 2 . Consequently denitrification typically occurs at
boundaries between low-O 2 and anoxic conditions
where the supply of NH 4
þ from the anoxic zone
sustains a high rate of NO 3
À production via nitrification to fuel- sustained NO 3
À respiration and denitrification.
Recently
a
new
group
of
microorganisms has been isolated that are capable of
simultaneously using both O 2 and NO 3
À /NO 2
À as
terminal electron acceptors. This process is termed
‘aerobic denitrification.’ Likewise, there are exceptional microorganisms that are able to carry out
anaerobic nitrification (oxidation of NH 4
þ in the
absence of O 2 ). It appears difficult to establish any
hard-and-fast rules regarding marine nitrogen cycle
processes.
N 2 Fixation
The ability to use N 2 as a growth substrate is restricted to a relatively small group of microorganisms. Open ocean ecosystems that are chronically
depleted in fixed nitrogen would appear to be ideal
habitats for the proliferation of N 2 -fixing microorganisms. However, the enzyme that is required for
reduction of N 2 to NH 4
þ is also inhibited by O 2 , so
specialized structural, molecular, and behavioral
adaptations have evolved to promote oceanic N 2
fixation.
Fixation of molecular nitrogen in the open ocean
may also be limited by the availability of iron, which
is an essential cofactor for the N 2 reduction enzyme
system. Changes in iron loading are caused by climate variations, in particular the areal extent of
global deserts, by the intensity of atmospheric circulation, and more recently by changes in land use
practices. Conversion of deserts into irrigated croplands may cause a change in the pattern and intensity
of dust production and, therefore, of iron transport
to the sea. Humanity is also altering the global nitrogen cycle by enhancing the fixation of N 2 by the
manufacturing of fertilizer. At the present time, the
industrial fixation of N 2 is approximately equivalent
to the pre-industrial, natural N 2 fixation rate.
Eventually some of this artificially fixed N 2 will
make its way to the sea, and this may lead to a
perturbation in the natural nitrogen cycle.
On a global scale and over relatively long timescales, the total rate of N 2 fixation is more or less in
balance with total denitrification, so that the nitrogen cycle is mass-balanced. However, significant net
deficits or excesses can be observed locally or even on
ocean basin space scales and on decade to century
timescales. These nitrogen imbalances may impact
the global carbon and phosphorus cycles as well,
including the net balance of CO 2 between the ocean
and the atmosphere.
Nitrogen Distributions in the Sea
Required growth nutrients, like nitrogen, typically
have uneven distributions in the open sea, with deficits in areas where net organic matter is produced
and exported, and excesses in areas where organic
matter is decomposed. For example, surface ocean
NO 3
À distributions in the Pacific basin reveal a coherent pattern with excess NO 3
À in high latitudes,
especially in the Southern Ocean (south of 601 S),
and along the Equator (especially east of the dateline), and generally depleted NO 3
À concentrations in
the middle latitudes of both hemispheres (Figure 3).
These distributions are a result of the balance between NO 3
À supply mostly by ocean mixing and
NO 3
À demand or net photosynthesis. The very large
NO 3
À inventory in the surface waters of the Southern Ocean implies that factors other than fixed nitrogen availability control photosynthesis in these
regions. It has been hypothesized that the availability
of iron is key in this and perhaps other regions of the
open ocean. The much smaller but very distinctive
band of elevated NO 3
À along the Equator is the result of upwelling of NO 3
À -enriched waters from
depth to the surface. This process has a large seasonal and, especially, interannual variability, and it is
almost absent during El Nin ˜ o conditions.
Excluding these high-latitude and equatorial regions, the remainder of the surface waters of the
North and South Pacific Oceans from about 401N to
401S are relatively depleted in NO 3
À . In fact surface
(0–50 m) NO 3
À concentrations in the North Pacific
subtropical gyre near Hawaii are typically below
0.01 mmol l
À1 (Figure 4). Within the upper 200 m, the
major pools of fixed nitrogen (e.g., NO 3
À , DON, and
PON) have different depth distributions. In the sunlit
surface zone, NO 3
À is removed to sustain organic
matter production and export. Beneath 100 m, there
is a steep concentration versus depth gradient (referred to as the nutricline), which reaches a
544 NITROGEN CYCLE
relative to O 2 -based metabolism. A majority of
marine bacteria have the ability for NO 3
À respiration
under the appropriate environmental conditions
(Table 1). Potential by-products of NO 3
À respiration
are NO 2
À , N 2 , and N 2 O; if a gas is formed (N 2 /N 2 O)
then the process is termed denitrification because the
net effect is to remove bioavailable nitrogen from the
local environment. The total rate of denitrification is
generally limited by the availability of NO 3
À , and a
continued supply of NO 3
À via nitrification is
dependent upon the availability of NH 4
þ and free
O 2 . Consequently denitrification typically occurs at
boundaries between low-O 2 and anoxic conditions
where the supply of NH 4
þ from the anoxic zone
sustains a high rate of NO 3
À production via nitrification to fuel- sustained NO 3
À respiration and denitrification.
Recently
a
new
group
of
microorganisms has been isolated that are capable of
simultaneously using both O 2 and NO 3
À /NO 2
À as
terminal electron acceptors. This process is termed
‘aerobic denitrification.’ Likewise, there are exceptional microorganisms that are able to carry out
anaerobic nitrification (oxidation of NH 4
þ in the
absence of O 2 ). It appears difficult to establish any
hard-and-fast rules regarding marine nitrogen cycle
processes.
N 2 Fixation
The ability to use N 2 as a growth substrate is restricted to a relatively small group of microorganisms. Open ocean ecosystems that are chronically
depleted in fixed nitrogen would appear to be ideal
habitats for the proliferation of N 2 -fixing microorganisms. However, the enzyme that is required for
reduction of N 2 to NH 4
þ is also inhibited by O 2 , so
specialized structural, molecular, and behavioral
adaptations have evolved to promote oceanic N 2
fixation.
Fixation of molecular nitrogen in the open ocean
may also be limited by the availability of iron, which
is an essential cofactor for the N 2 reduction enzyme
system. Changes in iron loading are caused by climate variations, in particular the areal extent of
global deserts, by the intensity of atmospheric circulation, and more recently by changes in land use
practices. Conversion of deserts into irrigated croplands may cause a change in the pattern and intensity
of dust production and, therefore, of iron transport
to the sea. Humanity is also altering the global nitrogen cycle by enhancing the fixation of N 2 by the
manufacturing of fertilizer. At the present time, the
industrial fixation of N 2 is approximately equivalent
to the pre-industrial, natural N 2 fixation rate.
Eventually some of this artificially fixed N 2 will
make its way to the sea, and this may lead to a
perturbation in the natural nitrogen cycle.
On a global scale and over relatively long timescales, the total rate of N 2 fixation is more or less in
balance with total denitrification, so that the nitrogen cycle is mass-balanced. However, significant net
deficits or excesses can be observed locally or even on
ocean basin space scales and on decade to century
timescales. These nitrogen imbalances may impact
the global carbon and phosphorus cycles as well,
including the net balance of CO 2 between the ocean
and the atmosphere.
Nitrogen Distributions in the Sea
Required growth nutrients, like nitrogen, typically
have uneven distributions in the open sea, with deficits in areas where net organic matter is produced
and exported, and excesses in areas where organic
matter is decomposed. For example, surface ocean
NO 3
À distributions in the Pacific basin reveal a coherent pattern with excess NO 3
À in high latitudes,
especially in the Southern Ocean (south of 601 S),
and along the Equator (especially east of the dateline), and generally depleted NO 3
À concentrations in
the middle latitudes of both hemispheres (Figure 3).
These distributions are a result of the balance between NO 3
À supply mostly by ocean mixing and
NO 3
À demand or net photosynthesis. The very large
NO 3
À inventory in the surface waters of the Southern Ocean implies that factors other than fixed nitrogen availability control photosynthesis in these
regions. It has been hypothesized that the availability
of iron is key in this and perhaps other regions of the
open ocean. The much smaller but very distinctive
band of elevated NO 3
À along the Equator is the result of upwelling of NO 3
À -enriched waters from
depth to the surface. This process has a large seasonal and, especially, interannual variability, and it is
almost absent during El Nin ˜ o conditions.
Excluding these high-latitude and equatorial regions, the remainder of the surface waters of the
North and South Pacific Oceans from about 401N to
401S are relatively depleted in NO 3
À . In fact surface
(0–50 m) NO 3
À concentrations in the North Pacific
subtropical gyre near Hawaii are typically below
0.01 mmol l
À1 (Figure 4). Within the upper 200 m, the
major pools of fixed nitrogen (e.g., NO 3
À , DON, and
PON) have different depth distributions. In the sunlit
surface zone, NO 3
À is removed to sustain organic
matter production and export. Beneath 100 m, there
is a steep concentration versus depth gradient (referred to as the nutricline), which reaches a
544 NITROGEN CYCLE