NITROGEN CYCLE
D. M. Karl, University of Hawaii at Manoa,
Honolulu, HI, USA
A. F. Michaels, University of Southern California, Los
Angeles, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The continued production of organic matter in the
sea requires the availability of the many building
blocks of life, including essential major elements
such as carbon (C), nitrogen (N), and phosphorus
(P); essential minor elements such as iron, zinc, and
cobalt; and, for many marine organisms, essential
trace organic nutrients that they cannot manufacture
themselves (e.g., amino acids and vitamins). These
required nutrients have diverse structural and metabolic function and, by definition, marine organisms
cannot survive in their absence.
The marine nitrogen cycle is part of the much larger
and interconnected hydrosphere–lithosphere–atmosphere–biosphere nitrogen cycle of the Earth. Furthermore, the oceanic cycles of carbon, nitrogen, and
phosphorus are inextricably linked together through
the production and remineralization of organic matter, especially near surface ocean phytoplankton production. This coordinated web of major bioelements
can be viewed as the nutrient ‘super-cycle.’
The dominant form of nitrogen in the sea is dissolved gaseous dinitrogen (N 2 ) which accounts for
more than 95% of the total nitrogen inventory.
However, the relative stability of the triple bond of
N 2 renders this form nearly inert. Although N 2 can
serve as a biologically-available nitrogen source for
specialized N 2 À fixing microorganisms, these organisms are relatively rare in most marine ecosystems.
Consequently, chemically ‘fixed’ or ‘reactive’ nitrogen
compounds such as nitrate (NO 3
À ), nitrate (NO 2
À ),
ammonium (NH 4
þ ), and dissolved and particulate
organic nitrogen (DON/PON) serve as the principal
sources of nitrogen to sustain biological processes.
For more than a century, oceanographers have
been concerned with the identification of growth-and
production-rate limiting factors. This has stimulated
investigations of the marine nitrogen cycle including
both inventory determinations and pathways and
controls of nitrogen transformations from one form
to another. Contemporaneous ocean investigations
have documented an inextricable link between nitrogen and phosphorus cycles, as well as the importance of trace inorganic nutrients. It now appears
almost certain that nitrogen is only one of several key
elements for life in the sea, neither more nor less
important than the others. Although the basic features of the marine nitrogen cycle were established
nearly 50 years ago, new pathways and novel
microorganisms continue to be discovered. Consequently, our conceptual view of the nitrogen cycle is
a flexible framework, always poised for readjustment.
Methods and Units
The analytical determinations of the various dissolved and particulate forms of nitrogen in the sea
rely largely on methods that have been in routine use
for several decades. Determinations of NO 3
À , NO 2
À ,
and NH 4
þ generally employ automated shipboard,
colorimetric assays, although surface waters of open
ocean ecosystems demand the use of modern highsensitivity chemiluminescence and fluorometric
detection systems. PON is measured by high-temperature combustion followed by chromatographic
detection of the by-product (N 2 ), usually with a
commercial C–N analyzer. Total dissolved nitrogen
(TDN) determination employs sample oxidation, by
chemical or photolytic means, followed by measurement of NO 3
À . DON is calculated as the difference
between TDN and the measured dissolved, reactive
inorganic forms of N (NO 3
À , NO 2
À , NH 4
þ ) present
in the original sample. Gaseous forms of nitrogen,
including N 2 , nitrous oxide (N 2 O), and nitric oxide
(NO) are generally measured by gas chromatography.
Nitrogen exists naturally as two stable isotopes,
14 N (99.6% by atoms) and
15 N (0.4% by atoms).
These isotopes can be used to study the marine nitrogen cycle by examination of natural variations in
the
14
N/
15
N ratio, or by the addition of specific
tracers that are artificially enriched in
15 N.
Most studies of oceanic nitrogen inventories or
transformations use either molar or mass units;
conversion between the two is straightforward
(1 mole N ¼ 14 g N, keeping in mind that the molecular weight of N 2 gas is 28).
Components of the Marine Nitrogen
Cycle
The systematic transformation of one form of nitrogen to another is referred to as the nitrogen cycle
(Figure 1). In the sea, the nitrogen cycle revolves
541
D. M. Karl, University of Hawaii at Manoa,
Honolulu, HI, USA
A. F. Michaels, University of Southern California, Los
Angeles, CA, USA
Copyright & 2001 Elsevier Ltd.
Introduction
The continued production of organic matter in the
sea requires the availability of the many building
blocks of life, including essential major elements
such as carbon (C), nitrogen (N), and phosphorus
(P); essential minor elements such as iron, zinc, and
cobalt; and, for many marine organisms, essential
trace organic nutrients that they cannot manufacture
themselves (e.g., amino acids and vitamins). These
required nutrients have diverse structural and metabolic function and, by definition, marine organisms
cannot survive in their absence.
The marine nitrogen cycle is part of the much larger
and interconnected hydrosphere–lithosphere–atmosphere–biosphere nitrogen cycle of the Earth. Furthermore, the oceanic cycles of carbon, nitrogen, and
phosphorus are inextricably linked together through
the production and remineralization of organic matter, especially near surface ocean phytoplankton production. This coordinated web of major bioelements
can be viewed as the nutrient ‘super-cycle.’
The dominant form of nitrogen in the sea is dissolved gaseous dinitrogen (N 2 ) which accounts for
more than 95% of the total nitrogen inventory.
However, the relative stability of the triple bond of
N 2 renders this form nearly inert. Although N 2 can
serve as a biologically-available nitrogen source for
specialized N 2 À fixing microorganisms, these organisms are relatively rare in most marine ecosystems.
Consequently, chemically ‘fixed’ or ‘reactive’ nitrogen
compounds such as nitrate (NO 3
À ), nitrate (NO 2
À ),
ammonium (NH 4
þ ), and dissolved and particulate
organic nitrogen (DON/PON) serve as the principal
sources of nitrogen to sustain biological processes.
For more than a century, oceanographers have
been concerned with the identification of growth-and
production-rate limiting factors. This has stimulated
investigations of the marine nitrogen cycle including
both inventory determinations and pathways and
controls of nitrogen transformations from one form
to another. Contemporaneous ocean investigations
have documented an inextricable link between nitrogen and phosphorus cycles, as well as the importance of trace inorganic nutrients. It now appears
almost certain that nitrogen is only one of several key
elements for life in the sea, neither more nor less
important than the others. Although the basic features of the marine nitrogen cycle were established
nearly 50 years ago, new pathways and novel
microorganisms continue to be discovered. Consequently, our conceptual view of the nitrogen cycle is
a flexible framework, always poised for readjustment.
Methods and Units
The analytical determinations of the various dissolved and particulate forms of nitrogen in the sea
rely largely on methods that have been in routine use
for several decades. Determinations of NO 3
À , NO 2
À ,
and NH 4
þ generally employ automated shipboard,
colorimetric assays, although surface waters of open
ocean ecosystems demand the use of modern highsensitivity chemiluminescence and fluorometric
detection systems. PON is measured by high-temperature combustion followed by chromatographic
detection of the by-product (N 2 ), usually with a
commercial C–N analyzer. Total dissolved nitrogen
(TDN) determination employs sample oxidation, by
chemical or photolytic means, followed by measurement of NO 3
À . DON is calculated as the difference
between TDN and the measured dissolved, reactive
inorganic forms of N (NO 3
À , NO 2
À , NH 4
þ ) present
in the original sample. Gaseous forms of nitrogen,
including N 2 , nitrous oxide (N 2 O), and nitric oxide
(NO) are generally measured by gas chromatography.
Nitrogen exists naturally as two stable isotopes,
14 N (99.6% by atoms) and
15 N (0.4% by atoms).
These isotopes can be used to study the marine nitrogen cycle by examination of natural variations in
the
14
N/
15
N ratio, or by the addition of specific
tracers that are artificially enriched in
15 N.
Most studies of oceanic nitrogen inventories or
transformations use either molar or mass units;
conversion between the two is straightforward
(1 mole N ¼ 14 g N, keeping in mind that the molecular weight of N 2 gas is 28).
Components of the Marine Nitrogen
Cycle
The systematic transformation of one form of nitrogen to another is referred to as the nitrogen cycle
(Figure 1). In the sea, the nitrogen cycle revolves
541