and organic ligands (complexing agents) or are adsorbed onto or bound within particles. Many trace
element nutrients (iron, copper, manganese, cobalt,
and selenium) cycle between different oxidation
states, which have quite different kinetic labilities
(reaction rates), solubilities, binding strengths with
organic ligands, and biological availabilities.
Nickel, zinc, and cadmium exist in normal oxygenated seawater as highly soluble divalent cations
that are complexed to varying degrees by inorganic
ligands (Cl
À , OH
À
, and CO 3
2À ) and organic chelators. Nickel is bound to only a small extent (0–30%)
by organic ligands. By contrast, B99% of the zinc
ions and 70% of the cadmium are heavily complexed
by unidentified strong organic ligands present at low
concentrations in surface waters of the North Pacific.
The strong chelation of zinc reduces the concentration of dissolved inorganic zinc to B1 pM in surface seawater, sufficiently low to limit the growth of
many algal species.
Manganese undergoes redox transformations, but
is minimally bound to organic ligands. The stable
redox species of manganese in oxygenated seawater,
Mn(IV) and Mn(III) oxides, are insoluble, although
Mn(III) can exist in some instances as soluble organic
chelates. Mn(III) and Mn(IV) can be reduced chemically, photochemically, or biologically to dissolved
Mn(II), which is fully soluble in seawater and is not
appreciably bound by organic ligands. Although
Mn(II) is unstable with respect to oxidation by molecular oxygen, the chemical kinetics of this reaction
are exceedingly slow in seawater. However, Mn(II)
oxidation is greatly accelerated by bacterial enzymes
that catalyze Mn(II) oxidation to Mn(IV) oxides. The
bacterial formation of Mn oxides, and subsequent
removal via coagulation and settling of oxide particles, results in short residence times (20–40 years in
the North Pacific) and low concentrations for manganese in deep-ocean waters (Figure 2(h)). Oxidation
is absent or greatly diminished in the ocean’s surface
mixed layer due to photo-inhibition of the Mn-oxidizing bacteria. The absence of bacterially mediated
oxidation of Mn(II) and minimal organic chelation
often results in high concentrations of Mn
2þ ions in
surface seawater (Table 1; Figure 2(h)), enhancing
the supply of Mn to phytoplankton.
Iron is the most biologically important trace metal
nutrient, and its chemical behavior is perhaps the
most complex. Its stable oxidation state in oxygencontaining waters is Fe(III), which forms sparingly
soluble iron hydroxide and oxide precipitates. This
oxide formation and the tendency of ferric ions to
adsorb onto particle surfaces results in the scavenging of iron from seawater via particulate aggregation and settling processes. This removal results in
short residence times for iron in deep-ocean waters
(B50–100 years) and low concentrations (0.4–
0.8 nM) despite the high crustal abundance of iron (it
is the fourth most abundant element by weight).
Most (499%) of the dissolved ferric iron in seawater is bound to organic ligands which minimizes
iron adsorption and precipitation, and thus reduces
the removal of iron from seawater by particulate
scavenging processes. Some of these organic ligands
may be strong ferric chelators (siderophores) produced by bacteria to solubilize iron and facilitate
intracellular iron uptake. Ferric iron can be reduced
in seawater to highly soluble Fe(II) (ferrous iron) by a
number of processes including photo-reduction of
organic chelates in surface waters, biological reduction of iron at cell surfaces, and reduction by
chemical reducing agents. Because ferrous iron binds
much more weakly to organic chelators than ferric
iron, the photo-reduction or biological reduction of
iron in ferric chelates often results in the dissociation
of iron from the chelates, which increases iron
availability for biological uptake (see below). The
released ferrous ions are unstable in oxygenated
seawater, and are reoxidized to soluble ferric hydrolysis species, and recomplexed by organic ligands
on timescales of minutes. Thus iron undergoes a
dynamic redox cycling in surface seawater, which
can greatly enhance its biological availability to
phytoplankton.
Other micronutrient metals such as copper and
cobalt also exist in multiple oxidation states and are
heavily complexed by organic chelators. Copper can
exist in seawater as thermodynamically stable copper(II), or as copper(I). Most (499%) of the copper
in near-surface seawater is heavily chelated by strong
organic ligands present at low concentrations (2–
3 nM in ocean waters). This chelation decreases free
cupric (copper II) ion concentrations to very low
levels (0.1–1 pM). Copper(II) can be reduced to Cu(I)
by photochemical and biological processes or by reaction with chemical reducing agents, such as sulfurcontaining organic ligands. The resultant Cu(I) can
be reoxidized by reaction with molecular oxygen,
but the effect of this redox cycling on the biological
availability of copper is currently unknown.
The chemistry of cobalt is also highly complex.
Cobalt exists in seawater as soluble cobalt(II) or as
cobalt(III), which forms insoluble oxides at the pH of
seawater. The formation of these oxides appears to
be microbially mediated and is largely responsible
for the removal of cobalt from deep-ocean waters
and for the resultant low deep-ocean concentrations
(Figure 3(d)). Much of the dissolved cobalt in seawater is strongly bound to organic ligands, and recent evidence suggests that this cobalt exists as
TRACE ELEMENT NUTRIENTS 21
element nutrients (iron, copper, manganese, cobalt,
and selenium) cycle between different oxidation
states, which have quite different kinetic labilities
(reaction rates), solubilities, binding strengths with
organic ligands, and biological availabilities.
Nickel, zinc, and cadmium exist in normal oxygenated seawater as highly soluble divalent cations
that are complexed to varying degrees by inorganic
ligands (Cl
À , OH
À
, and CO 3
2À ) and organic chelators. Nickel is bound to only a small extent (0–30%)
by organic ligands. By contrast, B99% of the zinc
ions and 70% of the cadmium are heavily complexed
by unidentified strong organic ligands present at low
concentrations in surface waters of the North Pacific.
The strong chelation of zinc reduces the concentration of dissolved inorganic zinc to B1 pM in surface seawater, sufficiently low to limit the growth of
many algal species.
Manganese undergoes redox transformations, but
is minimally bound to organic ligands. The stable
redox species of manganese in oxygenated seawater,
Mn(IV) and Mn(III) oxides, are insoluble, although
Mn(III) can exist in some instances as soluble organic
chelates. Mn(III) and Mn(IV) can be reduced chemically, photochemically, or biologically to dissolved
Mn(II), which is fully soluble in seawater and is not
appreciably bound by organic ligands. Although
Mn(II) is unstable with respect to oxidation by molecular oxygen, the chemical kinetics of this reaction
are exceedingly slow in seawater. However, Mn(II)
oxidation is greatly accelerated by bacterial enzymes
that catalyze Mn(II) oxidation to Mn(IV) oxides. The
bacterial formation of Mn oxides, and subsequent
removal via coagulation and settling of oxide particles, results in short residence times (20–40 years in
the North Pacific) and low concentrations for manganese in deep-ocean waters (Figure 2(h)). Oxidation
is absent or greatly diminished in the ocean’s surface
mixed layer due to photo-inhibition of the Mn-oxidizing bacteria. The absence of bacterially mediated
oxidation of Mn(II) and minimal organic chelation
often results in high concentrations of Mn
2þ ions in
surface seawater (Table 1; Figure 2(h)), enhancing
the supply of Mn to phytoplankton.
Iron is the most biologically important trace metal
nutrient, and its chemical behavior is perhaps the
most complex. Its stable oxidation state in oxygencontaining waters is Fe(III), which forms sparingly
soluble iron hydroxide and oxide precipitates. This
oxide formation and the tendency of ferric ions to
adsorb onto particle surfaces results in the scavenging of iron from seawater via particulate aggregation and settling processes. This removal results in
short residence times for iron in deep-ocean waters
(B50–100 years) and low concentrations (0.4–
0.8 nM) despite the high crustal abundance of iron (it
is the fourth most abundant element by weight).
Most (499%) of the dissolved ferric iron in seawater is bound to organic ligands which minimizes
iron adsorption and precipitation, and thus reduces
the removal of iron from seawater by particulate
scavenging processes. Some of these organic ligands
may be strong ferric chelators (siderophores) produced by bacteria to solubilize iron and facilitate
intracellular iron uptake. Ferric iron can be reduced
in seawater to highly soluble Fe(II) (ferrous iron) by a
number of processes including photo-reduction of
organic chelates in surface waters, biological reduction of iron at cell surfaces, and reduction by
chemical reducing agents. Because ferrous iron binds
much more weakly to organic chelators than ferric
iron, the photo-reduction or biological reduction of
iron in ferric chelates often results in the dissociation
of iron from the chelates, which increases iron
availability for biological uptake (see below). The
released ferrous ions are unstable in oxygenated
seawater, and are reoxidized to soluble ferric hydrolysis species, and recomplexed by organic ligands
on timescales of minutes. Thus iron undergoes a
dynamic redox cycling in surface seawater, which
can greatly enhance its biological availability to
phytoplankton.
Other micronutrient metals such as copper and
cobalt also exist in multiple oxidation states and are
heavily complexed by organic chelators. Copper can
exist in seawater as thermodynamically stable copper(II), or as copper(I). Most (499%) of the copper
in near-surface seawater is heavily chelated by strong
organic ligands present at low concentrations (2–
3 nM in ocean waters). This chelation decreases free
cupric (copper II) ion concentrations to very low
levels (0.1–1 pM). Copper(II) can be reduced to Cu(I)
by photochemical and biological processes or by reaction with chemical reducing agents, such as sulfurcontaining organic ligands. The resultant Cu(I) can
be reoxidized by reaction with molecular oxygen,
but the effect of this redox cycling on the biological
availability of copper is currently unknown.
The chemistry of cobalt is also highly complex.
Cobalt exists in seawater as soluble cobalt(II) or as
cobalt(III), which forms insoluble oxides at the pH of
seawater. The formation of these oxides appears to
be microbially mediated and is largely responsible
for the removal of cobalt from deep-ocean waters
and for the resultant low deep-ocean concentrations
(Figure 3(d)). Much of the dissolved cobalt in seawater is strongly bound to organic ligands, and recent evidence suggests that this cobalt exists as
TRACE ELEMENT NUTRIENTS 21