kinetically inert cobalt(III) chelates. There is also
evidence that these cobalt(III)-binding ligands are
produced by marine cyanobacteria and that these
ligands may facilitate microbial uptake of cobalt.
Selenium is a metalloid, which occurs immediately
below sulfur in the periodic table. Consequently, its
chemical behavior often mimics that of sulfur. Selenium exists in subsurface seawater primarily as
soluble oxyanions selenate (SeO 4
2À ; þ 6 oxidation
state) and selenite (SeO 3
2À ; þ 4 oxidation state).
Phytoplankton preferentially take up selenite which
depletes its concentration in surface seawater (Figure 4). Selenate is then taken up and depleted following the removal of selenite. The selenate and
selenite ions taken up by phytoplankton are metabolically reduced to the selenide ( À 2 oxidation
state) and used to synthesize selenomethionine and
selenocysteine, chemical analogs of the sulfur-containing amino acids methionine and cysteine. In
surface waters a majority of the selenium occurs as
biologically regenerated organic selenide compounds
of unknown chemical structure (Figure 4).
Biological Uptake
All trace elements are taken up intracellularly by
specialized transport proteins (enzymes) on the outer
membrane of plankton cells. Consequently, uptake
rates generally follow Michaelis–Menten enzyme
kinetics:
Uptake rate ¼ V max S=ðK s þ SÞ
V max is the maximum uptake rate, S is the concentration of the pool of chemical species that react with
receptor sites on the transport protein, and K s is
concentration of the substrate pool at which half of
the transport protein is bound, and the uptake rate is
half of V max . Virtually all of these proteins act as
pumps and require energy for intracellular uptake.
Each transport system reacts with a single chemical
species or group of related chemical species and thus
chemical speciation is extremely important in regulating cellular uptake. Uptake systems range from
simple to highly complex depending on the chemical
speciation of the nutrient element and its biological
demand (requirement) relative to its external
availability.
Uptake systems appear to be simplest for dissolved
Mn(II), which is taken up in phytoplankton by a
single high-affinity transport system that is under
negative feedback regulation. In this negative feedback, as the concentration of dissolved Mn(II) decreases, the V max of the transport system is increased
to maintain relatively constant Mn uptake rates and
intracellular concentrations.
Uptake systems for zinc, cadmium, cobalt(II), and
copper(II) are somewhat more complex. The phytoplankton species examined to date have at least two
separate zinc transport systems: a low-affinity system
whose V max is relatively constant, and an inducible
high-affinity system. The low-affinity system has high
V max and high K s values and transports zinc at high
zinc ion concentrations. The high-affinity system is
responsible for zinc uptake at low zinc ion concentrations, and has low K s , and variable V max values
that are under negative feedback regulation. At sufficiently low concentrations of dissolved inorganic
zinc species (B10 pM), the cellular uptake approaches limiting rates for the diffusion of labile inorganic zinc species to the cell surface. The existence
of high- and low-affinity transport systems results in
sigmoidal relationships between zinc uptake rates
(and cellular Zn:C ratios) and concentrations of
dissolved inorganic zinc species as seen in Figure 5
for an oceanic diatom.
Cobalt and sometimes cadmium can metabolically
substitute for zinc in many metalloenzymes. To facilitate this substitution, the uptake of these divalent
metals is increased by over 100-fold in diatoms with
decreasing dissolved inorganic zinc concentrations
and resulting decreases in cellular zinc uptake rates
(Figure 5). Uptake of Cd by this inducible transport
system is repressed at high intracellular zinc levels,
and under these conditions, cadmium leaks into the
cell through the cell’s Mn(II) transport system. Thus
cellular uptake of cadmium in the ocean is regulated
by complex interactions among dissolved inorganic
concentrations of Cd, Zn, and Mn. Likewise, since
cobalt uptake is repressed at high zinc ion concentrations, biological depletion of cobalt often does not
occur until after zinc is depleted, as observed in the
subarctic Pacific (Figure 6).
The binding and subsequent intracellular uptake
of the above divalent metals (Zn
2þ , Mn
2þ
, Cd
2þ ,
Co
2þ , and Cu
2þ ) by the various intracellular uptake
systems are regulated by the concentration of dissolved inorganic metal species (free aquated ions and
inorganic complexes with chloride ions, hydroxide
ions, etc.). Organic complexation of these metals
reduces their uptake by decreasing the concentration
of dissolved inorganic metal species. This effect can
be substantial in cases such as zinc, where up to 99%
or more of the metal is bound to organic ligands in
surface seawater.
Since iron is the most limiting of the trace element
nutrients and its chemistry the most complex, it is
perhaps not surprising that the transport systems for
iron are the most varied and complex. Iron is highly
22 TRACE ELEMENT NUTRIENTS
evidence that these cobalt(III)-binding ligands are
produced by marine cyanobacteria and that these
ligands may facilitate microbial uptake of cobalt.
Selenium is a metalloid, which occurs immediately
below sulfur in the periodic table. Consequently, its
chemical behavior often mimics that of sulfur. Selenium exists in subsurface seawater primarily as
soluble oxyanions selenate (SeO 4
2À ; þ 6 oxidation
state) and selenite (SeO 3
2À ; þ 4 oxidation state).
Phytoplankton preferentially take up selenite which
depletes its concentration in surface seawater (Figure 4). Selenate is then taken up and depleted following the removal of selenite. The selenate and
selenite ions taken up by phytoplankton are metabolically reduced to the selenide ( À 2 oxidation
state) and used to synthesize selenomethionine and
selenocysteine, chemical analogs of the sulfur-containing amino acids methionine and cysteine. In
surface waters a majority of the selenium occurs as
biologically regenerated organic selenide compounds
of unknown chemical structure (Figure 4).
Biological Uptake
All trace elements are taken up intracellularly by
specialized transport proteins (enzymes) on the outer
membrane of plankton cells. Consequently, uptake
rates generally follow Michaelis–Menten enzyme
kinetics:
Uptake rate ¼ V max S=ðK s þ SÞ
V max is the maximum uptake rate, S is the concentration of the pool of chemical species that react with
receptor sites on the transport protein, and K s is
concentration of the substrate pool at which half of
the transport protein is bound, and the uptake rate is
half of V max . Virtually all of these proteins act as
pumps and require energy for intracellular uptake.
Each transport system reacts with a single chemical
species or group of related chemical species and thus
chemical speciation is extremely important in regulating cellular uptake. Uptake systems range from
simple to highly complex depending on the chemical
speciation of the nutrient element and its biological
demand (requirement) relative to its external
availability.
Uptake systems appear to be simplest for dissolved
Mn(II), which is taken up in phytoplankton by a
single high-affinity transport system that is under
negative feedback regulation. In this negative feedback, as the concentration of dissolved Mn(II) decreases, the V max of the transport system is increased
to maintain relatively constant Mn uptake rates and
intracellular concentrations.
Uptake systems for zinc, cadmium, cobalt(II), and
copper(II) are somewhat more complex. The phytoplankton species examined to date have at least two
separate zinc transport systems: a low-affinity system
whose V max is relatively constant, and an inducible
high-affinity system. The low-affinity system has high
V max and high K s values and transports zinc at high
zinc ion concentrations. The high-affinity system is
responsible for zinc uptake at low zinc ion concentrations, and has low K s , and variable V max values
that are under negative feedback regulation. At sufficiently low concentrations of dissolved inorganic
zinc species (B10 pM), the cellular uptake approaches limiting rates for the diffusion of labile inorganic zinc species to the cell surface. The existence
of high- and low-affinity transport systems results in
sigmoidal relationships between zinc uptake rates
(and cellular Zn:C ratios) and concentrations of
dissolved inorganic zinc species as seen in Figure 5
for an oceanic diatom.
Cobalt and sometimes cadmium can metabolically
substitute for zinc in many metalloenzymes. To facilitate this substitution, the uptake of these divalent
metals is increased by over 100-fold in diatoms with
decreasing dissolved inorganic zinc concentrations
and resulting decreases in cellular zinc uptake rates
(Figure 5). Uptake of Cd by this inducible transport
system is repressed at high intracellular zinc levels,
and under these conditions, cadmium leaks into the
cell through the cell’s Mn(II) transport system. Thus
cellular uptake of cadmium in the ocean is regulated
by complex interactions among dissolved inorganic
concentrations of Cd, Zn, and Mn. Likewise, since
cobalt uptake is repressed at high zinc ion concentrations, biological depletion of cobalt often does not
occur until after zinc is depleted, as observed in the
subarctic Pacific (Figure 6).
The binding and subsequent intracellular uptake
of the above divalent metals (Zn
2þ , Mn
2þ
, Cd
2þ ,
Co
2þ , and Cu
2þ ) by the various intracellular uptake
systems are regulated by the concentration of dissolved inorganic metal species (free aquated ions and
inorganic complexes with chloride ions, hydroxide
ions, etc.). Organic complexation of these metals
reduces their uptake by decreasing the concentration
of dissolved inorganic metal species. This effect can
be substantial in cases such as zinc, where up to 99%
or more of the metal is bound to organic ligands in
surface seawater.
Since iron is the most limiting of the trace element
nutrients and its chemistry the most complex, it is
perhaps not surprising that the transport systems for
iron are the most varied and complex. Iron is highly
22 TRACE ELEMENT NUTRIENTS