nitrogen fixation. Because of its high concentration in
seawater (c. 105 nM), Mo does not appear to limit
algal growth or N 2 fixation in the ocean.
The metalloid selenium is also essential for the
growth of many marine phytoplankton. It occurs in
glutathione peroxidase, an enzyme that degrades
hydrogen peroxide, and thus is important in antioxidant protection. However, it is likely that selenium has other as-yet-unidentified metabolic
functions. The potential for selenium limitation in
the ocean is currently unknown.
Biological Feedback on Seawater
Chemistry
Trace elements not only influence the productivity
and species composition of planktonic communities,
but the plankton have a profound effect on the
chemistry and cycling of these elements on a variety
of temporal and spatial scales (Figure 1). The most
obvious example is the effect of algal uptake, particulate settling, and regeneration cycles on the vertical distribution and interocean transfer of trace
element nutrients (Fe, Zn, Cd, Ni, Cu, and Se; Figures 2–4). In addition, bacteria largely mediate the
removal of dissolved manganese and cobalt from
subsurface seawater via the formation of Mn(IV) and
Co(III) oxides.
There is evidence that the organic ligands that
strongly bind iron, copper, zinc, and cobalt are produced either directly or indirectly by the biota. In the
North Pacific, the organic ligands that strongly bind
copper occur at highest levels at the depth of maximum productivity, and decrease below the euphotic
zone. Ligands having the same copper-binding
strength are produced by Synechococcus, an abundant group of oceanic cyanobacteria. There is evidence that these organisms produce the chelators to
detoxify copper by decreasing free cupric ion concentrations. The organic ligands that strongly bind
iron(III), cobalt, and zinc also have a beneficial effect.
The iron ligands tightly bind ferric ions in soluble
chelates and thereby minimize the abiotic removal of
iron from seawater via the formation of insoluble
ferric oxides or ferric ion adsorption onto particulate
surfaces. Without such chelating ligands, iron concentrations would likely be much lower, and the
productivity of the ocean would be greatly reduced.
The Co(III)-binding ligands serve a similar function in
limiting the formation of insoluble Co(III) oxides, a
major mechanism for removal of cobalt from seawater. Recent culture experiments and seawater incubation experiments suggest that these ligands are
produced by the cyanobacterial genus Synechococcus,
whose growth may be limited by cobalt in some regions of the ocean.
Zinc chelators also serve a beneficial function, not
only by minimizing abiotic scavenging of zinc in
surface waters, but also by preventing the extremely
efficient uptake systems of eukaryotic phytoplankton
from completely depleting this essential micronutrient element from surface ocean waters.
Thus trace element nutrients and marine plankton
comprise an interactive system in the ocean in which
each exerts a controlling influence on the composition and dynamics of the other (Figure 1). On
longer geological timescales, the feedback interactions between the biota and trace metal chemistry
and availability have been profound. Currently, the
air we breathe and virtually the entire ocean, with
the exception of a few isolated anoxic basins (e.g.,
the Black Sea), contain free dioxygen molecules (O 2 ),
generated over billions of years from its release as a
byproduct of oxygenic photosynthesis. The presence
of free O 2 sets the redox state of modern ocean
toward oxidizing conditions, which as noted previously, limits the solubility of essential transition
metals (Fe, Co, and Mn) whose stable oxidation
states under these conditions are insoluble Co(III) and
Mn(IV) oxides or sparingly soluble Fe(III) oxides.
However, prior to the advent of oxygenic photosynthesis c. 3 billion years ago, the chemistry of the
ocean was far different from that which exists today.
There was no free oxygen and the entire ocean and
Earth’s surface was much more reducing. Under
these conditions, the stable redox state of Fe, Mn,
and Co was soluble Fe(II), Mn(II), and Co(II), and that
of copper was Cu(I). Furthermore, the stable redox
form of sulfur was sulfide (À 2 oxidation state), rather sulfate (þ 6 oxidation state), which occurs in
present-day seawater at a relatively high concentration (28 mM). The presence of moderate to high
levels of sulfide greatly restricted the availability of
zinc, copper, molybdenum, and cadmium, which
form insoluble sulfide precipitates; but it had a much
lesser impact on other metals (Mn
2þ , Fe
2þ , Co
2þ ,
and Ni
2þ ) whose sulfides are much more soluble.
Thus, early life in the ocean evolved in an environment of high availability of Fe, Mn, Co, and Ni and
low availabilities of Zn, Mo, Cu, and Cd, contrasting
the situation in the modern ocean. Given the utility
of Fe as a redox catalyst and its relative abundance in
the Earth’s crust and ancient ocean, it is perhaps not
surprising that this metal was utilized in the evolution of the major redox catylysts of life. It occurs in
high amounts in the redox centers of nitrogenase
responsible for dinitrogen fixation and in the various
proteins and protein complexes involved in oxygenic
photosynthesis (photosystem I, photosystem II,
26 TRACE ELEMENT NUTRIENTS
seawater (c. 105 nM), Mo does not appear to limit
algal growth or N 2 fixation in the ocean.
The metalloid selenium is also essential for the
growth of many marine phytoplankton. It occurs in
glutathione peroxidase, an enzyme that degrades
hydrogen peroxide, and thus is important in antioxidant protection. However, it is likely that selenium has other as-yet-unidentified metabolic
functions. The potential for selenium limitation in
the ocean is currently unknown.
Biological Feedback on Seawater
Chemistry
Trace elements not only influence the productivity
and species composition of planktonic communities,
but the plankton have a profound effect on the
chemistry and cycling of these elements on a variety
of temporal and spatial scales (Figure 1). The most
obvious example is the effect of algal uptake, particulate settling, and regeneration cycles on the vertical distribution and interocean transfer of trace
element nutrients (Fe, Zn, Cd, Ni, Cu, and Se; Figures 2–4). In addition, bacteria largely mediate the
removal of dissolved manganese and cobalt from
subsurface seawater via the formation of Mn(IV) and
Co(III) oxides.
There is evidence that the organic ligands that
strongly bind iron, copper, zinc, and cobalt are produced either directly or indirectly by the biota. In the
North Pacific, the organic ligands that strongly bind
copper occur at highest levels at the depth of maximum productivity, and decrease below the euphotic
zone. Ligands having the same copper-binding
strength are produced by Synechococcus, an abundant group of oceanic cyanobacteria. There is evidence that these organisms produce the chelators to
detoxify copper by decreasing free cupric ion concentrations. The organic ligands that strongly bind
iron(III), cobalt, and zinc also have a beneficial effect.
The iron ligands tightly bind ferric ions in soluble
chelates and thereby minimize the abiotic removal of
iron from seawater via the formation of insoluble
ferric oxides or ferric ion adsorption onto particulate
surfaces. Without such chelating ligands, iron concentrations would likely be much lower, and the
productivity of the ocean would be greatly reduced.
The Co(III)-binding ligands serve a similar function in
limiting the formation of insoluble Co(III) oxides, a
major mechanism for removal of cobalt from seawater. Recent culture experiments and seawater incubation experiments suggest that these ligands are
produced by the cyanobacterial genus Synechococcus,
whose growth may be limited by cobalt in some regions of the ocean.
Zinc chelators also serve a beneficial function, not
only by minimizing abiotic scavenging of zinc in
surface waters, but also by preventing the extremely
efficient uptake systems of eukaryotic phytoplankton
from completely depleting this essential micronutrient element from surface ocean waters.
Thus trace element nutrients and marine plankton
comprise an interactive system in the ocean in which
each exerts a controlling influence on the composition and dynamics of the other (Figure 1). On
longer geological timescales, the feedback interactions between the biota and trace metal chemistry
and availability have been profound. Currently, the
air we breathe and virtually the entire ocean, with
the exception of a few isolated anoxic basins (e.g.,
the Black Sea), contain free dioxygen molecules (O 2 ),
generated over billions of years from its release as a
byproduct of oxygenic photosynthesis. The presence
of free O 2 sets the redox state of modern ocean
toward oxidizing conditions, which as noted previously, limits the solubility of essential transition
metals (Fe, Co, and Mn) whose stable oxidation
states under these conditions are insoluble Co(III) and
Mn(IV) oxides or sparingly soluble Fe(III) oxides.
However, prior to the advent of oxygenic photosynthesis c. 3 billion years ago, the chemistry of the
ocean was far different from that which exists today.
There was no free oxygen and the entire ocean and
Earth’s surface was much more reducing. Under
these conditions, the stable redox state of Fe, Mn,
and Co was soluble Fe(II), Mn(II), and Co(II), and that
of copper was Cu(I). Furthermore, the stable redox
form of sulfur was sulfide (À 2 oxidation state), rather sulfate (þ 6 oxidation state), which occurs in
present-day seawater at a relatively high concentration (28 mM). The presence of moderate to high
levels of sulfide greatly restricted the availability of
zinc, copper, molybdenum, and cadmium, which
form insoluble sulfide precipitates; but it had a much
lesser impact on other metals (Mn
2þ , Fe
2þ , Co
2þ ,
and Ni
2þ ) whose sulfides are much more soluble.
Thus, early life in the ocean evolved in an environment of high availability of Fe, Mn, Co, and Ni and
low availabilities of Zn, Mo, Cu, and Cd, contrasting
the situation in the modern ocean. Given the utility
of Fe as a redox catalyst and its relative abundance in
the Earth’s crust and ancient ocean, it is perhaps not
surprising that this metal was utilized in the evolution of the major redox catylysts of life. It occurs in
high amounts in the redox centers of nitrogenase
responsible for dinitrogen fixation and in the various
proteins and protein complexes involved in oxygenic
photosynthesis (photosystem I, photosystem II,
26 TRACE ELEMENT NUTRIENTS