iron concentrations are much higher, phosphate is
the primary limiting nutrient. Due to iron limitation
of C fixation and N 2 fixation in major regions of
the ocean, iron plays a significant role in regulating
carbon and nitrogen cycles in the ocean. It thus helps
regulate the biological CO 2 pump discussed earlier,
which through transport of carbon to the deep
ocean, controls the ocean/atmosphere CO 2 balance
and CO 2 -linked greenhouse warming. There is evidence that climatically driven variations in the input
of iron-rich continental dust to the ocean has played
an important role in regulating glacial–interglacial
climate cycles.
Manganese occurs in the water-splitting complex
of photosystem II, and thus is essential for photosynthesis. Consequently, like iron, it is needed in
higher amounts for growth at low light. Manganese
also occurs in superoxide dismutase, an antioxidant
enzyme that removes toxic superoxide radicals,
produced as byproducts of photosynthesis. Because it
has fewer metabolic functions, its cellular growth
requirement is less than that of iron. Manganese
may limit algal growth in certain low-Mn environments such as the subarctic Pacific and Southern
Ocean, where manganese additions have been observed to stimulate algal growth in bottle incubation
experiments.
Zinc serves a variety of metabolic functions and
has a cellular requirement similar to that for manganese. It occurs in carbonic anhydrase (CA), an
enzyme critical to intracellular CO 2 transport and
fixation. Higher amounts of this enzyme are needed
at low CO 2 concentrations, leading to potential colimitation by zinc and CO 2 in the ocean. However,
the B35% increase in CO 2 in the atmosphere and
surface ocean waters from the burning of fossil fuels
makes Zn–CO 2 co-limitation less likely in the modern ocean than in preindustrial times. Zinc also occurs in zinc finger proteins, involved in DNA
transcription, and in alkaline phosphatase, needed to
acquire phosphorus from organic phosphate esters,
which dominate phosphate pools in low-phosphate
ocean waters. Consequently, Zn and P may co-limit
algal growth in regions where both nutrients occur at
low concentrations such as the central gyre of the
North Atlantic.
Cobalt, and sometimes cadmium, can substitute
for zinc in many zinc enzymes such as CA, leading
to complex interactions among the three metals in
marine algae (Figure 5). The presence of cadmium
in CA appears to explain its nutrient-like distribution
in ocean waters (Figure 2(e)), and the identification
of a unique Cd-CA enzyme in marine diatoms means
that it functions as a micronutrient in these organisms. Cobalt also occurs in vitamin B 12 , an essential
vitamin required for growth of many eukaryotic
algal species. This vitamin is synthesized only by
bacteria, resulting in potential interactions between
B 12 -producing bacteria and B 12 -requiring eukaryotic
algae in the ocean. A specific requirement for cobalt
not involving B 12 is seen in marine cyanobacteria
and bloom-forming prymnesiophytes (including
Emiliania huxleyi), but the biochemical basis for this
is not known. Both zinc and cobalt additions have
been shown to stimulate phytoplankton growth in
bottle incubation experiments in the subarctic Pacific
and in some coastal upwelling regimes along
the eastern margin of the Pacific, but the effects
were modest relative to those for added iron. However, zinc addition had a large effect on algal species
composition, and preferentially stimulated the
growth of coccolithophores, an algal group largely
responsible for calcium carbonate formation in the
ocean. Biogenic CaCO 3 formation helps regulate the
alkalinity (acid–base balance) of ocean water, which
in turn affects oceanic CO 2 concentrations, and air–
sea flux of this important greenhouse gas. By influencing the growth of coccolithophores, zinc could
indirectly affect atmospheric CO 2 levels and global
climate.
Copper occurs in cytochrome oxidase, a key protein in respiratory electron transport, and in plastocyanin, which substitutes for the iron protein
cytochrome c 6 in photosynthetic electron transport
in oceanic phytoplankton. It is also an essential
component of the high-affinity iron transport system
of many eukaryotic algae. Because copper is needed
for iron uptake and can metabolically substitute for
iron, co-limitations can occur for Cu and Fe, as observed in some diatoms.
Nickel and molybdenum, like iron, play important
roles in nitrogen assimilation. Nickel occurs in the
enzyme urease, and thus is required by phytoplankton grown on urea as a nitrogen source. It also
occurs in Ni-superoxide dismutase found in many
marine cyanobacteria, which, like the Mn and Fe
forms of the enzyme, removes harmful superoxide
radicals from cells. Little is currently known about
the potential for nickel limitation in the ocean.
Molybdenum occurs with iron in the enzymes
nitrate reductase and nitrite reductase and in nitrogenase, and consequently is utilized in nitrate assimilation and N 2 fixation. Along with the Fe–Mo
enzyme, there are two other isoforms of nitrogenase,
a primitive less-efficient form containing only iron in
its active center, and another which contains iron and
vanadium. Thus molybdenum is not absolutely essential for dinitrogen fixation, although the predominance of the more efficient Fe–Mo isoform in the
modern ocean helps to minimize iron limitation of
TRACE ELEMENT NUTRIENTS 25
the primary limiting nutrient. Due to iron limitation
of C fixation and N 2 fixation in major regions of
the ocean, iron plays a significant role in regulating
carbon and nitrogen cycles in the ocean. It thus helps
regulate the biological CO 2 pump discussed earlier,
which through transport of carbon to the deep
ocean, controls the ocean/atmosphere CO 2 balance
and CO 2 -linked greenhouse warming. There is evidence that climatically driven variations in the input
of iron-rich continental dust to the ocean has played
an important role in regulating glacial–interglacial
climate cycles.
Manganese occurs in the water-splitting complex
of photosystem II, and thus is essential for photosynthesis. Consequently, like iron, it is needed in
higher amounts for growth at low light. Manganese
also occurs in superoxide dismutase, an antioxidant
enzyme that removes toxic superoxide radicals,
produced as byproducts of photosynthesis. Because it
has fewer metabolic functions, its cellular growth
requirement is less than that of iron. Manganese
may limit algal growth in certain low-Mn environments such as the subarctic Pacific and Southern
Ocean, where manganese additions have been observed to stimulate algal growth in bottle incubation
experiments.
Zinc serves a variety of metabolic functions and
has a cellular requirement similar to that for manganese. It occurs in carbonic anhydrase (CA), an
enzyme critical to intracellular CO 2 transport and
fixation. Higher amounts of this enzyme are needed
at low CO 2 concentrations, leading to potential colimitation by zinc and CO 2 in the ocean. However,
the B35% increase in CO 2 in the atmosphere and
surface ocean waters from the burning of fossil fuels
makes Zn–CO 2 co-limitation less likely in the modern ocean than in preindustrial times. Zinc also occurs in zinc finger proteins, involved in DNA
transcription, and in alkaline phosphatase, needed to
acquire phosphorus from organic phosphate esters,
which dominate phosphate pools in low-phosphate
ocean waters. Consequently, Zn and P may co-limit
algal growth in regions where both nutrients occur at
low concentrations such as the central gyre of the
North Atlantic.
Cobalt, and sometimes cadmium, can substitute
for zinc in many zinc enzymes such as CA, leading
to complex interactions among the three metals in
marine algae (Figure 5). The presence of cadmium
in CA appears to explain its nutrient-like distribution
in ocean waters (Figure 2(e)), and the identification
of a unique Cd-CA enzyme in marine diatoms means
that it functions as a micronutrient in these organisms. Cobalt also occurs in vitamin B 12 , an essential
vitamin required for growth of many eukaryotic
algal species. This vitamin is synthesized only by
bacteria, resulting in potential interactions between
B 12 -producing bacteria and B 12 -requiring eukaryotic
algae in the ocean. A specific requirement for cobalt
not involving B 12 is seen in marine cyanobacteria
and bloom-forming prymnesiophytes (including
Emiliania huxleyi), but the biochemical basis for this
is not known. Both zinc and cobalt additions have
been shown to stimulate phytoplankton growth in
bottle incubation experiments in the subarctic Pacific
and in some coastal upwelling regimes along
the eastern margin of the Pacific, but the effects
were modest relative to those for added iron. However, zinc addition had a large effect on algal species
composition, and preferentially stimulated the
growth of coccolithophores, an algal group largely
responsible for calcium carbonate formation in the
ocean. Biogenic CaCO 3 formation helps regulate the
alkalinity (acid–base balance) of ocean water, which
in turn affects oceanic CO 2 concentrations, and air–
sea flux of this important greenhouse gas. By influencing the growth of coccolithophores, zinc could
indirectly affect atmospheric CO 2 levels and global
climate.
Copper occurs in cytochrome oxidase, a key protein in respiratory electron transport, and in plastocyanin, which substitutes for the iron protein
cytochrome c 6 in photosynthetic electron transport
in oceanic phytoplankton. It is also an essential
component of the high-affinity iron transport system
of many eukaryotic algae. Because copper is needed
for iron uptake and can metabolically substitute for
iron, co-limitations can occur for Cu and Fe, as observed in some diatoms.
Nickel and molybdenum, like iron, play important
roles in nitrogen assimilation. Nickel occurs in the
enzyme urease, and thus is required by phytoplankton grown on urea as a nitrogen source. It also
occurs in Ni-superoxide dismutase found in many
marine cyanobacteria, which, like the Mn and Fe
forms of the enzyme, removes harmful superoxide
radicals from cells. Little is currently known about
the potential for nickel limitation in the ocean.
Molybdenum occurs with iron in the enzymes
nitrate reductase and nitrite reductase and in nitrogenase, and consequently is utilized in nitrate assimilation and N 2 fixation. Along with the Fe–Mo
enzyme, there are two other isoforms of nitrogenase,
a primitive less-efficient form containing only iron in
its active center, and another which contains iron and
vanadium. Thus molybdenum is not absolutely essential for dinitrogen fixation, although the predominance of the more efficient Fe–Mo isoform in the
modern ocean helps to minimize iron limitation of
TRACE ELEMENT NUTRIENTS 25