phosphate, silicate, carbon dioxide partial pressure,
pH, alkalinity, total carbon dioxide, iron-binding
ligands,
234
Th :
238 U radioisotopic disequilibria (a
proxy for particle removal), relative fluorescence
(indicator of photosynthetic competence), primary
production, phytoplankton and zooplankton enumeration, grazing rates, nitrate uptake, and particulate and dissolved organic carbon and nitrogen.
These parameters allow for the general characterization of both the biological and geochemical response to added iron. From the results of the
equatorial enrichment experiments (IronEx I and II)
and the Southern Ocean Iron Enrichment Experiment (SOIREE), several general features have been
identified.
Findings to Date
Biophysical Response
The experiments to date have focused on the highnitrate, low-chlorophyll (HNLC) areas of the world’s
oceans, primarily in the Subarctic, equatorial Pacific
and Southern Ocean. In general, when light is
abundant many researchers find that HNLC systems
are iron-limited. The nature of this limitation is
similar between regions but manifests itself at different levels of the trophic structure in some characteristic ways. In general, all members of the HNLC
photosynthetic community are physiologically limited by iron availability. This observation is based
primarily on the examination of the efficiency of
photosystem II, the light-harvesting reaction centers.
At ambient levels of iron, light harvesting proceeds at
suboptimal rates. This has been attributed to the lack
of iron-dependent electron carrier proteins at low
iron concentrations. When iron concentrations are
increased by subnanomolar amounts, the efficiency
of light harvesting rapidly increases to maximum
levels. Using fast repetition rate fluorometry and
non-heme iron proteins, researchers have described
these observations in detail. What is notable about
these results is that iron limitation seems to affect the
photosynthetic energy conversion efficiency of even
the smallest of phytoplankton. This has been a
unique finding that stands in contrast to the hypothesis that, because of diffusion, smaller cells are
not iron limited but larger cells are.
Nitrate Uptake
As discussed above, iron is also required for the reduction (assimilation) of nitrate. In fact, a change of
oxidation state of five is required between nitrate
and the reduced forms of nitrogen found in amino
acids and proteins. Such a large and energetically
unfavorable redox process is only made possible by
substantial reducing power (in the form of NADPH)
made available through photosynthesis and active
nitrate reductase, an iron-requiring enzyme. Without
iron, plants cannot take up nitrate efficiently. This
provided original evidence implicating iron deficiency as the cause of the HNLC condition. When
phytoplankton communities are relieved from iron
deficiency, specific rates of nitrate uptake increase.
This has been observed in both the equatorial Pacific
and the Southern Ocean using isotopic tracers of
nitrate uptake and conversion. In addition, the accelerated uptake of nitrate has been observed in both
the mesoscale iron enrichment experiments to date,
IronEx and SOIREE.
Growth Response
When iron is present, phytoplankton growth rates
increase dramatically. Experiments over widely differing oceanographic regimes have demonstrated
that, when light and temperature are favorable,
phytoplankton growth rates in HNLC environments
increase to their maximum at dissolved iron concentrations generally below 0.5 nmol l
À1 . This observation is significant in that it indicates that
phytoplankton are adapted to very low levels of iron
and they do not grow faster if given iron at more
than 0.5 nmol l
À1
. Given that there is still some disagreement within the scientific community about the
validity of some iron measurements, this phytoplankton response provides a natural, environmental, and biogeochemical benchmark against
which to compare results.
The iron-induced transient imbalance between
phytoplankton growth and grazing in the equatorial
Pacific during IronEx II resulted in a 30-fold increase
in plant biomass (Figure 7). Similarly, a 6-fold increase was observed during the SOIREE experiment
in the Southern Ocean. These are perhaps the most
dramatic demonstrations of iron limitation of nutrient cycling, and phytoplankton growth to date and
has fortified the notion that iron fertilization may be
a useful strategy to sequester carbon in the oceans.
Heterotrophic Community
As the primary trophic levels increase in biomass,
growth in the small microflagellate and heterotrophic bacterial communities increase in kind. It
appears that these consumers of recently fixed carbon (both particulate and dissolved) respond to the
food source and not necessarily the iron (although
some have been found to be iron-limited). Because
their division rates are fast, heterotrophic bacteria,
ciliates, and flagellates can rapidly divide and
106 IRON FERTILIZATION
pH, alkalinity, total carbon dioxide, iron-binding
ligands,
234
Th :
238 U radioisotopic disequilibria (a
proxy for particle removal), relative fluorescence
(indicator of photosynthetic competence), primary
production, phytoplankton and zooplankton enumeration, grazing rates, nitrate uptake, and particulate and dissolved organic carbon and nitrogen.
These parameters allow for the general characterization of both the biological and geochemical response to added iron. From the results of the
equatorial enrichment experiments (IronEx I and II)
and the Southern Ocean Iron Enrichment Experiment (SOIREE), several general features have been
identified.
Findings to Date
Biophysical Response
The experiments to date have focused on the highnitrate, low-chlorophyll (HNLC) areas of the world’s
oceans, primarily in the Subarctic, equatorial Pacific
and Southern Ocean. In general, when light is
abundant many researchers find that HNLC systems
are iron-limited. The nature of this limitation is
similar between regions but manifests itself at different levels of the trophic structure in some characteristic ways. In general, all members of the HNLC
photosynthetic community are physiologically limited by iron availability. This observation is based
primarily on the examination of the efficiency of
photosystem II, the light-harvesting reaction centers.
At ambient levels of iron, light harvesting proceeds at
suboptimal rates. This has been attributed to the lack
of iron-dependent electron carrier proteins at low
iron concentrations. When iron concentrations are
increased by subnanomolar amounts, the efficiency
of light harvesting rapidly increases to maximum
levels. Using fast repetition rate fluorometry and
non-heme iron proteins, researchers have described
these observations in detail. What is notable about
these results is that iron limitation seems to affect the
photosynthetic energy conversion efficiency of even
the smallest of phytoplankton. This has been a
unique finding that stands in contrast to the hypothesis that, because of diffusion, smaller cells are
not iron limited but larger cells are.
Nitrate Uptake
As discussed above, iron is also required for the reduction (assimilation) of nitrate. In fact, a change of
oxidation state of five is required between nitrate
and the reduced forms of nitrogen found in amino
acids and proteins. Such a large and energetically
unfavorable redox process is only made possible by
substantial reducing power (in the form of NADPH)
made available through photosynthesis and active
nitrate reductase, an iron-requiring enzyme. Without
iron, plants cannot take up nitrate efficiently. This
provided original evidence implicating iron deficiency as the cause of the HNLC condition. When
phytoplankton communities are relieved from iron
deficiency, specific rates of nitrate uptake increase.
This has been observed in both the equatorial Pacific
and the Southern Ocean using isotopic tracers of
nitrate uptake and conversion. In addition, the accelerated uptake of nitrate has been observed in both
the mesoscale iron enrichment experiments to date,
IronEx and SOIREE.
Growth Response
When iron is present, phytoplankton growth rates
increase dramatically. Experiments over widely differing oceanographic regimes have demonstrated
that, when light and temperature are favorable,
phytoplankton growth rates in HNLC environments
increase to their maximum at dissolved iron concentrations generally below 0.5 nmol l
À1 . This observation is significant in that it indicates that
phytoplankton are adapted to very low levels of iron
and they do not grow faster if given iron at more
than 0.5 nmol l
À1
. Given that there is still some disagreement within the scientific community about the
validity of some iron measurements, this phytoplankton response provides a natural, environmental, and biogeochemical benchmark against
which to compare results.
The iron-induced transient imbalance between
phytoplankton growth and grazing in the equatorial
Pacific during IronEx II resulted in a 30-fold increase
in plant biomass (Figure 7). Similarly, a 6-fold increase was observed during the SOIREE experiment
in the Southern Ocean. These are perhaps the most
dramatic demonstrations of iron limitation of nutrient cycling, and phytoplankton growth to date and
has fortified the notion that iron fertilization may be
a useful strategy to sequester carbon in the oceans.
Heterotrophic Community
As the primary trophic levels increase in biomass,
growth in the small microflagellate and heterotrophic bacterial communities increase in kind. It
appears that these consumers of recently fixed carbon (both particulate and dissolved) respond to the
food source and not necessarily the iron (although
some have been found to be iron-limited). Because
their division rates are fast, heterotrophic bacteria,
ciliates, and flagellates can rapidly divide and
106 IRON FERTILIZATION