of many metals and metalloids, including reduction processes
and, more rarely, oxidation activities. Some microorganisms
are capable of growth by using reduced forms of arsenic and
selenium as electron donors. The case of arsenic to which
certain microorganisms oxidize arsenite (As(III)) to arsenate
(As(V)) is the best documented (Oremland and Stolz 2003)
and particularly interesting. Indeed, in the case of arsenic, the
reduced form, i.e. arsenite, is more toxic because its availability is higher compared with the oxidized form. Some
microorganisms that oxidize As(III) use this ability as a
resistance mechanism to detoxify As(III) to As (V), which is
less toxic and less prone to enter the cell. Other organisms,
chemolithotrophs, are capable of coupling the oxidation of As
(III) (electron donor) to the reduction of oxygen or nitrates
involved as terminal electrons acceptors.
Among the many metals which can be reduced by microorganisms, uranium U(VI) can be reduced to uranium U(IV)
or chromium Cr(VI) to chromium Cr(III), but arsenic (As),
selenium (Se), vanadium (V), molybdenum (Mo), copper
(Cu), gold (Au), mercury (Hg; Box 14.14), or silver (Ag)
Box 14.14: Iron in Ocean
Stephane Blain
The Iron Cycle in the Ocean
Concentrations of dissolved iron in the modern
ocean are extremely low (often less than 1 nanomole.
L
À1 ). This is partly due to the low solubility of this
element in seawater, which is an oxygenated lightly
alkaline environment (pH ¼ 8). However, over geological time this has not always been the case. Notably,
iron concentrations were much higher during the
Archean (3.8–2.5 Ma) and Proterozoic (2.5–0.5 Ma),
periods when oxygen was absent or found at very low
levels. The first living organisms appeared in the
oceans during these periods have therefore widely
incorporated iron into their metabolisms. Indeed, iron
has two oxidation states, Fe (II) and Fe (III), which
makes it particularly useful for electron transport within
the cells. Iron has become an essential element in many
biochemical functions, such as respiration, photosynthesis, dinitrogen fixation, and nitrate reduction.
The discovery of low levels of iron in the ocean is
relatively recent. Indeed, measuring iron concentrations in the ocean requires so-called ultra-clean
S. Blain (*)
Laboratoire d’Oce ´anographie Microbienne, LOMIC UMR
CNRS-UPMC, Observatoire Oce ´anologique de Banyuls, 66650
Banyuls-sur-Mer, France
(continued)
Box 14.14 (continued)
techniques first used in oceanography in the 80s
(Bruland et al. 1979). They are based on non-metallic
sampling equipments and they follow strict protocols
for storage and analysis of the samples. The low iron
supply to the ocean compared with the physiological
demand of the marine microorganisms results in a
limitation of photosynthesis and growth of phytoplankton in very large ocean areas (e.g., Southern
Ocean, Equatorial sub-Arctic Pacific). This has profound climatic implications because the process
called biological pump of CO 2 , which includes photosynthesis and transport of organic carbon in the
deep layers of the ocean and ultimately to the
sediments, is the main oceanic mechanism which
removes CO 2 from the atmosphere (Sarmiento and
Gruber 2006).
A major breakthrough in the demonstration of iron
limitation of biological production in the ocean has
clearly resulted from the realization of a dozen of
artificial fertilization experiments on a small scale
(approximately 100 km
2 ) between 1993 and 2009.
During these experiments, the addition of iron (several
tons) stimulates organic production in the surface layer
(Boyd et al. 2007). This results in an increase in the
concentration of chlorophyll and a decrease of the
partial pressure of CO 2 in water. Fertilization alters
the structure of the phytoplankton community in promoting growth diatoms. In some experiments, the
effect on the second trophic level (zooplankton) was
also observed. These experiments did not, however,
clearly demonstrated that the carbon was then transferred into the deep ocean. But, this was demonstrated
in the study of a region naturally fertilized by iron: the
Kerguelen Plateau in the Southern Ocean (Blain et al
2007). This region has high chlorophyll concentrations, while the surrounding waters are much poor
(Box Fig. 14.23). In this study, it was demonstrated
that the increase in biological activity was caused by
an enhanced iron supply, and that this led to an
increase in the deep export of carbon.
Natural fertilization of surface waters of the
Southern Ocean therefore enhanced the biological
pump of CO 2 .
In the present ocean, the main external source of
iron is the deposition of dust from natural (deserts) or
anthropogenic (industry, transport) origins. Two
types of depositions are distinguished: the wet deposition (rain) and the dry deposition (Jickells et al.
2005). The impact of these deposits on the biological
(continued)
14 Biogeochemical Cycles
601
and, more rarely, oxidation activities. Some microorganisms
are capable of growth by using reduced forms of arsenic and
selenium as electron donors. The case of arsenic to which
certain microorganisms oxidize arsenite (As(III)) to arsenate
(As(V)) is the best documented (Oremland and Stolz 2003)
and particularly interesting. Indeed, in the case of arsenic, the
reduced form, i.e. arsenite, is more toxic because its availability is higher compared with the oxidized form. Some
microorganisms that oxidize As(III) use this ability as a
resistance mechanism to detoxify As(III) to As (V), which is
less toxic and less prone to enter the cell. Other organisms,
chemolithotrophs, are capable of coupling the oxidation of As
(III) (electron donor) to the reduction of oxygen or nitrates
involved as terminal electrons acceptors.
Among the many metals which can be reduced by microorganisms, uranium U(VI) can be reduced to uranium U(IV)
or chromium Cr(VI) to chromium Cr(III), but arsenic (As),
selenium (Se), vanadium (V), molybdenum (Mo), copper
(Cu), gold (Au), mercury (Hg; Box 14.14), or silver (Ag)
Box 14.14: Iron in Ocean
Stephane Blain
The Iron Cycle in the Ocean
Concentrations of dissolved iron in the modern
ocean are extremely low (often less than 1 nanomole.
L
À1 ). This is partly due to the low solubility of this
element in seawater, which is an oxygenated lightly
alkaline environment (pH ¼ 8). However, over geological time this has not always been the case. Notably,
iron concentrations were much higher during the
Archean (3.8–2.5 Ma) and Proterozoic (2.5–0.5 Ma),
periods when oxygen was absent or found at very low
levels. The first living organisms appeared in the
oceans during these periods have therefore widely
incorporated iron into their metabolisms. Indeed, iron
has two oxidation states, Fe (II) and Fe (III), which
makes it particularly useful for electron transport within
the cells. Iron has become an essential element in many
biochemical functions, such as respiration, photosynthesis, dinitrogen fixation, and nitrate reduction.
The discovery of low levels of iron in the ocean is
relatively recent. Indeed, measuring iron concentrations in the ocean requires so-called ultra-clean
S. Blain (*)
Laboratoire d’Oce ´anographie Microbienne, LOMIC UMR
CNRS-UPMC, Observatoire Oce ´anologique de Banyuls, 66650
Banyuls-sur-Mer, France
(continued)
Box 14.14 (continued)
techniques first used in oceanography in the 80s
(Bruland et al. 1979). They are based on non-metallic
sampling equipments and they follow strict protocols
for storage and analysis of the samples. The low iron
supply to the ocean compared with the physiological
demand of the marine microorganisms results in a
limitation of photosynthesis and growth of phytoplankton in very large ocean areas (e.g., Southern
Ocean, Equatorial sub-Arctic Pacific). This has profound climatic implications because the process
called biological pump of CO 2 , which includes photosynthesis and transport of organic carbon in the
deep layers of the ocean and ultimately to the
sediments, is the main oceanic mechanism which
removes CO 2 from the atmosphere (Sarmiento and
Gruber 2006).
A major breakthrough in the demonstration of iron
limitation of biological production in the ocean has
clearly resulted from the realization of a dozen of
artificial fertilization experiments on a small scale
(approximately 100 km
2 ) between 1993 and 2009.
During these experiments, the addition of iron (several
tons) stimulates organic production in the surface layer
(Boyd et al. 2007). This results in an increase in the
concentration of chlorophyll and a decrease of the
partial pressure of CO 2 in water. Fertilization alters
the structure of the phytoplankton community in promoting growth diatoms. In some experiments, the
effect on the second trophic level (zooplankton) was
also observed. These experiments did not, however,
clearly demonstrated that the carbon was then transferred into the deep ocean. But, this was demonstrated
in the study of a region naturally fertilized by iron: the
Kerguelen Plateau in the Southern Ocean (Blain et al
2007). This region has high chlorophyll concentrations, while the surrounding waters are much poor
(Box Fig. 14.23). In this study, it was demonstrated
that the increase in biological activity was caused by
an enhanced iron supply, and that this led to an
increase in the deep export of carbon.
Natural fertilization of surface waters of the
Southern Ocean therefore enhanced the biological
pump of CO 2 .
In the present ocean, the main external source of
iron is the deposition of dust from natural (deserts) or
anthropogenic (industry, transport) origins. Two
types of depositions are distinguished: the wet deposition (rain) and the dry deposition (Jickells et al.
2005). The impact of these deposits on the biological
(continued)
14 Biogeochemical Cycles
601