can also be reduced. These reduction processes can be
related to energetic metabolisms (dissimilatory reduction),
or can be the result of an indirect reduction (reduction by
reduced iron or sulfide). The reduction of these elements is
interesting in the case of treatment processes of
contaminated sites, because the reduced forms are often
less toxic and/or less soluble (unlike iron and manganese).
14.5.2 The Cycle of Iron
14.5.2.1 Iron in the Environment
Iron is quantitatively the fourth constitutive element of the
earth’s crust. It is abundant in the continents and rare in the
oceans where iron resources are often a limiting factor for
primary production (Box 14.15).
Iron is present in the environment as oxides, carbonates,
sulfides, and hydroxides. It exists in different oxidation
states, ranging from 0 to +VI (ferrate), but only two of
these states are involved in biological pathways: Fe
2+ (II)
which is the reduced ferrous form, and Fe
3+ (III) which is
the oxidized ferric form. Iron is an essential element in some
enzymes: cytochromes, catalases, peroxidases, nitrite
reductases, and iron-sulfur metalloprotein complexes. It is
therefore an essential element in respiratory and photosynthetic processes.
The greater part of iron found in the environment is
insoluble, the solubility of ferric ion being very low at
physiological pH. The solubility of Fe(II) form is substantially greater, but in anoxic conditions. However, under
these conditions, iron is usually precipitated as black iron
sulfide (FeS) or even as pyrite (FeS 2 ), the last one forming a
crystalline structure, highly insoluble and very difficult to
oxidize (Fig. 14.45, pathways 1 and 3).
The oxidation of pyrite requires the sum of chemical and
biological reactions involving two electron acceptors:
dioxygen and ferric ions. In the presence of sulfide (S
2À
)
and when anaerobic conditions prevail, ferric iron (Fe
3+ ) is
chemically (i.e. by abiotic processes) reduced to sulfide
(FeS) and ferrous iron (Fe
2+
) (Fig. 14.45, pathways 1 and
2). During this reaction, a small portion of sulfide (S
2À ) is
oxidized to sulfur (S
) which participates in the formation of
pyrite by association with sulfide iron (Fig. 14.45, pathway
3). Sulfide iron can be chemically oxidized, in the presence
of oxygen, to ferric iron by releasing sulfur and thiosulfate
(S 2 O 3
2À ) (Fig. 14.45, pathway 4).
14.5.2.2 Iron Assimilation by Microorganisms
The inventory of biological functions of iron shows that this
element is essential to life. In the cell, the iron must be in the
Fe
2+ state. Due to its low solubility, living organisms have
developed systems to capture and incorporate iron
(Fig. 14.45, pathway 5). Indeed, many microorganisms
produce a wide variety of small organic molecules,
siderophores, capable of fixing Fe
3+ with a very high efficiency, ensuring its transport within the cell where it is
reduced and incorporated into the organic material.
Some aquatic bacteria, described as magnetotactic
microorganisms, such as Aquaspirillum magnetotacticum,
Magnetotacticum bavaricum, and Magnetospirillum
magnetotacticum, can transform iron extracellularly to crystal of magnetite (Fe 3 O 4 ) and construct “intracellular magnetic compasses” called magnetosomes (Bazylinski and
Frankel 2004), which are included in cytoplasmic vesicles
(Figure 14.46). Some magnetotactic bacteria do not synthesize magnetite but greigite (Fe 3 S 4 ) (Farina et al. 1990). With
this structure, these motile bacteria can move to areas rich in
organic matter (water-sediment interface), by following the
Earth’s magnetic field. High concentrations of magnetotactic
bacteria were detected in anoxic marine sediments at
3,000 m depth (Petermann and Bleil 1993).
Box 14.14 (continued)
All of these mechanisms are not yet known but
some pathways have been described (Morel et al.
2008). When iron is limiting, a number of bacteria
excrete molecules, siderophores (cf. Sect. 14.5.2) that
forms strong complex with iron. That allows them to
extract iron from other chemical species present in the
environment. The iron-siderophore complex is then
taken up by bacteria (Butler 1998). Eukaryotes seem
to have adopted a different strategy, which consists in
reducing Fe (III) at the surface of the cell to form iron
(II). This facilitates the dissociation of the organic
complexes and allows the transport iron into the cell.
Other mechanisms are likely to be discovered in the
future, such as direct transport of other iron complex
like heme-type. Detailed analysis of the growing number of genomes available for marine microorganisms
should allow rapid progress in this direction.
The Iron Cycle and Climate Geoengineering
The major role of iron in controlling the biological
pump of CO 2 have inspired geo engeneering proposals
describing large scale iron fertilization of the ocean to
mitigate the atmospheric CO 2 increase. A very large
majority the scientific community does not support this
idea. Indeed, the efficiency of a such manipulation of
the ocean is not firmly proven. The amount of carbon
that would be eventually stored would be very difficult
to assess. In addition, the side effects of such large scale
manipulations of a global ecosystem are not known and
could be dangerous (Strong et al. 2009).
14 Biogeochemical Cycles
603
related to energetic metabolisms (dissimilatory reduction),
or can be the result of an indirect reduction (reduction by
reduced iron or sulfide). The reduction of these elements is
interesting in the case of treatment processes of
contaminated sites, because the reduced forms are often
less toxic and/or less soluble (unlike iron and manganese).
14.5.2 The Cycle of Iron
14.5.2.1 Iron in the Environment
Iron is quantitatively the fourth constitutive element of the
earth’s crust. It is abundant in the continents and rare in the
oceans where iron resources are often a limiting factor for
primary production (Box 14.15).
Iron is present in the environment as oxides, carbonates,
sulfides, and hydroxides. It exists in different oxidation
states, ranging from 0 to +VI (ferrate), but only two of
these states are involved in biological pathways: Fe
2+ (II)
which is the reduced ferrous form, and Fe
3+ (III) which is
the oxidized ferric form. Iron is an essential element in some
enzymes: cytochromes, catalases, peroxidases, nitrite
reductases, and iron-sulfur metalloprotein complexes. It is
therefore an essential element in respiratory and photosynthetic processes.
The greater part of iron found in the environment is
insoluble, the solubility of ferric ion being very low at
physiological pH. The solubility of Fe(II) form is substantially greater, but in anoxic conditions. However, under
these conditions, iron is usually precipitated as black iron
sulfide (FeS) or even as pyrite (FeS 2 ), the last one forming a
crystalline structure, highly insoluble and very difficult to
oxidize (Fig. 14.45, pathways 1 and 3).
The oxidation of pyrite requires the sum of chemical and
biological reactions involving two electron acceptors:
dioxygen and ferric ions. In the presence of sulfide (S
2À
)
and when anaerobic conditions prevail, ferric iron (Fe
3+ ) is
chemically (i.e. by abiotic processes) reduced to sulfide
(FeS) and ferrous iron (Fe
2+
) (Fig. 14.45, pathways 1 and
2). During this reaction, a small portion of sulfide (S
2À ) is
oxidized to sulfur (S
) which participates in the formation of
pyrite by association with sulfide iron (Fig. 14.45, pathway
3). Sulfide iron can be chemically oxidized, in the presence
of oxygen, to ferric iron by releasing sulfur and thiosulfate
(S 2 O 3
2À ) (Fig. 14.45, pathway 4).
14.5.2.2 Iron Assimilation by Microorganisms
The inventory of biological functions of iron shows that this
element is essential to life. In the cell, the iron must be in the
Fe
2+ state. Due to its low solubility, living organisms have
developed systems to capture and incorporate iron
(Fig. 14.45, pathway 5). Indeed, many microorganisms
produce a wide variety of small organic molecules,
siderophores, capable of fixing Fe
3+ with a very high efficiency, ensuring its transport within the cell where it is
reduced and incorporated into the organic material.
Some aquatic bacteria, described as magnetotactic
microorganisms, such as Aquaspirillum magnetotacticum,
Magnetotacticum bavaricum, and Magnetospirillum
magnetotacticum, can transform iron extracellularly to crystal of magnetite (Fe 3 O 4 ) and construct “intracellular magnetic compasses” called magnetosomes (Bazylinski and
Frankel 2004), which are included in cytoplasmic vesicles
(Figure 14.46). Some magnetotactic bacteria do not synthesize magnetite but greigite (Fe 3 S 4 ) (Farina et al. 1990). With
this structure, these motile bacteria can move to areas rich in
organic matter (water-sediment interface), by following the
Earth’s magnetic field. High concentrations of magnetotactic
bacteria were detected in anoxic marine sediments at
3,000 m depth (Petermann and Bleil 1993).
Box 14.14 (continued)
All of these mechanisms are not yet known but
some pathways have been described (Morel et al.
2008). When iron is limiting, a number of bacteria
excrete molecules, siderophores (cf. Sect. 14.5.2) that
forms strong complex with iron. That allows them to
extract iron from other chemical species present in the
environment. The iron-siderophore complex is then
taken up by bacteria (Butler 1998). Eukaryotes seem
to have adopted a different strategy, which consists in
reducing Fe (III) at the surface of the cell to form iron
(II). This facilitates the dissociation of the organic
complexes and allows the transport iron into the cell.
Other mechanisms are likely to be discovered in the
future, such as direct transport of other iron complex
like heme-type. Detailed analysis of the growing number of genomes available for marine microorganisms
should allow rapid progress in this direction.
The Iron Cycle and Climate Geoengineering
The major role of iron in controlling the biological
pump of CO 2 have inspired geo engeneering proposals
describing large scale iron fertilization of the ocean to
mitigate the atmospheric CO 2 increase. A very large
majority the scientific community does not support this
idea. Indeed, the efficiency of a such manipulation of
the ocean is not firmly proven. The amount of carbon
that would be eventually stored would be very difficult
to assess. In addition, the side effects of such large scale
manipulations of a global ecosystem are not known and
could be dangerous (Strong et al. 2009).
14 Biogeochemical Cycles
603