(formation of iron metal sulfides, manganese, zinc, copper),
differentiating black from white smokers. These sources
are also enriched in oxidized (CO, CO 2 ) and reduced
(CH 4 ) carbon, and in calcium which precipitates in the
form of CaSO 4 in contact with oceanic waters. This
CaSO 4 is at the origin of the chimney wall formation of
smokers, which then are enriched in metal sulfides and in
iron and manganese oxides.
Ecosystems that develop in deep hydrothermal vent
receive no solar radiation and biomass production is due
to chemosynthetic processes (Jannasch and Mottl 1985).
Autotrophic microorganisms that grow in these ecosystems
used reduced sulfur compounds (sulfur-oxidizing
chemolithoautotrophs) and other reduced mineral
compounds (such as NH 3 , H 2 , Fe
2+ , or Mn
2+ ) as energy
sources. Methylotrophic microorganisms also colonize
these environments. Many hyperthermophilic and thermophilic microorganisms grow directly on the walls of smokers
in the form of microbial mats, or in the water around the
plume of the hydrothermal source. Chemosynthetic
microorganisms are at the origin of a food chain involving
planktonic or benthic animals, filter-feeding organisms and
grazers, including worms, crustaceans, molluscs, and fishes.
The distribution of animal populations is established
according to the distance to the smoker, the different species
being arranged in concentric rings around the sourceejection depending on the prevailing extreme conditions
(temperature, pH, concentrations of sulfide, and metal).
In addition, in these ecosystems, symbioses between
chemolithotrophic sulfur-oxidizing microorganisms and
metazoans (worms, bivalves) have been described (cf. Sect.
10.3.1). For example, red giant tubeworms (Riftia
pachyptila), which lack digestive tube, feed through the
symbiotic activity of sulfur-oxidizing microorganisms
(endobionts) housed in a particular organ, the trophosome.
The substrates required for the growth of microorganisms
(H 2 S, CO 2 , and O 2 ) are provided by the circulatory system of
the worm, and part of the organic matter synthesized by
bacteria is used by worms for their diet. Other symbioses
involving exobiontes, often located in the gills of aquatic
organisms in bacteriocytes, have also been described.
Such symbioses also exist in coastal marine sediments
between bivalves (clams) and chemosynthetic sulfuroxidizing bacteria.
Because of the total absence of solar light, deep hydrothermal ecosystems therefore seem to exclusively depend
on biomass production by chemosynthetic processes.
However, it was discovered that these areas also contain
phototrophic green sulfur bacteria (Prosthecochloris sp.)
which use geothermal radiations (near-infrared radiations between 750 and 1,050 nm) as energy source.
The question of their exact role still remains open (Beatty
et al. 2005).
14.5 Cycles of Metals and Metalloids
14.5.1 Introduction
Many microorganisms are involved in the biogeochemical
cycles of metals and metalloids, primarily by modifying their
oxidation level and/or their chemical forms. Among these
elements, some are essential to life (iron, manganese, cobalt)
while others are toxic at low doses (mercury, cadmium, arsenic). Their biological transformations are typically related
either to energetic metabolisms (donor or electron acceptor)
or to resistance and detoxification mechanisms. They can also
be due to incidental or indirect mechanisms (biomethylation,
indirect reduction). Metals and metalloids are also subject to
non-enzymatic redox reactions and it is often difficult to distinguish between changes of biotic origin and those generated
by strictly physico-chemical processes. The relative importance of these biotic versus abiotic transformations depends on
environmental conditions, such as the presence or absence of
oxygen, and the prevailing pH (cf. Chap. 16.9).
This chapter will focus on the biogeochemical cycles of
iron and manganese which are among the most abundant
metals on the planet, and whose cycles are closely related.
The standard potential of the couple Mn(IV)/Mn(II) is at
+770 mV, and that of the couple Fe(III)/Fe(II) at +380 mV.
The oxidized forms of iron Fe(III) and manganese Mn(IV)
are excellent respiratory electron acceptors. Thus, the degradation of many organic compounds is coupled to the dissimilatory reduction of Fe(III) and Mn(IV), which play an
important role in the anaerobic degradation of organic matter (cf. Sect. 3.3.2). Microorganisms involved in the dissimilatory reduction of these two metals are frequently the same.
For example, Shewanella putrefaciens produces the energy
necessary to its growth by coupling the oxidation of formate
into CO 2 to the reduction of iron or manganese:
Formate
À
þ 2 Fe III
ð Þ þ H 2 O ! HCO 3
À
þ 2 Fe II
ð Þ þ 2H
þ
Formate
À
þ 2 Mn IV
ð Þ þ H 2 O ! HCO 3
À
þ 2 Mn II
ð Þ þ 2H
þ
In the presence of these electron acceptors, lactate and
pyruvate are also oxidized but not completely mineralized
(acetate production). In part against the dissimilatory reduction, many bacterial species derive their energy from the
oxidation of iron and manganese. These organisms can be
either aerobic (Thiobacillus spp. for example) or denitrifying
anaerobic chemolithotrophs, or anoxygenic phototrophs
(Rhodovulum robiginosum and Rhodovulum iodosum).
Because of their abundance, iron and manganese have
been the most extensively studied metals, but the spectrum
of metals used by microorganisms is much larger.
Microorganisms are involved in the biogeochemical cycles
600
J.-C. Bertrand et al.
differentiating black from white smokers. These sources
are also enriched in oxidized (CO, CO 2 ) and reduced
(CH 4 ) carbon, and in calcium which precipitates in the
form of CaSO 4 in contact with oceanic waters. This
CaSO 4 is at the origin of the chimney wall formation of
smokers, which then are enriched in metal sulfides and in
iron and manganese oxides.
Ecosystems that develop in deep hydrothermal vent
receive no solar radiation and biomass production is due
to chemosynthetic processes (Jannasch and Mottl 1985).
Autotrophic microorganisms that grow in these ecosystems
used reduced sulfur compounds (sulfur-oxidizing
chemolithoautotrophs) and other reduced mineral
compounds (such as NH 3 , H 2 , Fe
2+ , or Mn
2+ ) as energy
sources. Methylotrophic microorganisms also colonize
these environments. Many hyperthermophilic and thermophilic microorganisms grow directly on the walls of smokers
in the form of microbial mats, or in the water around the
plume of the hydrothermal source. Chemosynthetic
microorganisms are at the origin of a food chain involving
planktonic or benthic animals, filter-feeding organisms and
grazers, including worms, crustaceans, molluscs, and fishes.
The distribution of animal populations is established
according to the distance to the smoker, the different species
being arranged in concentric rings around the sourceejection depending on the prevailing extreme conditions
(temperature, pH, concentrations of sulfide, and metal).
In addition, in these ecosystems, symbioses between
chemolithotrophic sulfur-oxidizing microorganisms and
metazoans (worms, bivalves) have been described (cf. Sect.
10.3.1). For example, red giant tubeworms (Riftia
pachyptila), which lack digestive tube, feed through the
symbiotic activity of sulfur-oxidizing microorganisms
(endobionts) housed in a particular organ, the trophosome.
The substrates required for the growth of microorganisms
(H 2 S, CO 2 , and O 2 ) are provided by the circulatory system of
the worm, and part of the organic matter synthesized by
bacteria is used by worms for their diet. Other symbioses
involving exobiontes, often located in the gills of aquatic
organisms in bacteriocytes, have also been described.
Such symbioses also exist in coastal marine sediments
between bivalves (clams) and chemosynthetic sulfuroxidizing bacteria.
Because of the total absence of solar light, deep hydrothermal ecosystems therefore seem to exclusively depend
on biomass production by chemosynthetic processes.
However, it was discovered that these areas also contain
phototrophic green sulfur bacteria (Prosthecochloris sp.)
which use geothermal radiations (near-infrared radiations between 750 and 1,050 nm) as energy source.
The question of their exact role still remains open (Beatty
et al. 2005).
14.5 Cycles of Metals and Metalloids
14.5.1 Introduction
Many microorganisms are involved in the biogeochemical
cycles of metals and metalloids, primarily by modifying their
oxidation level and/or their chemical forms. Among these
elements, some are essential to life (iron, manganese, cobalt)
while others are toxic at low doses (mercury, cadmium, arsenic). Their biological transformations are typically related
either to energetic metabolisms (donor or electron acceptor)
or to resistance and detoxification mechanisms. They can also
be due to incidental or indirect mechanisms (biomethylation,
indirect reduction). Metals and metalloids are also subject to
non-enzymatic redox reactions and it is often difficult to distinguish between changes of biotic origin and those generated
by strictly physico-chemical processes. The relative importance of these biotic versus abiotic transformations depends on
environmental conditions, such as the presence or absence of
oxygen, and the prevailing pH (cf. Chap. 16.9).
This chapter will focus on the biogeochemical cycles of
iron and manganese which are among the most abundant
metals on the planet, and whose cycles are closely related.
The standard potential of the couple Mn(IV)/Mn(II) is at
+770 mV, and that of the couple Fe(III)/Fe(II) at +380 mV.
The oxidized forms of iron Fe(III) and manganese Mn(IV)
are excellent respiratory electron acceptors. Thus, the degradation of many organic compounds is coupled to the dissimilatory reduction of Fe(III) and Mn(IV), which play an
important role in the anaerobic degradation of organic matter (cf. Sect. 3.3.2). Microorganisms involved in the dissimilatory reduction of these two metals are frequently the same.
For example, Shewanella putrefaciens produces the energy
necessary to its growth by coupling the oxidation of formate
into CO 2 to the reduction of iron or manganese:
Formate
À
þ 2 Fe III
ð Þ þ H 2 O ! HCO 3
À
þ 2 Fe II
ð Þ þ 2H
þ
Formate
À
þ 2 Mn IV
ð Þ þ H 2 O ! HCO 3
À
þ 2 Mn II
ð Þ þ 2H
þ
In the presence of these electron acceptors, lactate and
pyruvate are also oxidized but not completely mineralized
(acetate production). In part against the dissimilatory reduction, many bacterial species derive their energy from the
oxidation of iron and manganese. These organisms can be
either aerobic (Thiobacillus spp. for example) or denitrifying
anaerobic chemolithotrophs, or anoxygenic phototrophs
(Rhodovulum robiginosum and Rhodovulum iodosum).
Because of their abundance, iron and manganese have
been the most extensively studied metals, but the spectrum
of metals used by microorganisms is much larger.
Microorganisms are involved in the biogeochemical cycles
600
J.-C. Bertrand et al.