Within acidophilic Gram-positive bacteria, Desulfosporosinus spp. are able to use sulfate as an electron acceptor
for anaerobic respiration (commonly named sulfatereducing bacteria (SRB)). The presence of SRB in acidic
environments has been clearly established (Gonza ´lez-Toril
et al. 2003, 2005) where they are likely to use hydrogen and
glycerol as energy sources. It is the case of the first acidophilic sulfate-reducing bacterium which has been recently
isolated and named Desulfosporosinus acidophilus (Alazard
et al. 2010); metabolism of these microorganisms is very
appropriate to develop methodologies for metal removal
from industrial contaminated waters.
10.6.2.2 Archaea Domain
Acidophilic aerobic or facultative anaerobic archaea that
colonize biotopes such as AMD and solfatara fields, where
iron and sulfur are in reduced forms, are represented by two
different phylogenetic groups, the orders Sulfolobales and
Thermoplasmatales. These groups differ in their phenotypic
properties. Some representatives of the Sulfolobales,
e.g., Acidianus brierleyi or members of the genus
Metallosphaera, obtain energy by oxidizing sulfur, sulfidic
minerals, and reduced iron. Other species of the order
Sulfolobales, e.g., Sulfolobus acidocaldarius and
S. solfataricus, utilize only sulfur and reduced sulfur
compounds. Sulfolobus metallicus exploits sulfidic ores and
elemental sulfur as energy sources. Although other members
of the Sulfolobales are able to grow chemolithoautotrophically, S. metallicus and Acidianus ambivalens are the
only obligate chemolithoautotrophs known. In contrast, in
the order Thermoplasmatales, only species of the genus
Ferroplasma, F. acidiphilum and F. acidarmanus, are acidophilic chemolithotrophs capable to oxidize iron, although
they require traces of yeast extract for autotrophic growth.
F. acidarmanus, able to grow at pH near zero, has been
isolated from biofilms of pyritic sediments in the Iron
Mountain mine in California (Edwards et al. 2000).
Ferroplasma-like microorganisms are detected recurrently in
different commercial bioleaching plants, indicating that they
must play an important role in this type of artificial ecosystem.
The rest of the genus Thermoplasma and Picrophilus
corresponds to heterotrophic archaea. In the Tinto ecosystem, only members of the Thermoplasmatales order,
Thermoplasma acidophilum, and Ferroplasma acidiphilum
have been identified.
10.6.2.3 Eukarya Domain
An important level of eukaryotic diversity has been reported
associated to acidic environments, mainly AMD (AmaralZettler et al. 2003). Unfortunately there is scarce information
about the eukaryotes that are able to deal with a proton
gradient of five orders of magnitude through their
membranes, most of them without any extracellular structure
that could help to control the acidity of the habitat in which
they develop. In addition, they are exposed, so they must be
resistant, to extremely high concentrations of toxic heavy
metals, normally present in acidic environments. In the Tinto
Basin,
members
of
the
phylum
Chlorophyta
(Chlamydomonas, Chlorella, and Euglena) are the most
frequent species followed by two filamentous algae belonging to the genera Klebsormidium and Zygnemopsis. The
most acidic part of the river is inhabited by eukaryotic
community dominated by two species of the genera
Dunaliella and Cyanidium, well known for their acidity
and heavy-metal tolerance. Among the eukaryotic
decomposers, fungi are very abundant and exhibit great
diversity, including yeast and filamentous forms; the
mixotrophic community is dominated by cercomonads and
stramenopiles. The protistan consumer community is
characterized by two species of ciliates, tentatively assigned
to the genera Oxytrichia and Euplotes. Amoebas related to
the genera Valhkampfia and Noegleria can be found in the
most acidic part of the river, and one species of heliozoan
belonging to the genus Actinophyris seems to be the most
characteristic predator of the benthic food chain of the river.
10.6.3 Microbial Ecology of an Extreme Acidic
Environment: Rio Tinto
The combined use of conventional (enrichment cultures, isolation, and phenotypic characterization) and molecular ecology methods (DGGE, FISH, and cloning) has led to identify
the most representative microorganisms of the Rı ´o Tinto Basin
(Gonza ´lez-Toril et al. 2003). Eighty percent of the diversity of
the water column corresponds to microorganisms belonging to
two species, Leptospirillum ferrooxidans, Acidithiobacillus
ferrooxidans, and several species of the genus Acidiphilium,
all conspicuous members of the iron cycle.
All L. ferrooxidans isolates from Rı ´o Tinto are aerobic
iron oxidizers. A. ferrooxidans can oxidize iron aerobically
and reduce it anaerobically. All Acidiphilium isolates can
oxidize reduced organic compounds using ferric iron as an
electron acceptor (anaerobic respiration) and some isolates
can do so in the presence of oxygen. Although other ironoxidizing (Ferroplasma spp., Thermoplasma acidophilum,
Acidimicrobium spp., and Ferrimicrobium spp.) or ironreducing (Acidimicrobium spp. and Ferrimicrobium spp.)
microorganisms have been detected in the Tinto ecosystem,
their low numbers suggest that they play a minor role in the
operation of the iron cycle, at least in the water column.
Concerning the sulfur cycle, only A. ferrooxidans is
found in significant numbers in the water column of the
Tinto Basin. This bacterium can oxidize ferrous iron and
reduced sulfur compounds. The oxidation of reduced sulfur
compounds can be carried out aerobically and anaerobically.
10 The Extreme Conditions of Life on the Planet and Exobiology
383
for anaerobic respiration (commonly named sulfatereducing bacteria (SRB)). The presence of SRB in acidic
environments has been clearly established (Gonza ´lez-Toril
et al. 2003, 2005) where they are likely to use hydrogen and
glycerol as energy sources. It is the case of the first acidophilic sulfate-reducing bacterium which has been recently
isolated and named Desulfosporosinus acidophilus (Alazard
et al. 2010); metabolism of these microorganisms is very
appropriate to develop methodologies for metal removal
from industrial contaminated waters.
10.6.2.2 Archaea Domain
Acidophilic aerobic or facultative anaerobic archaea that
colonize biotopes such as AMD and solfatara fields, where
iron and sulfur are in reduced forms, are represented by two
different phylogenetic groups, the orders Sulfolobales and
Thermoplasmatales. These groups differ in their phenotypic
properties. Some representatives of the Sulfolobales,
e.g., Acidianus brierleyi or members of the genus
Metallosphaera, obtain energy by oxidizing sulfur, sulfidic
minerals, and reduced iron. Other species of the order
Sulfolobales, e.g., Sulfolobus acidocaldarius and
S. solfataricus, utilize only sulfur and reduced sulfur
compounds. Sulfolobus metallicus exploits sulfidic ores and
elemental sulfur as energy sources. Although other members
of the Sulfolobales are able to grow chemolithoautotrophically, S. metallicus and Acidianus ambivalens are the
only obligate chemolithoautotrophs known. In contrast, in
the order Thermoplasmatales, only species of the genus
Ferroplasma, F. acidiphilum and F. acidarmanus, are acidophilic chemolithotrophs capable to oxidize iron, although
they require traces of yeast extract for autotrophic growth.
F. acidarmanus, able to grow at pH near zero, has been
isolated from biofilms of pyritic sediments in the Iron
Mountain mine in California (Edwards et al. 2000).
Ferroplasma-like microorganisms are detected recurrently in
different commercial bioleaching plants, indicating that they
must play an important role in this type of artificial ecosystem.
The rest of the genus Thermoplasma and Picrophilus
corresponds to heterotrophic archaea. In the Tinto ecosystem, only members of the Thermoplasmatales order,
Thermoplasma acidophilum, and Ferroplasma acidiphilum
have been identified.
10.6.2.3 Eukarya Domain
An important level of eukaryotic diversity has been reported
associated to acidic environments, mainly AMD (AmaralZettler et al. 2003). Unfortunately there is scarce information
about the eukaryotes that are able to deal with a proton
gradient of five orders of magnitude through their
membranes, most of them without any extracellular structure
that could help to control the acidity of the habitat in which
they develop. In addition, they are exposed, so they must be
resistant, to extremely high concentrations of toxic heavy
metals, normally present in acidic environments. In the Tinto
Basin,
members
of
the
phylum
Chlorophyta
(Chlamydomonas, Chlorella, and Euglena) are the most
frequent species followed by two filamentous algae belonging to the genera Klebsormidium and Zygnemopsis. The
most acidic part of the river is inhabited by eukaryotic
community dominated by two species of the genera
Dunaliella and Cyanidium, well known for their acidity
and heavy-metal tolerance. Among the eukaryotic
decomposers, fungi are very abundant and exhibit great
diversity, including yeast and filamentous forms; the
mixotrophic community is dominated by cercomonads and
stramenopiles. The protistan consumer community is
characterized by two species of ciliates, tentatively assigned
to the genera Oxytrichia and Euplotes. Amoebas related to
the genera Valhkampfia and Noegleria can be found in the
most acidic part of the river, and one species of heliozoan
belonging to the genus Actinophyris seems to be the most
characteristic predator of the benthic food chain of the river.
10.6.3 Microbial Ecology of an Extreme Acidic
Environment: Rio Tinto
The combined use of conventional (enrichment cultures, isolation, and phenotypic characterization) and molecular ecology methods (DGGE, FISH, and cloning) has led to identify
the most representative microorganisms of the Rı ´o Tinto Basin
(Gonza ´lez-Toril et al. 2003). Eighty percent of the diversity of
the water column corresponds to microorganisms belonging to
two species, Leptospirillum ferrooxidans, Acidithiobacillus
ferrooxidans, and several species of the genus Acidiphilium,
all conspicuous members of the iron cycle.
All L. ferrooxidans isolates from Rı ´o Tinto are aerobic
iron oxidizers. A. ferrooxidans can oxidize iron aerobically
and reduce it anaerobically. All Acidiphilium isolates can
oxidize reduced organic compounds using ferric iron as an
electron acceptor (anaerobic respiration) and some isolates
can do so in the presence of oxygen. Although other ironoxidizing (Ferroplasma spp., Thermoplasma acidophilum,
Acidimicrobium spp., and Ferrimicrobium spp.) or ironreducing (Acidimicrobium spp. and Ferrimicrobium spp.)
microorganisms have been detected in the Tinto ecosystem,
their low numbers suggest that they play a minor role in the
operation of the iron cycle, at least in the water column.
Concerning the sulfur cycle, only A. ferrooxidans is
found in significant numbers in the water column of the
Tinto Basin. This bacterium can oxidize ferrous iron and
reduced sulfur compounds. The oxidation of reduced sulfur
compounds can be carried out aerobically and anaerobically.
10 The Extreme Conditions of Life on the Planet and Exobiology
383