and oxidizing H 2 or S
0 ; lithoautotrophically by oxidizing
sulfidic ores (pyrite, sphalerite, and chalcopyrite) or S
0 ; or
chemolithotrophically to chemoheterotrophically by using
complex organic compounds and amino acids and by
oxidizing S
0
. Finally some of them are also able to oxidize
Fe
2+ to Fe
3+ . They are mainly found in terrestrial volcanic
hot springs. Sulfolobus are very popular models for studies
among others on informational processes (i.e., translation,
transcription, replication, and repair), cell division, RNA
processing, and metabolism and have been developed for
genetic experiments (Leigh et al. 2011). A neighbor genus,
Acidianus, harbors facultative aerobes that can be either obligate lithoautotrophs or facultative chemolithoautotrophs.
Under aerobic conditions, they oxidize sulfur compounds
as S
0 , sulfidic ores, H 2 , and Fe
2+ , while under anaerobic
conditions, they use H 2 as electron donor to reduce S
0 ,
leading to the formation of H 2 S as metabolic end product
(S
0 -H 2 autotrophy). Metallosphaera are aerobic facultative
chemolithoautotrophs that oxidize sulfur compounds as S
0 ,
sulfidic ores, metal sulfides (FeS), H 2 S, S 4 O 6
2À , H 2 and can
also grow on complex organic substrates. In contrast,
Stygiolobus are obligate chemolithoautotrophs growing
under anaerobic conditions via the S
0 -H 2 autotrophy.
Sulfurisphaera are facultative anaerobes that grow
mixotrophically or heterotrophically through S
0 -H 2 autotrophy or the oxidation of complex organic compounds.
Sulfurococcus are aerobic facultative chemolithoautotrophs
which oxidize sulfidic ores or S
0 and use complex organic
compounds, various sugars, and amino acids. Some of them
are also able to oxidize Fe
2+ to Fe
3+ . For additional information on Sulfolobales, see Dworkin and collaborators (2006).
The Desulfurococcales
This order was defined by Harald Huber and Karl Stetter in
2001. Desulfurococcales cells are regular to irregular
cocci (of about 0.5–15 μm), discs or dishes, which occur
singly, in pairs, short chains, or aggregates. Some of them
are flagellated. Most are neutrophilic or weakly acidophilic
hyperthermophiles, living at temperatures ranging from
85 to 106
C. Desulfurococcales occur mainly in hot
marine environments, such as shallow marine sediments,
springs, and venting waters (Aeropyrum, Ignicoccus,
Staphylothermus, Stetteria, Thermodiscus, Pyrodictium,
and Hyperthermus) or deep-sea hydrothermal systems and
black smokers (Ignicoccus, Staphylothermus, Pyrodictium,
and Pyrolobus). However, some representatives are also
found in hot volcanic terrestrial ecosystems with low salinity
and acidity to slightly alkaline pH values, such as hot
springs, mud holes, and soils of continental solfataric fields
(Desulfurococcus, Sulfophobococcus, Thermosphaera, and
Acidilobus). Desulfurococcales gathers anaerobic, facultative anaerobic, or aerobic organisms. Under autotrophic
conditions, they oxidize H 2 using S
0 , S 2 O 3
2À , NO 3
À , or
NO 2
À as electron acceptor and CO 2 as a carbon source.
Alternatively, they are also able to grow organotrophically
through aerobic respiration, anaerobic sulfur respiration, or
fermentation of organic substrates. The most hyperthermophilic organisms known to date belong to Desulfurococcales, among which Pyrolobus fumarii has an optimal
growth temperature of 106
C. It is able to grow at 113
C
but not below 90
C. In contrast, even if some bacterial
spores (e.g., Morella, Firmicutes) are able to resist
121
C, no bacterial cells are able to survive above 100
C.
Desulfurococcales are divided into two main families: the
Pyrodictiaceae and the Desulfurococcaceae. Pyrodictiaceae
form a rather coherent cluster containing the genera
Geogemma (which contain the famous strain 121 which
was claimed to grow at 121
C), Hyperthermus,
Pyrodictium, and Pyrolobus, all having optimal growth temperature ranging from 95 to 106
C. Pyrodictium forms
networks of hollow cannulae, in which the disk- or dishshapes cells are embedded. They are marine obligate
anaerobes that can be chemolithoautotrophs to mixotrophs
able to use S
0 or S 2 O 3
2À and H 2 under autotrophy with grow
on complex organic compounds, or obligate heterotrophs
fermenting complex organic compounds or peptides.
Hyperthermus are obligate marine anaerobic heterotrophic
cocci that ferment peptides products leading to the formation
of organic acids, butanol, and CO 2 . Finally Pyrolobus are
marine anaerobic or microaerobic, obligately chemolithoautotrophic irregular cocci. They reduce NO 3
À
, S 2 O 3
2À , or O 2
with H 2 . In contrast, Desulfurococcaceae grow optimally at
85–95
C and are much more diverse than Pyrodictiaceae.
They encompass the genera Aeropyrum, Desulfurococcus,
Ignicoccus (the host of Nanoarchaeum equitans),
Ignisphaera, Staphylothermus, Stetteria, Sulfophobococcus,
Thermodiscus, Thermogladius, and Thermosphaera.
Desulfurococcus are anaerobic cocci isolated from continental environments. They grow mixotrophically or heterotrophically by respiring S
0 or fermenting complex organic
compounds, peptides, amino acids, starch, or glycogen.
Aeropyrum are the only aerobic members of Desulfurococcaceae. These cocci are marine heterotrophs that respire
complex organic compounds with O 2 . Ignicoccus are marine
anaerobic cocci which grow lithoautotrophically via S
0
-H 2
autotrophy and produce exclusively H 2 S as the metabolic
end product. Staphylothermus are anaerobic marine coccoid
organisms that grow in aggregates and gain energy by fermentation of complex organic substrates in the presence of S
0
.
Stetteria are coccoid marine anaerobic mixotrophs that respire
S
0 or S 2 O 3
2À on complex organic compounds in presence of
H 2 . Sulfophobococcus are continental anaerobic obligate heterotrophic cocci in which growth is inhibited by S
0
.
Thermodiscus are dish- or disk-shaped anaerobic marine obligate heterotrophs, which carry out S
0 respiration and fermentation of complex organic compounds. Thermosphaera forms
166
P. Caumette et al.
0 ; lithoautotrophically by oxidizing
sulfidic ores (pyrite, sphalerite, and chalcopyrite) or S
0 ; or
chemolithotrophically to chemoheterotrophically by using
complex organic compounds and amino acids and by
oxidizing S
0
. Finally some of them are also able to oxidize
Fe
2+ to Fe
3+ . They are mainly found in terrestrial volcanic
hot springs. Sulfolobus are very popular models for studies
among others on informational processes (i.e., translation,
transcription, replication, and repair), cell division, RNA
processing, and metabolism and have been developed for
genetic experiments (Leigh et al. 2011). A neighbor genus,
Acidianus, harbors facultative aerobes that can be either obligate lithoautotrophs or facultative chemolithoautotrophs.
Under aerobic conditions, they oxidize sulfur compounds
as S
0 , sulfidic ores, H 2 , and Fe
2+ , while under anaerobic
conditions, they use H 2 as electron donor to reduce S
0 ,
leading to the formation of H 2 S as metabolic end product
(S
0 -H 2 autotrophy). Metallosphaera are aerobic facultative
chemolithoautotrophs that oxidize sulfur compounds as S
0 ,
sulfidic ores, metal sulfides (FeS), H 2 S, S 4 O 6
2À , H 2 and can
also grow on complex organic substrates. In contrast,
Stygiolobus are obligate chemolithoautotrophs growing
under anaerobic conditions via the S
0 -H 2 autotrophy.
Sulfurisphaera are facultative anaerobes that grow
mixotrophically or heterotrophically through S
0 -H 2 autotrophy or the oxidation of complex organic compounds.
Sulfurococcus are aerobic facultative chemolithoautotrophs
which oxidize sulfidic ores or S
0 and use complex organic
compounds, various sugars, and amino acids. Some of them
are also able to oxidize Fe
2+ to Fe
3+ . For additional information on Sulfolobales, see Dworkin and collaborators (2006).
The Desulfurococcales
This order was defined by Harald Huber and Karl Stetter in
2001. Desulfurococcales cells are regular to irregular
cocci (of about 0.5–15 μm), discs or dishes, which occur
singly, in pairs, short chains, or aggregates. Some of them
are flagellated. Most are neutrophilic or weakly acidophilic
hyperthermophiles, living at temperatures ranging from
85 to 106
C. Desulfurococcales occur mainly in hot
marine environments, such as shallow marine sediments,
springs, and venting waters (Aeropyrum, Ignicoccus,
Staphylothermus, Stetteria, Thermodiscus, Pyrodictium,
and Hyperthermus) or deep-sea hydrothermal systems and
black smokers (Ignicoccus, Staphylothermus, Pyrodictium,
and Pyrolobus). However, some representatives are also
found in hot volcanic terrestrial ecosystems with low salinity
and acidity to slightly alkaline pH values, such as hot
springs, mud holes, and soils of continental solfataric fields
(Desulfurococcus, Sulfophobococcus, Thermosphaera, and
Acidilobus). Desulfurococcales gathers anaerobic, facultative anaerobic, or aerobic organisms. Under autotrophic
conditions, they oxidize H 2 using S
0 , S 2 O 3
2À , NO 3
À , or
NO 2
À as electron acceptor and CO 2 as a carbon source.
Alternatively, they are also able to grow organotrophically
through aerobic respiration, anaerobic sulfur respiration, or
fermentation of organic substrates. The most hyperthermophilic organisms known to date belong to Desulfurococcales, among which Pyrolobus fumarii has an optimal
growth temperature of 106
C. It is able to grow at 113
C
but not below 90
C. In contrast, even if some bacterial
spores (e.g., Morella, Firmicutes) are able to resist
121
C, no bacterial cells are able to survive above 100
C.
Desulfurococcales are divided into two main families: the
Pyrodictiaceae and the Desulfurococcaceae. Pyrodictiaceae
form a rather coherent cluster containing the genera
Geogemma (which contain the famous strain 121 which
was claimed to grow at 121
C), Hyperthermus,
Pyrodictium, and Pyrolobus, all having optimal growth temperature ranging from 95 to 106
C. Pyrodictium forms
networks of hollow cannulae, in which the disk- or dishshapes cells are embedded. They are marine obligate
anaerobes that can be chemolithoautotrophs to mixotrophs
able to use S
0 or S 2 O 3
2À and H 2 under autotrophy with grow
on complex organic compounds, or obligate heterotrophs
fermenting complex organic compounds or peptides.
Hyperthermus are obligate marine anaerobic heterotrophic
cocci that ferment peptides products leading to the formation
of organic acids, butanol, and CO 2 . Finally Pyrolobus are
marine anaerobic or microaerobic, obligately chemolithoautotrophic irregular cocci. They reduce NO 3
À
, S 2 O 3
2À , or O 2
with H 2 . In contrast, Desulfurococcaceae grow optimally at
85–95
C and are much more diverse than Pyrodictiaceae.
They encompass the genera Aeropyrum, Desulfurococcus,
Ignicoccus (the host of Nanoarchaeum equitans),
Ignisphaera, Staphylothermus, Stetteria, Sulfophobococcus,
Thermodiscus, Thermogladius, and Thermosphaera.
Desulfurococcus are anaerobic cocci isolated from continental environments. They grow mixotrophically or heterotrophically by respiring S
0 or fermenting complex organic
compounds, peptides, amino acids, starch, or glycogen.
Aeropyrum are the only aerobic members of Desulfurococcaceae. These cocci are marine heterotrophs that respire
complex organic compounds with O 2 . Ignicoccus are marine
anaerobic cocci which grow lithoautotrophically via S
0
-H 2
autotrophy and produce exclusively H 2 S as the metabolic
end product. Staphylothermus are anaerobic marine coccoid
organisms that grow in aggregates and gain energy by fermentation of complex organic substrates in the presence of S
0
.
Stetteria are coccoid marine anaerobic mixotrophs that respire
S
0 or S 2 O 3
2À on complex organic compounds in presence of
H 2 . Sulfophobococcus are continental anaerobic obligate heterotrophic cocci in which growth is inhibited by S
0
.
Thermodiscus are dish- or disk-shaped anaerobic marine obligate heterotrophs, which carry out S
0 respiration and fermentation of complex organic compounds. Thermosphaera forms
166
P. Caumette et al.