Box 9.2 (continued)
The Ramlibacter Bacterium and Its Cell Cycle
In 1931, a meteorite was found near the town of
Tataouine, in southern Tunisia. The largest fragments
of the meteorite were sent the next day to the National
Museum of Natural History in Paris (Lacroix 1931).
New samples of small fragments of the meteorite were
recovered in 1994. Analysis by scanning electron
microscopy revealed areas of dissolution on the
surface of these fragments, associated with the precipitation of calcite crystals, although such areas were
absent from the fragments preserved in the Museum
(Barrat et al. 1998; Benzerara et al. 2003). A search for
microbes responsible for this alteration permitted to
isolate a candidate bacterium (strain TTB310) that was
identified as belonging to a new genus and therefore a
new bacterial species: Ramlibacter tataouinensis
(Raml-sand in Arabic; a bacterium from the Tataouine
sand) (Heulin et al. 2003). We have shown that this
bacterium was actually able to colonize and alter
fragments of pyroxene rock (meteoritic silicate mineral component) and to induce biomineralization
under controlled conditions (Benzerara et al. 2004).
The originality of this bacterium is that it has a complex cell cycle involving two very different cell types:
(1) a spherical form with a diameter of 0.8 μm with
bacterial cyst properties and (2) a mobile rod-shaped
form (0.2 μm in diameter and 2–3 μm in length)
permitting dissemination (Gommeaux et al. 2005).
The cell cycle consists of two phases that can explain
its mode of reproduction and dissemination: (1) a
multiplication phase during which a cyst divides to
yield two cysts and (2) a dissemination phase during
which a cyst differentiates into rods, which can divide
and move in search of more favorable conditions,
followed by redifferentiation of rods into cysts. The
cysts can then divide and form a new satellite colony.
The cell cycle is an adaptation to extreme conditions of
hot deserts. The main factor limiting growth and reproduction of living beings in such a medium is water. For
this bacterium, the form sensitive to desiccation (rods)
permits dissemination, while the form tolerant to desiccation (cysts) is able to divide, which optimizes the
narrow “window” of availability of water (dew mainly
late at night during winter). Its genome has recently been
studied (De Luca et al. 2011).
Rhizobia and the Desert
Many studies mention the presence of rhizobia in
desert soils, and, in particular, strains can nodulate
acacias (Zerhari et al. 2000). One adaptive response
(continued)
Box 9.2 (continued)
often noted in these bacteria is the biosynthesis of
osmoprotecting molecules. Looking for the dominant
bacteria that produce exopolysaccharides (EPS) in
many desert soils in Algeria, Kaci et al. (2005) have
highlighted the presence of Rhizobium producing
heteroglycans. These studies suggest that EPS production is stimulated in Rhizobium by the rhizosphere of
plants other than legumes.
Cyanobacteria and the Desert
A study of cyanobacterial diversity in “crusts” developing at the surface of a Utah desert has highlighted
the existence of a cluster named “Xeronema”
consisting of thin-walled cyanobacteria, close to
Phormidium (Garcia-Pichel et al. 2001). This diversity
of cyanobacteria was found in the meteorite
Tataouine, with genera Oscillatoria, Anabaena, Nostoc, and Symploca (Benzerara et al. 2006).
Conclusion
Far from being barren, hot deserts, both low in
nutrients (carbon, nitrogen) and subjected to various
stresses (water, UV, temperature), harbor a great
diversity of microorganisms. This diversity reveals
only very few bacterial species indicating an adaptation of thermophilic microorganisms to water stress,
but little or no adaptation to temperature (though very
high in this ecosystem). Water being available, and
thus growth of microorganisms being only possible
during the few cool hours of the day (dew, late at
night during the winter). Bacteria in fact, have not
developed mechanisms of heat tolerance: when it is
hot, bacteria “sleep.” Within this diversity, families
and genera of bacteria have developed mechanisms
of desiccation tolerance: sporulation (firmicutes
endospores and actinobacteria sporophores), DNA
repair (Rubrobacter, Deinococcus), encystment
(Ramlibacter, Azotobacter), or the production of
exopolysaccharides and osmoprotectants (Rhizobium).
In contrast, these studies show that:
• Bacterial genera described for their desiccation
tolerance (e.g., Arthrobacter, Cystobacter), but for
which involved mechanisms remain unknown
• Bacterial genera already known (most genera
belonging to Proteobacteria: e.g., Chelatococcus,
Pseudomonas, Agrobacterium) but with so far
unknown tolerance to water stress
• A very large number of bacteria not belonging to
any species described so far
(continued)
338
P. Normand et al.
The Ramlibacter Bacterium and Its Cell Cycle
In 1931, a meteorite was found near the town of
Tataouine, in southern Tunisia. The largest fragments
of the meteorite were sent the next day to the National
Museum of Natural History in Paris (Lacroix 1931).
New samples of small fragments of the meteorite were
recovered in 1994. Analysis by scanning electron
microscopy revealed areas of dissolution on the
surface of these fragments, associated with the precipitation of calcite crystals, although such areas were
absent from the fragments preserved in the Museum
(Barrat et al. 1998; Benzerara et al. 2003). A search for
microbes responsible for this alteration permitted to
isolate a candidate bacterium (strain TTB310) that was
identified as belonging to a new genus and therefore a
new bacterial species: Ramlibacter tataouinensis
(Raml-sand in Arabic; a bacterium from the Tataouine
sand) (Heulin et al. 2003). We have shown that this
bacterium was actually able to colonize and alter
fragments of pyroxene rock (meteoritic silicate mineral component) and to induce biomineralization
under controlled conditions (Benzerara et al. 2004).
The originality of this bacterium is that it has a complex cell cycle involving two very different cell types:
(1) a spherical form with a diameter of 0.8 μm with
bacterial cyst properties and (2) a mobile rod-shaped
form (0.2 μm in diameter and 2–3 μm in length)
permitting dissemination (Gommeaux et al. 2005).
The cell cycle consists of two phases that can explain
its mode of reproduction and dissemination: (1) a
multiplication phase during which a cyst divides to
yield two cysts and (2) a dissemination phase during
which a cyst differentiates into rods, which can divide
and move in search of more favorable conditions,
followed by redifferentiation of rods into cysts. The
cysts can then divide and form a new satellite colony.
The cell cycle is an adaptation to extreme conditions of
hot deserts. The main factor limiting growth and reproduction of living beings in such a medium is water. For
this bacterium, the form sensitive to desiccation (rods)
permits dissemination, while the form tolerant to desiccation (cysts) is able to divide, which optimizes the
narrow “window” of availability of water (dew mainly
late at night during winter). Its genome has recently been
studied (De Luca et al. 2011).
Rhizobia and the Desert
Many studies mention the presence of rhizobia in
desert soils, and, in particular, strains can nodulate
acacias (Zerhari et al. 2000). One adaptive response
(continued)
Box 9.2 (continued)
often noted in these bacteria is the biosynthesis of
osmoprotecting molecules. Looking for the dominant
bacteria that produce exopolysaccharides (EPS) in
many desert soils in Algeria, Kaci et al. (2005) have
highlighted the presence of Rhizobium producing
heteroglycans. These studies suggest that EPS production is stimulated in Rhizobium by the rhizosphere of
plants other than legumes.
Cyanobacteria and the Desert
A study of cyanobacterial diversity in “crusts” developing at the surface of a Utah desert has highlighted
the existence of a cluster named “Xeronema”
consisting of thin-walled cyanobacteria, close to
Phormidium (Garcia-Pichel et al. 2001). This diversity
of cyanobacteria was found in the meteorite
Tataouine, with genera Oscillatoria, Anabaena, Nostoc, and Symploca (Benzerara et al. 2006).
Conclusion
Far from being barren, hot deserts, both low in
nutrients (carbon, nitrogen) and subjected to various
stresses (water, UV, temperature), harbor a great
diversity of microorganisms. This diversity reveals
only very few bacterial species indicating an adaptation of thermophilic microorganisms to water stress,
but little or no adaptation to temperature (though very
high in this ecosystem). Water being available, and
thus growth of microorganisms being only possible
during the few cool hours of the day (dew, late at
night during the winter). Bacteria in fact, have not
developed mechanisms of heat tolerance: when it is
hot, bacteria “sleep.” Within this diversity, families
and genera of bacteria have developed mechanisms
of desiccation tolerance: sporulation (firmicutes
endospores and actinobacteria sporophores), DNA
repair (Rubrobacter, Deinococcus), encystment
(Ramlibacter, Azotobacter), or the production of
exopolysaccharides and osmoprotectants (Rhizobium).
In contrast, these studies show that:
• Bacterial genera described for their desiccation
tolerance (e.g., Arthrobacter, Cystobacter), but for
which involved mechanisms remain unknown
• Bacterial genera already known (most genera
belonging to Proteobacteria: e.g., Chelatococcus,
Pseudomonas, Agrobacterium) but with so far
unknown tolerance to water stress
• A very large number of bacteria not belonging to
any species described so far
(continued)
338
P. Normand et al.