Box 9.2: Microbial Life in the Deserts
Thierry Heulin
Introduction
The desert is a paradigm of an extreme environment
for life, to the extent that water limits the growth and
development of living organisms during much of the
day. In hot deserts, this lack of water is compounded
by excessive heat and light during the day.
The main features of a hot desert are:
• Scarce and irregular precipitation [average less
than 200 mm per year (Miller 1961)]
• Morning dew forming the only source of water for
living species
• Evaporation exceeding precipitation and large
temperature range between day and night
• Poor soil organic matter (sandy or rocky)
• Flora and fauna scarce, consisting of a few adapted
species
Some Characteristics of the Sahara
If the cold deserts of the polar regions are true deserts
because of a lack of available water due to freezing,
hot deserts are the most extensive and mainly located
near the tropics of Cancer and Capricorn (about 50
million km
2 on the surface of the Earth, one-third of
land area). In Africa, the Kalahari and Namib deserts
are located in the south and the Sahara to the north.
The Sahara is by far the largest hot desert on the
face of the earth (9,000,000 km
2 ) extending from
Mauritania to Egypt, extending through semidesert or
desert northeastward, to the Gobi Desert. The sand is
of sedimentary origin and comes from the destruction
of other rocks mainly through erosion.
Microbial Communities in Hot Deserts
A desert like the Sahara is far from “sterile,” as shown by
several studies of microbial ecology published recently.
Some Results from the Project “Treasures
of the Sahara”
On the site of Merzouga (Morocco), the sand is of the
homometric type, mainly composed of quartz with a
mean grain diameter of 300 μm. One gram of sand
contains about 20,000 grains of sand. On the surface of
T. Heulin
UMR 7265 CNRS-CEA-Aix Marseille University,
Institute of Environmental Biology and Biotechnology
IBEB/DSV/CEA, CEA Cadarache,
13108 Saint-Paul-lez-Durance, France
(continued)
Box 9.2 (continued)
each grain, an average of 10 bacteria was detected by
fluorescence (approximately 200,000 bacteria per g of
sand), with approximately 14 % of bacteria that could
be cultured by analyzing the sand grain by grain (130
grains analyzed) (Gommeaux et al. 2005). The overall
analysis of the diversity of culturable bacteria in the
form of colonies growing on nutrient media, or by
cloning-sequencing of 16S rDNA gene, shows that
the following are dominant bacterial groups:
Firmicutes, Actinobacteria, and Proteobacteria with
other groups Flexibacter-Bacteroides-Cytophaga
(FBC), GNS bacteria (green non-sulfur), Acidobacteria, and Planctomycetes at much lower frequencies (Gommeaux et al. 2005). Importance of diversity
was estimated between 400 and 1,400 equivalent species, indicating that in this extreme environment, the
total number of bacteria (2 Â 10
5 per gram of soil,
direct observation) was much lower than in a
cultivated soil of temperate areas (10
8 bacteria per
gram of soil), while diversity is roughly comparable
(Torsvik et al. 2002). On the site of Tataouine
(Tunisia), conclusions about the wide variety of bacterial species are identical. These species belong mostly
to the Firmicutes, Actinobacteria, Proteobacteria,
and CFB. On these samples, the presence of
non-thermophilic archaea has also been demonstrated
(Chanal et al. 2006). A search for bacteria tolerant to
ionizing radiation was undertaken among these
desiccation-tolerant bacteria. A sample of Tataouine
sand was subjected to a gamma irradiation of 15,000
Gy, which revealed the presence of spore-forming
bacteria (Bacillus), of Proteobacteria (Chelatococcus), and, as expected, of Deinococcus (de Groot
et al. 2005, 2009; Chanal et al. 2006). Some
similarities between the two sites are worth
highlighting:
• About 70–80 % of 16S rDNA sequences revealed
by the molecular approach (diversity of culturable
bacteria and non-culturable) do not match any bacterial species described to date.
• Bacterial diversity is dominated by bacteria
belonging to phyla and genera with known
mechanisms of desiccation tolerance, Firmicutes
and Actinobacteria (sporulation) and Deinococcus
(DNA repair), but also soma bacteria in the phylum
Proteobacteria that we do not know the mechanisms
of adaptation to desiccation, with the exception of
encystment. A new bacterial genus, Ramlibacter
isolated from the Tataouine sand, has this property.
(continued)
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
337
Thierry Heulin
Introduction
The desert is a paradigm of an extreme environment
for life, to the extent that water limits the growth and
development of living organisms during much of the
day. In hot deserts, this lack of water is compounded
by excessive heat and light during the day.
The main features of a hot desert are:
• Scarce and irregular precipitation [average less
than 200 mm per year (Miller 1961)]
• Morning dew forming the only source of water for
living species
• Evaporation exceeding precipitation and large
temperature range between day and night
• Poor soil organic matter (sandy or rocky)
• Flora and fauna scarce, consisting of a few adapted
species
Some Characteristics of the Sahara
If the cold deserts of the polar regions are true deserts
because of a lack of available water due to freezing,
hot deserts are the most extensive and mainly located
near the tropics of Cancer and Capricorn (about 50
million km
2 on the surface of the Earth, one-third of
land area). In Africa, the Kalahari and Namib deserts
are located in the south and the Sahara to the north.
The Sahara is by far the largest hot desert on the
face of the earth (9,000,000 km
2 ) extending from
Mauritania to Egypt, extending through semidesert or
desert northeastward, to the Gobi Desert. The sand is
of sedimentary origin and comes from the destruction
of other rocks mainly through erosion.
Microbial Communities in Hot Deserts
A desert like the Sahara is far from “sterile,” as shown by
several studies of microbial ecology published recently.
Some Results from the Project “Treasures
of the Sahara”
On the site of Merzouga (Morocco), the sand is of the
homometric type, mainly composed of quartz with a
mean grain diameter of 300 μm. One gram of sand
contains about 20,000 grains of sand. On the surface of
T. Heulin
UMR 7265 CNRS-CEA-Aix Marseille University,
Institute of Environmental Biology and Biotechnology
IBEB/DSV/CEA, CEA Cadarache,
13108 Saint-Paul-lez-Durance, France
(continued)
Box 9.2 (continued)
each grain, an average of 10 bacteria was detected by
fluorescence (approximately 200,000 bacteria per g of
sand), with approximately 14 % of bacteria that could
be cultured by analyzing the sand grain by grain (130
grains analyzed) (Gommeaux et al. 2005). The overall
analysis of the diversity of culturable bacteria in the
form of colonies growing on nutrient media, or by
cloning-sequencing of 16S rDNA gene, shows that
the following are dominant bacterial groups:
Firmicutes, Actinobacteria, and Proteobacteria with
other groups Flexibacter-Bacteroides-Cytophaga
(FBC), GNS bacteria (green non-sulfur), Acidobacteria, and Planctomycetes at much lower frequencies (Gommeaux et al. 2005). Importance of diversity
was estimated between 400 and 1,400 equivalent species, indicating that in this extreme environment, the
total number of bacteria (2 Â 10
5 per gram of soil,
direct observation) was much lower than in a
cultivated soil of temperate areas (10
8 bacteria per
gram of soil), while diversity is roughly comparable
(Torsvik et al. 2002). On the site of Tataouine
(Tunisia), conclusions about the wide variety of bacterial species are identical. These species belong mostly
to the Firmicutes, Actinobacteria, Proteobacteria,
and CFB. On these samples, the presence of
non-thermophilic archaea has also been demonstrated
(Chanal et al. 2006). A search for bacteria tolerant to
ionizing radiation was undertaken among these
desiccation-tolerant bacteria. A sample of Tataouine
sand was subjected to a gamma irradiation of 15,000
Gy, which revealed the presence of spore-forming
bacteria (Bacillus), of Proteobacteria (Chelatococcus), and, as expected, of Deinococcus (de Groot
et al. 2005, 2009; Chanal et al. 2006). Some
similarities between the two sites are worth
highlighting:
• About 70–80 % of 16S rDNA sequences revealed
by the molecular approach (diversity of culturable
bacteria and non-culturable) do not match any bacterial species described to date.
• Bacterial diversity is dominated by bacteria
belonging to phyla and genera with known
mechanisms of desiccation tolerance, Firmicutes
and Actinobacteria (sporulation) and Deinococcus
(DNA repair), but also soma bacteria in the phylum
Proteobacteria that we do not know the mechanisms
of adaptation to desiccation, with the exception of
encystment. A new bacterial genus, Ramlibacter
isolated from the Tataouine sand, has this property.
(continued)
9 Adaptations of Prokaryotes to Their Biotopes and to Physicochemical Conditions. . .
337