carbonate-rich waters also promote photosynthesis. The
main contributors to primary production of organic matter
in the soda lakes are prokaryotic microorganisms:
cyanobacteria and anaerobic phototrophic bacteria also
called purple bacteria.
Filamentous cyanobacteria (Cyanospira) and unicellular
organisms (Chroococcus and Pleurocapsa) have been
described. In some soda lakes, algal mats result mainly
from development of filamentous cyanobacteria of the
genera Spirulina, Anabaenopsis, and Arthrospira.
Cyanobacteria also contribute in primary production by
their ability to atmospheric nitrogen fixation.
Weakly alkaline lakes are generally dominated by large
clusters of cyanobacteria while hyper-alkaline lakes such as
Lake Magadi contain both cyanobacteria and anoxygenic
phototrophic bacteria belonging to the genera Ectothiorhodospira and Halorhodospira. Primary productivity of
these haloalkaliphilic phototrophic bacteria is probably at
the origin of all biological processes in soda lakes. In the
mid-1990s, new anoxygenic phototrophic bacteria have been
isolated from soda lakes. These bacteria include not only
purple bacteria but also heliobacteria (Clostridia). The latter
differ from haloalkaliphilic species such as Halorhodospira,
both in terms of phylogeny and physiology, by requiring in
particular low concentrations of NaCl for growth. These
isolates are obligate alkaliphilic. Many of these isolates
belong to the genera Rhodobaca, Thiorhodospira and
Thioalkalicoccus (all purple bacteria), and Heliorestis
(Heliobacteria). Members of the genus Rhodobaca present
a particular interest because:
1. They are devoid of light collecting device type II antena,
considered as an original property in non-sulfur purple
bacteria.
2. These phototrophic microorganisms synthesize several
original carotenoids responsible for their yellow color in
cultures.
Highly salted soda lakes (from 20 % NaCl to NaCl
saturation) have a different primary microflora.
Cyanobacteria are missing and it is Ectothiorhodospira
spp., which are responsible for primary production. The
dominant microorganisms are haloalkaliphilic Archaea,
with population reaching up to 10
7 –10
8 archaeons per mL.
Haloalkaliphilic archaea are classified in two genera:
Natronobacterium and Natronococcus, containing 5 and 4
species, respectively.
10.7.3.2 Alkaliphilic Aerobic Microflora
Although the alkaline environments are usually shallow and
limited in dioxygen (eutrophic), they have very dense
populations of aerobic non-phototrophic organotrophic bacteria that use the products of photosynthesis as well as the
products of anaerobic degradation. Relatively few isolates
were identified and deposited in public collections of
microorganisms. Regarding the Gram-negative bacteria,
they belong to the Gammaproteobacteria and include
many proteolytic microorganisms related to the genus
Halomonas. Proteolytic Gram-negative isolates were also
affiliated to genera Pseudomonas and Stenotrophomonas,
while other isolates were phylogenetically related to typical
aquatic bacteria such as Aeromonas, Vibrio, and
Alteromonas. Recent studies on bacterial communities of
low saline lakes of Siberia and lakes of the Rift Valley in
Kenya have revealed the presence of aerobic
chemolithotrophic sulfur-oxidizing bacteria of the genera
Thioalkalimicrobium, Thioalkalivibrio, and Thioalkalispira
belonging to the Gammaproteobacteria. More recently, a
new aerobic heterotrophic bacterium, Alkalilimnicola
halodurans of the family Ectorhodospirillaceae, was
isolated from sediments of Lake Natron. Furthermore, several alkaliphilic methanotrophic bacteria of the family
Methylococcaceae were isolated from alkaline moderately
saline lakes of Central Asia and Kenya. They include
Methylomicrobium
alcaliphilum,
Methylomicrobium
buryatense, and Methylomicrobium kenyense. At high pH
values (pH 10–10.5), M. kenyense was shown to oxidize
ammonia to nitrite and organic sulfur compounds.
Chemolithotrophic nitrite-oxidizing bacteria (Nitrobacter
alkalicus) belonging to the Alphaproteobacteria were
isolated from soda lakes. These nitrifying bacteria play an
important role in the nitrogen cycle by converting inorganic
nitrogen into nitrate.
Aerobic Gram-positive bacteria of the high and low
G + C% phyla have been described. Bacteria with low
G + C% mainly belong to the genus Bacillus while those
with high G + C% were affiliated to Dietza, Arthrobacter,
and Terrabacter genera. Several Bacillus species have
recently been reclassified into new genera phylogenetically
related to the genus Bacillus. They include Anoxybacillus,
Natronobacillus, and Oceanobacillus genera. Members of
genus Natronobacillus are capable of dinitrogen fixation.
Alkalispirillum and Alkalilimnicola genera were originally described as aerobic non-photosynthetic heterotrophic
microorganisms with remarkable tolerance to alkalinity.
Although the possibility of anaerobic growth with nitrate
was briefly mentioned for Alkalilimnicola, the denitrification
potential has not been studied in detail, nor the capacity to
grow lithoautotrophically. A study of the arsenic cycle in
Mono Lake in California revealed a new chemolithoautotrophic metabolism with arsenic as the sole electron
donor and nitrate as electron acceptor. During this process,
arsenite is oxidized to arsenate with the simultaneous reduction of nitrate to nitrite. The bacterium responsible for this
reaction is strain MLHE-1. It is a new member of the group
Alkalispirillum-Alkalilimnicola. In addition, this bacterium
is able to grow lithoautotrophically with sulfide and
10 The Extreme Conditions of Life on the Planet and Exobiology
389
main contributors to primary production of organic matter
in the soda lakes are prokaryotic microorganisms:
cyanobacteria and anaerobic phototrophic bacteria also
called purple bacteria.
Filamentous cyanobacteria (Cyanospira) and unicellular
organisms (Chroococcus and Pleurocapsa) have been
described. In some soda lakes, algal mats result mainly
from development of filamentous cyanobacteria of the
genera Spirulina, Anabaenopsis, and Arthrospira.
Cyanobacteria also contribute in primary production by
their ability to atmospheric nitrogen fixation.
Weakly alkaline lakes are generally dominated by large
clusters of cyanobacteria while hyper-alkaline lakes such as
Lake Magadi contain both cyanobacteria and anoxygenic
phototrophic bacteria belonging to the genera Ectothiorhodospira and Halorhodospira. Primary productivity of
these haloalkaliphilic phototrophic bacteria is probably at
the origin of all biological processes in soda lakes. In the
mid-1990s, new anoxygenic phototrophic bacteria have been
isolated from soda lakes. These bacteria include not only
purple bacteria but also heliobacteria (Clostridia). The latter
differ from haloalkaliphilic species such as Halorhodospira,
both in terms of phylogeny and physiology, by requiring in
particular low concentrations of NaCl for growth. These
isolates are obligate alkaliphilic. Many of these isolates
belong to the genera Rhodobaca, Thiorhodospira and
Thioalkalicoccus (all purple bacteria), and Heliorestis
(Heliobacteria). Members of the genus Rhodobaca present
a particular interest because:
1. They are devoid of light collecting device type II antena,
considered as an original property in non-sulfur purple
bacteria.
2. These phototrophic microorganisms synthesize several
original carotenoids responsible for their yellow color in
cultures.
Highly salted soda lakes (from 20 % NaCl to NaCl
saturation) have a different primary microflora.
Cyanobacteria are missing and it is Ectothiorhodospira
spp., which are responsible for primary production. The
dominant microorganisms are haloalkaliphilic Archaea,
with population reaching up to 10
7 –10
8 archaeons per mL.
Haloalkaliphilic archaea are classified in two genera:
Natronobacterium and Natronococcus, containing 5 and 4
species, respectively.
10.7.3.2 Alkaliphilic Aerobic Microflora
Although the alkaline environments are usually shallow and
limited in dioxygen (eutrophic), they have very dense
populations of aerobic non-phototrophic organotrophic bacteria that use the products of photosynthesis as well as the
products of anaerobic degradation. Relatively few isolates
were identified and deposited in public collections of
microorganisms. Regarding the Gram-negative bacteria,
they belong to the Gammaproteobacteria and include
many proteolytic microorganisms related to the genus
Halomonas. Proteolytic Gram-negative isolates were also
affiliated to genera Pseudomonas and Stenotrophomonas,
while other isolates were phylogenetically related to typical
aquatic bacteria such as Aeromonas, Vibrio, and
Alteromonas. Recent studies on bacterial communities of
low saline lakes of Siberia and lakes of the Rift Valley in
Kenya have revealed the presence of aerobic
chemolithotrophic sulfur-oxidizing bacteria of the genera
Thioalkalimicrobium, Thioalkalivibrio, and Thioalkalispira
belonging to the Gammaproteobacteria. More recently, a
new aerobic heterotrophic bacterium, Alkalilimnicola
halodurans of the family Ectorhodospirillaceae, was
isolated from sediments of Lake Natron. Furthermore, several alkaliphilic methanotrophic bacteria of the family
Methylococcaceae were isolated from alkaline moderately
saline lakes of Central Asia and Kenya. They include
Methylomicrobium
alcaliphilum,
Methylomicrobium
buryatense, and Methylomicrobium kenyense. At high pH
values (pH 10–10.5), M. kenyense was shown to oxidize
ammonia to nitrite and organic sulfur compounds.
Chemolithotrophic nitrite-oxidizing bacteria (Nitrobacter
alkalicus) belonging to the Alphaproteobacteria were
isolated from soda lakes. These nitrifying bacteria play an
important role in the nitrogen cycle by converting inorganic
nitrogen into nitrate.
Aerobic Gram-positive bacteria of the high and low
G + C% phyla have been described. Bacteria with low
G + C% mainly belong to the genus Bacillus while those
with high G + C% were affiliated to Dietza, Arthrobacter,
and Terrabacter genera. Several Bacillus species have
recently been reclassified into new genera phylogenetically
related to the genus Bacillus. They include Anoxybacillus,
Natronobacillus, and Oceanobacillus genera. Members of
genus Natronobacillus are capable of dinitrogen fixation.
Alkalispirillum and Alkalilimnicola genera were originally described as aerobic non-photosynthetic heterotrophic
microorganisms with remarkable tolerance to alkalinity.
Although the possibility of anaerobic growth with nitrate
was briefly mentioned for Alkalilimnicola, the denitrification
potential has not been studied in detail, nor the capacity to
grow lithoautotrophically. A study of the arsenic cycle in
Mono Lake in California revealed a new chemolithoautotrophic metabolism with arsenic as the sole electron
donor and nitrate as electron acceptor. During this process,
arsenite is oxidized to arsenate with the simultaneous reduction of nitrate to nitrite. The bacterium responsible for this
reaction is strain MLHE-1. It is a new member of the group
Alkalispirillum-Alkalilimnicola. In addition, this bacterium
is able to grow lithoautotrophically with sulfide and
10 The Extreme Conditions of Life on the Planet and Exobiology
389