mechanisms, including genes for the CO 2 -uptake NAD(P)H dehydrogenase type
1 complexes NDH-1 3 and NDH-1 4 , as well as the ccmKLMNO cluster, which codes
for the structural proteins of the carboxysomes. Conversely, A. platensis NIES-39
and A. platensis sp. paraca were shown to contain a gene encoding the high-affinity
HCO 3
À transporter StbA, in addition with the low-affinity transporter BicA, which
was shared by all the genomes analysed. The presence of both transporters in these
strains might provide a selective advantage in highly alkaline environments where
HCO 3
À concentrations are high [11].
Alkaliphilic anaerobes thriving on the products from the primary producers can
be expected to metabolize a range of complex carbohydrates. This is the case with
the obligately alkaliphilic anaerobe, Clostridium alkalicellum, which was isolated
from a cellulose-decomposing community in the Verkhnee Beloe soda lake in
Russia. This bacterium was found to be very specialized, with a strictly fermentative
metabolism capable of degrading xylan, cellulose, and cellobiose into hydrogen,
ethanol, acetate, and lactate [31]. Other anaerobic heterotrophic bacteria found in
soda lakes, such as Halonatronum saccharophilum, Amphibacillus fermentum,
and Amphibacillus tropicus, isolated from Lake Magadi in Kenya, use a purely
fermentative type of metabolism in which mono-, di-, and polysaccharides are
catabolized through the fructose bisphosphate pathway into acetate, ethanol, and
CO 2 [32]. Microarray studies on another hemicellulose degrading facultative anaerobe isolated from the Wudunur Soda Lake in China, Bacillus sp., N16-5, revealed a
complex hierarchical pattern of sugar metabolism in which glucose was the preferred
substrate, followed by components of complex hemicellulose polysaccharides such
as xylan, pectin, and galactomannan. Glucose was found to be the primary repressor
of expression of gene clusters involved in the degradation of polysaccharides, which
are only partially degraded by extracellular glycoside hydrolases before being
transported intracellularly by oligopeptide transporters to be further processed
[33]. This is thought to be a genetic adaptation to allow responses to the carbohydrate source fluctuations in the environment [33].
Unlike acetogens from more neutrophilic habitats, acetogenic bacteria found in
soda lakes cannot grow chemolithotropically on hydrogen, carbon monoxide, and
carbon dioxide mixtures and are trophically limited to products from primary
anaerobes in the community such as lactate, histidine, and ethanol [8]. In these
bacteria, the Wood-Ljungdahl acetyl-CoA pathway, in which the CO 2 generated
from the catabolism of the primary substrates is converted to acetate, is coupled with
proton-dependent ATP synthesis by generating the electron and proton gradient
required by F 1 F 0 -type ATP synthases [34]. This is the case with the bacteria
Natroniella acetigena and Natronincola histidinovorans, while in T. magadiensis
the fermentation of arginine through the ornithine cycle and subsequent conversion
of carbamoyl phosphate by carbamate kinase is coupled to ATP synthesis
[34]. Genome analysis of the strictly anaerobic and sulphate-reducing bacterium
Thermodesulfovibrio sp. N1 showed that it lacked the complete TCA and WoodLjundahl pathways and would therefore not be capable of fully oxidizing organic
substrates [35]. Instead, it can reversibly decarboxylate pyruvate to acetyl-CoA,
which is further converted into acetate by the acetyl-CoA synthetase, generating
ATP in the process. Several other dehydrogenases present in the periplasmic space,
144
P. H. Lebre and D. A. Cowan
1 complexes NDH-1 3 and NDH-1 4 , as well as the ccmKLMNO cluster, which codes
for the structural proteins of the carboxysomes. Conversely, A. platensis NIES-39
and A. platensis sp. paraca were shown to contain a gene encoding the high-affinity
HCO 3
À transporter StbA, in addition with the low-affinity transporter BicA, which
was shared by all the genomes analysed. The presence of both transporters in these
strains might provide a selective advantage in highly alkaline environments where
HCO 3
À concentrations are high [11].
Alkaliphilic anaerobes thriving on the products from the primary producers can
be expected to metabolize a range of complex carbohydrates. This is the case with
the obligately alkaliphilic anaerobe, Clostridium alkalicellum, which was isolated
from a cellulose-decomposing community in the Verkhnee Beloe soda lake in
Russia. This bacterium was found to be very specialized, with a strictly fermentative
metabolism capable of degrading xylan, cellulose, and cellobiose into hydrogen,
ethanol, acetate, and lactate [31]. Other anaerobic heterotrophic bacteria found in
soda lakes, such as Halonatronum saccharophilum, Amphibacillus fermentum,
and Amphibacillus tropicus, isolated from Lake Magadi in Kenya, use a purely
fermentative type of metabolism in which mono-, di-, and polysaccharides are
catabolized through the fructose bisphosphate pathway into acetate, ethanol, and
CO 2 [32]. Microarray studies on another hemicellulose degrading facultative anaerobe isolated from the Wudunur Soda Lake in China, Bacillus sp., N16-5, revealed a
complex hierarchical pattern of sugar metabolism in which glucose was the preferred
substrate, followed by components of complex hemicellulose polysaccharides such
as xylan, pectin, and galactomannan. Glucose was found to be the primary repressor
of expression of gene clusters involved in the degradation of polysaccharides, which
are only partially degraded by extracellular glycoside hydrolases before being
transported intracellularly by oligopeptide transporters to be further processed
[33]. This is thought to be a genetic adaptation to allow responses to the carbohydrate source fluctuations in the environment [33].
Unlike acetogens from more neutrophilic habitats, acetogenic bacteria found in
soda lakes cannot grow chemolithotropically on hydrogen, carbon monoxide, and
carbon dioxide mixtures and are trophically limited to products from primary
anaerobes in the community such as lactate, histidine, and ethanol [8]. In these
bacteria, the Wood-Ljungdahl acetyl-CoA pathway, in which the CO 2 generated
from the catabolism of the primary substrates is converted to acetate, is coupled with
proton-dependent ATP synthesis by generating the electron and proton gradient
required by F 1 F 0 -type ATP synthases [34]. This is the case with the bacteria
Natroniella acetigena and Natronincola histidinovorans, while in T. magadiensis
the fermentation of arginine through the ornithine cycle and subsequent conversion
of carbamoyl phosphate by carbamate kinase is coupled to ATP synthesis
[34]. Genome analysis of the strictly anaerobic and sulphate-reducing bacterium
Thermodesulfovibrio sp. N1 showed that it lacked the complete TCA and WoodLjundahl pathways and would therefore not be capable of fully oxidizing organic
substrates [35]. Instead, it can reversibly decarboxylate pyruvate to acetyl-CoA,
which is further converted into acetate by the acetyl-CoA synthetase, generating
ATP in the process. Several other dehydrogenases present in the periplasmic space,
144
P. H. Lebre and D. A. Cowan