c-554, and c-551, with cytochrome-c552 making up more than 60% of the total
soluble cytochrome c content [77]. The midpoint potential of cytochrome-c552 in
this bacterium is similar to that of neutrophilic cytochromes-c (+218 mV at pH 9 and
10), which suggests that the bioenergetic state of the periplasm of Pseudomonas
alcaliphila is akin to of its more neutrophilic counterparts. Despite this, cytochromec552 was shown to play an important role for growth of this bacteria at high pH by
working as a proton reservoir in the periplasmic space to compensate for the low
proton concentrations in the extracellular medium [77].
7 Concluding Remarks
Alkaliphiles exist in one of the accepted “extreme environments,” where selective
pressures preclude most other organisms. In alkaline niches such as soda lakes and
saline freshwater, a combination of abiotic pressures, including low CO 2 and metal
ions concentrations, low proton gradient, and high salinity, impose a thermodynamic
burden on essential biological functions such as carbon fixation, motility, and
oxidative phosphorylation. Despite apparent thermodynamic limitations,
alkaliphiles comprise a diverse group of microorganisms with divergent genetic
origins, metabolic requirements, and functional capabilities. This genetic diversity,
observed across eukaryotic, archaeal, and bacterial phyla, suggests that alkalinity is
not a “narrow” driver of the evolution of organisms, in that alkaliphilicity is a
derived property across a very wide taxonomic space.
Alkaliphiles share two main features: the ability to maintain pH homeostasis and
the ability to perform bioenergetics processes in an environment with an inverse, or
“reversed”, chemical gradient. Accordingly, many alkaliphile genomes are enriched
in genes for cation/proton antiporters and cation/substrate symporters. The enrichment of such transporters reflects the need to balance both PMF and SMF in order
achieve pH homeostasis and fuel energetically expensive processes such as substrate
acquisition and chemotaxis. In addition, alkaliphile genomes encode a large number
of membrane-localized proton and electron-retaining proteins such as cytochrome
oxidases, which allow for the maintenance of a PMF across the membrane that
favours efficient proton-coupled ATP synthesis. Some level of metabolic specialization can also be observed as a possible response to secondary pressures exerted in
an alkaline environment. For instance, alkaliphilic cyanobacteria express genes
involved in CO 2 -concentration mechanisms, which allow for efficient carbon fixation in environments where the levels of CO 2 are low. Other alkaliphilic bacteria
resort to using other gases and inorganic compounds such as H 2 and sulphites as
energy sources, although such trophic characteristics are not limited to high pH
environments. In turn, heterotrophic aerobes and aerobes utilize on the products of
the primary autotrophs. As is typical of all complex communities, microbial communities in alkaline niches live as multilayered tiers of microorganisms that are
functionally and metabolically interdependent, where such interdependency is
expressed as a high level of genetic diversity.
Genomics of Alkaliphiles
151
soluble cytochrome c content [77]. The midpoint potential of cytochrome-c552 in
this bacterium is similar to that of neutrophilic cytochromes-c (+218 mV at pH 9 and
10), which suggests that the bioenergetic state of the periplasm of Pseudomonas
alcaliphila is akin to of its more neutrophilic counterparts. Despite this, cytochromec552 was shown to play an important role for growth of this bacteria at high pH by
working as a proton reservoir in the periplasmic space to compensate for the low
proton concentrations in the extracellular medium [77].
7 Concluding Remarks
Alkaliphiles exist in one of the accepted “extreme environments,” where selective
pressures preclude most other organisms. In alkaline niches such as soda lakes and
saline freshwater, a combination of abiotic pressures, including low CO 2 and metal
ions concentrations, low proton gradient, and high salinity, impose a thermodynamic
burden on essential biological functions such as carbon fixation, motility, and
oxidative phosphorylation. Despite apparent thermodynamic limitations,
alkaliphiles comprise a diverse group of microorganisms with divergent genetic
origins, metabolic requirements, and functional capabilities. This genetic diversity,
observed across eukaryotic, archaeal, and bacterial phyla, suggests that alkalinity is
not a “narrow” driver of the evolution of organisms, in that alkaliphilicity is a
derived property across a very wide taxonomic space.
Alkaliphiles share two main features: the ability to maintain pH homeostasis and
the ability to perform bioenergetics processes in an environment with an inverse, or
“reversed”, chemical gradient. Accordingly, many alkaliphile genomes are enriched
in genes for cation/proton antiporters and cation/substrate symporters. The enrichment of such transporters reflects the need to balance both PMF and SMF in order
achieve pH homeostasis and fuel energetically expensive processes such as substrate
acquisition and chemotaxis. In addition, alkaliphile genomes encode a large number
of membrane-localized proton and electron-retaining proteins such as cytochrome
oxidases, which allow for the maintenance of a PMF across the membrane that
favours efficient proton-coupled ATP synthesis. Some level of metabolic specialization can also be observed as a possible response to secondary pressures exerted in
an alkaline environment. For instance, alkaliphilic cyanobacteria express genes
involved in CO 2 -concentration mechanisms, which allow for efficient carbon fixation in environments where the levels of CO 2 are low. Other alkaliphilic bacteria
resort to using other gases and inorganic compounds such as H 2 and sulphites as
energy sources, although such trophic characteristics are not limited to high pH
environments. In turn, heterotrophic aerobes and aerobes utilize on the products of
the primary autotrophs. As is typical of all complex communities, microbial communities in alkaline niches live as multilayered tiers of microorganisms that are
functionally and metabolically interdependent, where such interdependency is
expressed as a high level of genetic diversity.
Genomics of Alkaliphiles
151