environments, it is necessary to evolve mechanisms that effectively generate energy
carriers under this thermodynamically challenging condition.
High pH is known to degrade biological entities by disrupting molecular bonds
between atoms in biomolecules. Thus, one of the challenges to thrive in alkaline
environments is the maintenance of the cell structural integrity at high pH. Proteins,
lipids, carbohydrates, and aromatic structures such as lignin which are the important
structural components of different organisms are susceptible to alkaline conditions.
For instance, keratin, a tough structural protein of hair, feather, horn, and nails
decomposes at high pH [29]. This type of alkaline-mediated proteolysis of cellular
proteins and peptides is sometimes referred to as liquefactive denaturation [23]. Studies on the degradation of plant biomass in aquatic environments have also revealed
that the decomposition rate is linearly related to pH [30]. At high pH the plant cell
wall cementing substance, lignin, decomposes, and the solubility of the hemicellulose fraction increases which results in degradation of plant biomass. Such susceptibility of structural biomolecules to high pH has led to the emergence of applications
that use alkaline treatments to break down biological materials in various processes.
Alkaline treatments are used in pulp and paper industry to break down the lignin and
hemicellulose fractions of the plant biomass (Kraft pulping process), in molecular
biology to digest bacterial cell wall during DNA (e.g., plasmid) extraction, in leather
tanning to dehair skin and hides, in waste management to decompose keratin (e.g.,
feather) waste, etc. Lipids, which are important structural components of cells such
as membranes, are also labile to high pH. The degradation process of lipids is known
as saponification [23]. Degradation of structural components such as the cell membrane is lethal as it compromises the integrity of cells. In fact, alkaline solutions have
long known for their disinfectant properties and are used as antimicrobial agents
[31–34]. Alkaline solutions are also widely used as cleaning agents due to their
ability of removing (by degrading and solubilizing) organic matter such as protein,
lipid, and nucleic acids [32]. Thus, organisms that colonize alkaline habitats must
evolve mechanisms which protect the cell integrity from the adverse effect of the
extreme pH.
The challenges of life at high pH habitats are not restricted only to cell associated
structures, but it also involves the structural and functional integrity of extracellular
products. Cells produce and secrete various biomolecules to the extracellular environment to perform different tasks such as exopolysaccharides for protection,
adhesion and biofilm formation, chemical signal molecules for cell-to-cell communication, enzymes for nutrient acquiring and recycling, bioactive compounds for
defense and competition, etc. [35–40]. The efficiency of these biomolecules influences the success of the organism in colonizing a habitat. To fulfill the desired tasks,
these products should be operationally stable in the habitat condition. Thus, the
success of colonizing high pH habitats, at least partly, depends on the operational
stability of the extracellular products. For instance, extracellular enzymes are very
important to acquire nutrients by breaking down polymeric substrates to smaller
pieces that can be transported to the cytoplasm across the cell envelope. But enzymes
are optimally active and stable within a certain range of pH and can be denatured
and cease to function outside this range. Since the extracellular biomolecules of
Challenges and Adaptations of Life in Alkaline Habitats
91
carriers under this thermodynamically challenging condition.
High pH is known to degrade biological entities by disrupting molecular bonds
between atoms in biomolecules. Thus, one of the challenges to thrive in alkaline
environments is the maintenance of the cell structural integrity at high pH. Proteins,
lipids, carbohydrates, and aromatic structures such as lignin which are the important
structural components of different organisms are susceptible to alkaline conditions.
For instance, keratin, a tough structural protein of hair, feather, horn, and nails
decomposes at high pH [29]. This type of alkaline-mediated proteolysis of cellular
proteins and peptides is sometimes referred to as liquefactive denaturation [23]. Studies on the degradation of plant biomass in aquatic environments have also revealed
that the decomposition rate is linearly related to pH [30]. At high pH the plant cell
wall cementing substance, lignin, decomposes, and the solubility of the hemicellulose fraction increases which results in degradation of plant biomass. Such susceptibility of structural biomolecules to high pH has led to the emergence of applications
that use alkaline treatments to break down biological materials in various processes.
Alkaline treatments are used in pulp and paper industry to break down the lignin and
hemicellulose fractions of the plant biomass (Kraft pulping process), in molecular
biology to digest bacterial cell wall during DNA (e.g., plasmid) extraction, in leather
tanning to dehair skin and hides, in waste management to decompose keratin (e.g.,
feather) waste, etc. Lipids, which are important structural components of cells such
as membranes, are also labile to high pH. The degradation process of lipids is known
as saponification [23]. Degradation of structural components such as the cell membrane is lethal as it compromises the integrity of cells. In fact, alkaline solutions have
long known for their disinfectant properties and are used as antimicrobial agents
[31–34]. Alkaline solutions are also widely used as cleaning agents due to their
ability of removing (by degrading and solubilizing) organic matter such as protein,
lipid, and nucleic acids [32]. Thus, organisms that colonize alkaline habitats must
evolve mechanisms which protect the cell integrity from the adverse effect of the
extreme pH.
The challenges of life at high pH habitats are not restricted only to cell associated
structures, but it also involves the structural and functional integrity of extracellular
products. Cells produce and secrete various biomolecules to the extracellular environment to perform different tasks such as exopolysaccharides for protection,
adhesion and biofilm formation, chemical signal molecules for cell-to-cell communication, enzymes for nutrient acquiring and recycling, bioactive compounds for
defense and competition, etc. [35–40]. The efficiency of these biomolecules influences the success of the organism in colonizing a habitat. To fulfill the desired tasks,
these products should be operationally stable in the habitat condition. Thus, the
success of colonizing high pH habitats, at least partly, depends on the operational
stability of the extracellular products. For instance, extracellular enzymes are very
important to acquire nutrients by breaking down polymeric substrates to smaller
pieces that can be transported to the cytoplasm across the cell envelope. But enzymes
are optimally active and stable within a certain range of pH and can be denatured
and cease to function outside this range. Since the extracellular biomolecules of
Challenges and Adaptations of Life in Alkaline Habitats
91