2 The Grand Challenges to Thrive in Alkaline Habitats
from a Neutralophilic Standpoint
A wide range of “bizarre” environments exist in the biosphere. Hot and frozen
environments, sulfurous springs, solfataras, the deep-sea black smoker vents and
cold seeps, acidic environments of anthropogenic and natural origin, and salt lakes
are some among the many that fall in this category. These environments have their
own challenges for life to thrive in. However, such sites are often found inhabited by
organisms, which have specific adaptive solutions to the challenges of the respective
extreme habitats. Likewise, alkaline environments have their own challenges, and
some of the most important ones are discussed below.
The biochemical reactions of life are not spontaneous or self-driven; rather they
are highly regulated and are mediated by specific enzymes which are operationally
stable within a range of pH. The cytoplasmic pH of cells from various organisms is
known to be within the neutral range [22], and the enzymes that catalyze the myriad
biochemical reactions occurring inside cells are evolved to work optimally around
this pH, neutrality. As the pH drifts away from the neutral range, the catalytic
efficiency of the enzymes dwindles, and the cellular functional integrity drops.
Like the functional integrity, the structural integrity of intracellular biomolecules
is tuned to the cytoplasmic pH. The integrity of at least some of the important
macromolecules such as proteins, lipids, and genetic materials can be labile at
elevated pH, and the molecules become prone to precipitation or breakdown [23–
25]. This structural and functional integrity impairment can be fatal and bring
cellular demise. Thus, for biochemical reactions to proceed without a flaw and
ensure survival, the intracellular pH should be maintained in the neutral range.
However, when organisms are exposed to high pH condition, maintaining their
cytoplasmic pH within the neutral range becomes difficult, and an upward drift in
the cytoplasmic pH can occur. If this cytoplasmic pH rise remains unchecked, it
ultimately kills the cell/organism. Thus, thriving in high pH environment requires an
effective way of maintaining the intracellular pH close to neutrality and ability to
withstand some degree of alkalinization. This process of maintaining pH within
physiologically favorable range regardless of the extracellular environment is known
as pH homeostasis.
In addition to pH homeostasis, at least non-photosynthetic aerobic prokaryotic
life forms face another daunting task in high pH environment, bioenergetics. Living
organisms require energy to perform the phenomena of life such as growth, reproduction, structure maintenance, movement, etc. Moreover, cells maintain order
against chaos/randomness with expenditure of energy. If there is no energy that a
cell uses to maintain order, chaos reign, and it loses viability. Thus, life-sustaining
metabolic processes enable organisms to generate and store energy. In this regard,
ATP is the most vital molecule which lays at the center of cellular bioenergetics. It is
known as the energy currency of life which can store and shuttle chemical energy
within cells. This energy-rich molecule can be produced by various cellular processes, most typically by F 1 F 0 -ATP synthase-mediated oxidative phosphorylation
Challenges and Adaptations of Life in Alkaline Habitats
89
from a Neutralophilic Standpoint
A wide range of “bizarre” environments exist in the biosphere. Hot and frozen
environments, sulfurous springs, solfataras, the deep-sea black smoker vents and
cold seeps, acidic environments of anthropogenic and natural origin, and salt lakes
are some among the many that fall in this category. These environments have their
own challenges for life to thrive in. However, such sites are often found inhabited by
organisms, which have specific adaptive solutions to the challenges of the respective
extreme habitats. Likewise, alkaline environments have their own challenges, and
some of the most important ones are discussed below.
The biochemical reactions of life are not spontaneous or self-driven; rather they
are highly regulated and are mediated by specific enzymes which are operationally
stable within a range of pH. The cytoplasmic pH of cells from various organisms is
known to be within the neutral range [22], and the enzymes that catalyze the myriad
biochemical reactions occurring inside cells are evolved to work optimally around
this pH, neutrality. As the pH drifts away from the neutral range, the catalytic
efficiency of the enzymes dwindles, and the cellular functional integrity drops.
Like the functional integrity, the structural integrity of intracellular biomolecules
is tuned to the cytoplasmic pH. The integrity of at least some of the important
macromolecules such as proteins, lipids, and genetic materials can be labile at
elevated pH, and the molecules become prone to precipitation or breakdown [23–
25]. This structural and functional integrity impairment can be fatal and bring
cellular demise. Thus, for biochemical reactions to proceed without a flaw and
ensure survival, the intracellular pH should be maintained in the neutral range.
However, when organisms are exposed to high pH condition, maintaining their
cytoplasmic pH within the neutral range becomes difficult, and an upward drift in
the cytoplasmic pH can occur. If this cytoplasmic pH rise remains unchecked, it
ultimately kills the cell/organism. Thus, thriving in high pH environment requires an
effective way of maintaining the intracellular pH close to neutrality and ability to
withstand some degree of alkalinization. This process of maintaining pH within
physiologically favorable range regardless of the extracellular environment is known
as pH homeostasis.
In addition to pH homeostasis, at least non-photosynthetic aerobic prokaryotic
life forms face another daunting task in high pH environment, bioenergetics. Living
organisms require energy to perform the phenomena of life such as growth, reproduction, structure maintenance, movement, etc. Moreover, cells maintain order
against chaos/randomness with expenditure of energy. If there is no energy that a
cell uses to maintain order, chaos reign, and it loses viability. Thus, life-sustaining
metabolic processes enable organisms to generate and store energy. In this regard,
ATP is the most vital molecule which lays at the center of cellular bioenergetics. It is
known as the energy currency of life which can store and shuttle chemical energy
within cells. This energy-rich molecule can be produced by various cellular processes, most typically by F 1 F 0 -ATP synthase-mediated oxidative phosphorylation
Challenges and Adaptations of Life in Alkaline Habitats
89