Alkaline environments, particularly the East African Rift Valley soda lakes are
the most productive lakes on the planet and are supporting huge flocks of birds,
especially flamingos (Phoenicopterus roseus and Phoeniconaias minor). These
birds are associated with the soda lakes, wade and swim to feed on the cyanobacteria
Arthrospira (previously known as Spirulina). However, these lakes are very alkaline
and are hostile to practically all other forms of non-adapted life including humans. It
is believed that the birds adapted to these hostile lakes have special tough skin and
scales on their legs which prevent them from the alkali attack. However, detailed
studies on how exactly this scale protects the birds’ leg from the alkali effect are still
lacking. The same holds true for other eukaryotes such as crustaceans, flagellates,
insects, etc. that are thriving in high pH habitats. Relatively, fish adapted to alkaline
lakes attracted attention which seems more due to economic importance than interest
in basic understanding.
Although not all, some soda lakes are known for their fish. Fish such as the lake
Magadi tilapia (Alcolapia graham) are adapted to thrive in hypersaline alkaline
water that can kill other fish within minutes [237]. Studies made so far on high pH
adaptation of fish revolved around two challenges, blood pH maintenance and
ammonia excretion. When fish get transferred from neutral to alkaline water, the
blood pH increased rapidly [238–241]. This is not mainly due to direct translocation
of alkali to blood but driven by solubility of CO 2 . Above pH 8.5, almost all CO 2 in
water converts to bicarbonate (HCO 3
À ) and carbonate (CO 3
2À
), and this results in a
CO 2 deficiency around the gill. This creates a faster diffusion of CO 2 from the blood
of the fish [242]. The rapid loss of CO 2 from the blood leads to the rise in blood pH
which is known as respiratory alkalosis [238–241]. Moreover, the abundant OH
À
and HCO 3
À of the alkaline environment create an electrical gradient which facilitates
the exchange of blood H
+ for environmental HCO 3
À [243] and this may potentially
contribute to the blood pH rise. However, studies made on tilapia that live in a pH 10
soda lake revealed that it involved mechanisms that reduce gill permeability to
HCO 3
À [244]. Moreover, these fish are adapted to handle high plasma pH [242,
245]. It has also been reported that fish adapted to high pH habitats lower the blood
pH to physiological range by exchanging the blood Na
+ and HCO 3
À
, respectively,
for H
+ and Cl
À of the aquatic body [240, 241, 246].
The other potential challenge for fish to adapt high pH environment is accumulation of ammonia in the blood caused by its unfavorable passive diffusion across the
gills [240]. Since protein catabolism continuously generates ammonia [247], if it is
not removed effectively, it tends to accumulate in the body. High level of NH 3 is
toxic as it binds to the brain N-methyl-D-aspartate (NMDA) receptors and cause an
over-excitation and ultimate death [248, 249]. Terrestrial animals convert NH 3 to
urea or uric acid at the expense of energy and then remove it from their bodies.
Whereas fish, living in aquatic environment have become ammonotelic and directly
discharge NH 3 through the surface of their gills without energy expenditure. However, such removal of NH 3 at gill surface is unfavorable in high pH environment.
Fish adapted to high pH habitats are able to reduce NH 3 load by converting it to
urea and discharge it through urea transporter A (UT-A) at the gills surface
[246, 250]. This indicates that the fish adapted to high pH aquatic environments
have the necessary biochemical machineries to process NH 3 into urea and
Challenges and Adaptations of Life in Alkaline Habitats
119
the most productive lakes on the planet and are supporting huge flocks of birds,
especially flamingos (Phoenicopterus roseus and Phoeniconaias minor). These
birds are associated with the soda lakes, wade and swim to feed on the cyanobacteria
Arthrospira (previously known as Spirulina). However, these lakes are very alkaline
and are hostile to practically all other forms of non-adapted life including humans. It
is believed that the birds adapted to these hostile lakes have special tough skin and
scales on their legs which prevent them from the alkali attack. However, detailed
studies on how exactly this scale protects the birds’ leg from the alkali effect are still
lacking. The same holds true for other eukaryotes such as crustaceans, flagellates,
insects, etc. that are thriving in high pH habitats. Relatively, fish adapted to alkaline
lakes attracted attention which seems more due to economic importance than interest
in basic understanding.
Although not all, some soda lakes are known for their fish. Fish such as the lake
Magadi tilapia (Alcolapia graham) are adapted to thrive in hypersaline alkaline
water that can kill other fish within minutes [237]. Studies made so far on high pH
adaptation of fish revolved around two challenges, blood pH maintenance and
ammonia excretion. When fish get transferred from neutral to alkaline water, the
blood pH increased rapidly [238–241]. This is not mainly due to direct translocation
of alkali to blood but driven by solubility of CO 2 . Above pH 8.5, almost all CO 2 in
water converts to bicarbonate (HCO 3
À ) and carbonate (CO 3
2À
), and this results in a
CO 2 deficiency around the gill. This creates a faster diffusion of CO 2 from the blood
of the fish [242]. The rapid loss of CO 2 from the blood leads to the rise in blood pH
which is known as respiratory alkalosis [238–241]. Moreover, the abundant OH
À
and HCO 3
À of the alkaline environment create an electrical gradient which facilitates
the exchange of blood H
+ for environmental HCO 3
À [243] and this may potentially
contribute to the blood pH rise. However, studies made on tilapia that live in a pH 10
soda lake revealed that it involved mechanisms that reduce gill permeability to
HCO 3
À [244]. Moreover, these fish are adapted to handle high plasma pH [242,
245]. It has also been reported that fish adapted to high pH habitats lower the blood
pH to physiological range by exchanging the blood Na
+ and HCO 3
À
, respectively,
for H
+ and Cl
À of the aquatic body [240, 241, 246].
The other potential challenge for fish to adapt high pH environment is accumulation of ammonia in the blood caused by its unfavorable passive diffusion across the
gills [240]. Since protein catabolism continuously generates ammonia [247], if it is
not removed effectively, it tends to accumulate in the body. High level of NH 3 is
toxic as it binds to the brain N-methyl-D-aspartate (NMDA) receptors and cause an
over-excitation and ultimate death [248, 249]. Terrestrial animals convert NH 3 to
urea or uric acid at the expense of energy and then remove it from their bodies.
Whereas fish, living in aquatic environment have become ammonotelic and directly
discharge NH 3 through the surface of their gills without energy expenditure. However, such removal of NH 3 at gill surface is unfavorable in high pH environment.
Fish adapted to high pH habitats are able to reduce NH 3 load by converting it to
urea and discharge it through urea transporter A (UT-A) at the gills surface
[246, 250]. This indicates that the fish adapted to high pH aquatic environments
have the necessary biochemical machineries to process NH 3 into urea and
Challenges and Adaptations of Life in Alkaline Habitats
119