pH 7 [229]. Considering the extreme scarcity of iron at high pH, one expects that
alkaliphiles evolved a very efficient sequestering mechanism. Indeed, studies
revealed that alkaliphiles produce very effective iron-binding chelators,
siderophores. It is believed that siderophore-assisted iron acquisition is one of the
critical adaptations of alkaliphiles in high pH habitats [229–231]. Studies made so
far are limited to production (of siderophores). Detail biochemical characterization
and structural analysis of these siderophores can be beneficial to advance our
understanding on alkaliphiles adaptation and may also yield new siderophores of
biotechnological importance. It is interesting that the first structural analysis proves
the potential of alkaliphiles as sources of novel siderophores [229].
It is not only the metal ions’ availability that is limited at highly alkaline
conditions [232], other major nutrients such as nitrogen and phosphate could also
be growth-limiting factors [16, 42, 233]. For example, NH 4
+ which serves as
nitrogen source for a wide variety of organisms is mostly converted to volatile and
toxic NH 3 and becomes unavailable in alkaline habitats of pH 10 and above. Studies
have shown that the poor availability of nitrogen sources in some alkaline habitats
makes the inhabitant alkaliphiles resort to utilization of certain unconventional
resources such as cyanide and its derivatives as nitrogen source [230, 234].
In general, the limited work done so far indicates that there are challenges and
associated adaptations regarding the bioavailability of certain nutrients in high pH
habitats. It may be attractive for basic and applied areas to extend studies in this
direction.
4 Adaptations of Eukaryotes to High pH Environments
There are numerous unicellular and multicellular eukaryotes such as ciliates, dinoflagellates, diatoms, fungi, green algae, invertebrates, fish, etc. that flourish in high
pH habitats. Although there are very interesting studies on the diversity, taxonomy,
population dynamics, ecological role, etc. of these eukaryotes [235, 236], there is
very little information on how these organisms are adapted to their respective high
pH habitats. Nearly all the studies regarding high pH adaptations of life have been
focused on microorganisms (bacteria, archaea, and to some extent fungi). This may
be due to several reasons such as their dominance/abundance, biotechnological
interests, relatively easy handling, etc. On the other hand, studies on high pH
adaptation of eukaryotes not only improve our understanding but also may enlighten
us with new mechanisms. For instance, the cell membrane of most protozoans such
as ciliates adapted to high pH environment may be exposed to the alkaline environment. It is of great interest to know how this membrane shields the cytoplasm
effectively from the effect of the extreme pH. Moreover, the ciliates inner part of
the cell membrane facing the cytoplasm is lined with proteinaceous structure known
as pellicle. Although one expects that this pellicle may play an important role in the
adaptation, there is no available information how this remarkable structure contributes in adapting alkaline habitats.
118
G. Mamo
alkaliphiles evolved a very efficient sequestering mechanism. Indeed, studies
revealed that alkaliphiles produce very effective iron-binding chelators,
siderophores. It is believed that siderophore-assisted iron acquisition is one of the
critical adaptations of alkaliphiles in high pH habitats [229–231]. Studies made so
far are limited to production (of siderophores). Detail biochemical characterization
and structural analysis of these siderophores can be beneficial to advance our
understanding on alkaliphiles adaptation and may also yield new siderophores of
biotechnological importance. It is interesting that the first structural analysis proves
the potential of alkaliphiles as sources of novel siderophores [229].
It is not only the metal ions’ availability that is limited at highly alkaline
conditions [232], other major nutrients such as nitrogen and phosphate could also
be growth-limiting factors [16, 42, 233]. For example, NH 4
+ which serves as
nitrogen source for a wide variety of organisms is mostly converted to volatile and
toxic NH 3 and becomes unavailable in alkaline habitats of pH 10 and above. Studies
have shown that the poor availability of nitrogen sources in some alkaline habitats
makes the inhabitant alkaliphiles resort to utilization of certain unconventional
resources such as cyanide and its derivatives as nitrogen source [230, 234].
In general, the limited work done so far indicates that there are challenges and
associated adaptations regarding the bioavailability of certain nutrients in high pH
habitats. It may be attractive for basic and applied areas to extend studies in this
direction.
4 Adaptations of Eukaryotes to High pH Environments
There are numerous unicellular and multicellular eukaryotes such as ciliates, dinoflagellates, diatoms, fungi, green algae, invertebrates, fish, etc. that flourish in high
pH habitats. Although there are very interesting studies on the diversity, taxonomy,
population dynamics, ecological role, etc. of these eukaryotes [235, 236], there is
very little information on how these organisms are adapted to their respective high
pH habitats. Nearly all the studies regarding high pH adaptations of life have been
focused on microorganisms (bacteria, archaea, and to some extent fungi). This may
be due to several reasons such as their dominance/abundance, biotechnological
interests, relatively easy handling, etc. On the other hand, studies on high pH
adaptation of eukaryotes not only improve our understanding but also may enlighten
us with new mechanisms. For instance, the cell membrane of most protozoans such
as ciliates adapted to high pH environment may be exposed to the alkaline environment. It is of great interest to know how this membrane shields the cytoplasm
effectively from the effect of the extreme pH. Moreover, the ciliates inner part of
the cell membrane facing the cytoplasm is lined with proteinaceous structure known
as pellicle. Although one expects that this pellicle may play an important role in the
adaptation, there is no available information how this remarkable structure contributes in adapting alkaline habitats.
118
G. Mamo