only ways to reintroduce this critical element into the biosphere. A possible advantage for the eukaryotes thriving in
the extreme conditions of the Tinto Basin is an unlimited
iron supply provided by the chemolithotrophs growing on
the rich iron sulfides of the Iberian Pyritic Belt.
Has this iron-limiting scenario been a constant in the
history of life on Earth? According to different authors, the
Archaean oceans held important concentrations of soluble
iron with extremely low concentration of oxygen. In these
conditions, it is reasonable to postulate the appearance of
microorganisms able to obtain energy oxidizing or reducing
iron, in other words, maintaining operative an anaerobic iron
cycle. It has been suggested that the sulfur cycle was not
fully operative at that time. Although microbial sulfur oxidation was probably active, different authors consider that
sulfate-reducing activity was not operative until the Proterozoic, which gave rise to a massive sequestering of heavy
metals and a correspondent severe nitrogen crisis with
important evolutionary consequences.
As can be seen the history of iron in the biosphere is still
an open question. There is no doubt about the importance of
microorganisms in iron mobilization. At present, it is not
clear whether the Tinto Basin and the related ecosystems are
a relic of the period in the history of life on our planet when
iron was freely available or simple a recent adaptation to an
extreme environment product of chemolithotrophy and able
to maintain enough ferric iron in solution to generate a
constant acidic pH environment. Concerning adaptation,
preliminary physiological studies of acidophilic photosynthetic protists suggest that they can adapt rather easily to
extreme acidic environments. If complex cellular systems
can do it, it is reasonable to assume that less complex
systems, like prokaryotes, should not have any problem. As
mentioned, the level of eukaryotic diversity in the Tinto
ecosystem seems to be much higher than the prokaryotic
one. This apparent contradiction could result from the
limitations of our preliminary knowledge of the system. It
could well be that prokaryotic diversity is much higher in
the unexplored anaerobic sediments or in the subsurface of
the Iberian Pyritic Belt. In any case, it should be kept in
mind that we are dealing with extant ecosystems. Conveniently addressed questions should facilitate a deeper characterization of these systems, which could help to clarify its
origin and the role of the different components of the
habitat.
In conclusion, the exploration of different extreme acidic
environments has been able to answer some basic questions
concerning:
1. The microbial generation of the extreme conditions of the
habitats
2. The isolation and characterization of the iron- and sulfurmetabolizing microorganisms responsible for the active
iron and sulfur cycles fully operative in these systems
3. The identification of an unexpected level of eukaryotic
diversity
4. The characterization of iron bioformations
Understanding these ecosystems is necessary not only to
increase our knowledge of unusual habitats with
implications in different areas – geomicrobiology, paleontology, evolution, astrobiology, etc. – but to improve the
performance of biotechnology processes, such as biomining,
and to search for new applications for the microorganisms
thriving at extreme low pH.
10.7 Alkaliphilic Microorganisms
10.7.1 Presentation
Microorganisms whose development is favored at high pH
values can be divided into two categories: (1) alkalinetolerant microorganisms able to grow at a pH greater than
or equal to 9, with optimal at neutral pH, and (2) alkaliphilic
microorganisms with optimal growth at pH above 9 (cf. Sect.
9.6.3). Alkaliphiles can be further subdivided into facultative alkaliphiles and obligate alkaliphiles. The former group
of bacteria grows at a pH greater than 9 but also in the
neutral pH range, while the latter group of bacteria cannot
grow at pH below 8.5–9.
Some alkaliphiles are also halophiles (haloalkaliphiles);
they are able to grow in the presence of NaCl concentration
up to saturation. Alkaliphilic microorganisms are ubiquitous; unlike other extremophiles found mainly in the
corresponding extreme environment, alkaliphiles can be
found not only in high pH environments but also in neutral
pH environments. Alkaliphiles are of considerable interest
regarding the production of enzymes.
There are also big challenges in discovering novel
microorganisms and novel mechanisms that they use to
adapt to these harsh conditions of life.
10.7.2 Alkaline Environments
The alkalinity of the natural alkaline environments results
from the presence of high levels of sodium carbonate. Globally there are two types of alkaline environments differing
by their salinity (NaCl): the alkaline saline environments and
the weakly salted alkaline environments. The formers are
represented by the soda lakes; they correspond to the most
stable natural alkaline environments on Earth. They are
found in closed ponds situated in the dry zones where high
rates of evaporation contribute to concentrate salts and
carbonates. Their salinity can reach 35 % in NaCl and pH
values range between 8 and 12. They are characterized by
10 The Extreme Conditions of Life on the Planet and Exobiology
385
the extreme conditions of the Tinto Basin is an unlimited
iron supply provided by the chemolithotrophs growing on
the rich iron sulfides of the Iberian Pyritic Belt.
Has this iron-limiting scenario been a constant in the
history of life on Earth? According to different authors, the
Archaean oceans held important concentrations of soluble
iron with extremely low concentration of oxygen. In these
conditions, it is reasonable to postulate the appearance of
microorganisms able to obtain energy oxidizing or reducing
iron, in other words, maintaining operative an anaerobic iron
cycle. It has been suggested that the sulfur cycle was not
fully operative at that time. Although microbial sulfur oxidation was probably active, different authors consider that
sulfate-reducing activity was not operative until the Proterozoic, which gave rise to a massive sequestering of heavy
metals and a correspondent severe nitrogen crisis with
important evolutionary consequences.
As can be seen the history of iron in the biosphere is still
an open question. There is no doubt about the importance of
microorganisms in iron mobilization. At present, it is not
clear whether the Tinto Basin and the related ecosystems are
a relic of the period in the history of life on our planet when
iron was freely available or simple a recent adaptation to an
extreme environment product of chemolithotrophy and able
to maintain enough ferric iron in solution to generate a
constant acidic pH environment. Concerning adaptation,
preliminary physiological studies of acidophilic photosynthetic protists suggest that they can adapt rather easily to
extreme acidic environments. If complex cellular systems
can do it, it is reasonable to assume that less complex
systems, like prokaryotes, should not have any problem. As
mentioned, the level of eukaryotic diversity in the Tinto
ecosystem seems to be much higher than the prokaryotic
one. This apparent contradiction could result from the
limitations of our preliminary knowledge of the system. It
could well be that prokaryotic diversity is much higher in
the unexplored anaerobic sediments or in the subsurface of
the Iberian Pyritic Belt. In any case, it should be kept in
mind that we are dealing with extant ecosystems. Conveniently addressed questions should facilitate a deeper characterization of these systems, which could help to clarify its
origin and the role of the different components of the
habitat.
In conclusion, the exploration of different extreme acidic
environments has been able to answer some basic questions
concerning:
1. The microbial generation of the extreme conditions of the
habitats
2. The isolation and characterization of the iron- and sulfurmetabolizing microorganisms responsible for the active
iron and sulfur cycles fully operative in these systems
3. The identification of an unexpected level of eukaryotic
diversity
4. The characterization of iron bioformations
Understanding these ecosystems is necessary not only to
increase our knowledge of unusual habitats with
implications in different areas – geomicrobiology, paleontology, evolution, astrobiology, etc. – but to improve the
performance of biotechnology processes, such as biomining,
and to search for new applications for the microorganisms
thriving at extreme low pH.
10.7 Alkaliphilic Microorganisms
10.7.1 Presentation
Microorganisms whose development is favored at high pH
values can be divided into two categories: (1) alkalinetolerant microorganisms able to grow at a pH greater than
or equal to 9, with optimal at neutral pH, and (2) alkaliphilic
microorganisms with optimal growth at pH above 9 (cf. Sect.
9.6.3). Alkaliphiles can be further subdivided into facultative alkaliphiles and obligate alkaliphiles. The former group
of bacteria grows at a pH greater than 9 but also in the
neutral pH range, while the latter group of bacteria cannot
grow at pH below 8.5–9.
Some alkaliphiles are also halophiles (haloalkaliphiles);
they are able to grow in the presence of NaCl concentration
up to saturation. Alkaliphilic microorganisms are ubiquitous; unlike other extremophiles found mainly in the
corresponding extreme environment, alkaliphiles can be
found not only in high pH environments but also in neutral
pH environments. Alkaliphiles are of considerable interest
regarding the production of enzymes.
There are also big challenges in discovering novel
microorganisms and novel mechanisms that they use to
adapt to these harsh conditions of life.
10.7.2 Alkaline Environments
The alkalinity of the natural alkaline environments results
from the presence of high levels of sodium carbonate. Globally there are two types of alkaline environments differing
by their salinity (NaCl): the alkaline saline environments and
the weakly salted alkaline environments. The formers are
represented by the soda lakes; they correspond to the most
stable natural alkaline environments on Earth. They are
found in closed ponds situated in the dry zones where high
rates of evaporation contribute to concentrate salts and
carbonates. Their salinity can reach 35 % in NaCl and pH
values range between 8 and 12. They are characterized by
10 The Extreme Conditions of Life on the Planet and Exobiology
385