Acidophiles
are microorganisms found in environments of excessive pH.Ý More specifically, in acidic
environments that are less than a pH of 5.Ý
These types of environments can be found naturally, such as the acid
hot springs found in Yellow Stone National Park. Pictured is Norris, an
acid spring that has an abundance of the element known as iron.ÝÝ It is the iron that some acidophiles
use metabolically to survive.Ý Two
examples of acidophiles that oxidize iron are Thiobacillus ferrooxidans and Leptospirillum ferrooxidans. Ý
Figure 1 Ýhttp://www.bact.wisc.edu/bact303/b13
Even
still, some acidophiles seem to be those that benefit from or directly
create their own acidic environment from metabolic pathways. One such
microorganism is A.Coyptum, which
is associated with T. ferrooxidians.
Perhaps
the most remarkable feature of acidophiles is their ability to thrive in
environments of high acidity while maintaining a neutral pH
intracellularly.Ý The reason why the
organism must maintain neutrality within the cytoplasm concerns the
survival and function of the biomolecules.Ý
The enzymes and metabolic processes found in acidophiles are
designed to function at neutrality, like other neutral microorganisms.Ý These enzymes are not adapted to acid
tolerance and most would denature if exposed to the surrounding outer
habitat. In addition, the tertiary and quaternary structures of proteins
can be destroyed by the exposure.Ý
This is because the actual pH stability of a protein is a specific
feature and it has been reported that if, in an acidophile, the cytoplasm
pH reaches about 5.0, the cell is non-viable and cannot recover (Edwards,
ed., 37).ÝÝ What
enables the acidophiles to maintain an internal neutral pH is the osmotic
barrier between the two pH environments.Ý
The osmotic barrier is achieved by the cytoplasmic membrane, in
which one side is exposed to the acidic medium, and the other to the
neutral cytoplasm.Ý The process by
which the acidophile is able to maintain this difference in pH is through a
complex chemiosmotic process.Ý The
chemiosmotic process involves the movement of protons across the
membrane.Ý It is the extremozymes
that enable this process to occur.Ý
It is these enzymes that are highly researched for their industrial
purposes. Ý
Figure 3
Bingham Canyon copper mineÝÝÝÝÝÝÝÝÝ http://www.brass.org/Megab2/General/pub121/section6.htm
Acidophiles have been utilized in
microbiological mining, which involves the extraction of metals, called
mineral leaching.Ý Iron and sulfur
oxidizing acidophiles are used to enhance the extraction of certain metals
in the heap leaching process of copper, uranium, and gold-bearing sulfide
mines.Ý A list of some of the
microorganisms involved is found in Figure
4.Ý
Partial Listing of
Acidophilic bacteria
of Interest
in Metal Leaching Processes
Type
|
Main mode of metabolism |
Species
designation
|
|
Mesophiles |
Inorganic
carbon source |
Thiobacillus
ferrooxidians, T.
thiooxidans, T. prosperous, Leptospirillum ferrooxidans |
|
|
Organic
carbon and energy source |
Acidophilium
cryptum and
related heterotrophs |
|
|
Inorganic
or organic carbon and energy source (facultative heterotrophs) |
T.
acidophilus, T. organovorus, T. cuprinus |
|
Moderate
thermo- Acidophiles |
Inorganic
carbon and energy
source |
Sulfobacillus
thermosulfidooxidans, Metallospaera sedula, TH strains
and several other unnamed isolates from thermal springs and from samples
derived from sulfide ore and coal deposits |
|
Extreme Thermo- acidophiles |
Inorganic
carbon and energy source |
Sulfolobus solfaraticus,
S.acidocaldarius, Acidianus brierleyi, A. infernus |
The
extraction of copper by microbiological leaching today is carried out on a
large scale.Ý There are numerous
companies that now offer services of bioleaching.Ý One such organization is BacTech and their process of
bioleaching can be found on the web at http://www.bactech.com.Ý The underlying technology is to utilize
bacteria to break down the mineral matrix by oxidation, which releases the
metals for recovery.Ý This allows
for Gold to be freed by the process of oxidation and once in residue can be
recovered by cyanidation.Ý Base
metals such as copper pass into solution and can be recovered using solvent
extraction.ÝÝ Pictured in Figure 5 is a bacterium BacTech
uses in the extraction of sulfides.
Acidophiles
are the focus of recent research, especially in the area of molecular
genetics.Ý With the availability of
DNA sequencing and PCR, thanks to another extremophile, the theromophile T. aquaticus (link here to Mikeís
page), DNA sequences of acidophiles are possible.Ý Check out these web sites for cutting edge research on
genetic studies of acidophiles: http://link.springer.de/link/service/journals/00792/bibs/9003003/90030/99.htm http://link.springer.de/link/service/journal/00792/bibs/8002002/80020067.htm http://link.springer.de/service/journal/00792/journal/00792/index.htm
Alkaliphiles are microorganisms that
are also found in environments of excessive pH, however, the pH level in
which they thrive is basic.Ý More
specifically, environments with a pH level above 9.Ý Obligate alkaliphiles are incapable of
growth at a neutral pH and have the optimal growth at pH around 9.5.Ý However, these obligate alkaliphiles are
able to sustain life at pH as high as 11.Ý
There must be a distinction made between a microorganism that is an
alkaliphile and one that is alkalotolerant.Ý It is the alkaliphile that has optimum growth at least two
units above neutrality (pH 7).Ý
However, an alkalotolerant microorganism is one that can survive in
an environment of 9.0 or higher, but it has optimal growth at neutrality or
lower. Environments that have a low pH can
be found naturally throughout the world.Ý
These areas are rich in certain elements that make them
alkaline.Ý One of the elements is
calcium.Ý High calcium environments
can be found in ground waters located in California, Oman and Jordan.Ý However, the most dominant alkalitic
environments are those rich in calcium carbonate.Ý These environments consist of soda lakes and deserts located
throughout the world.Ý For example, Figure 6 is a soda dry lake found
on the edge of the Mojave National Reserve.ÝÝ It is also the site of the Desert Studies Center, an
affiliate of California State University. Ý