Thriving in cold polar seas, volcanic springs more than 100°C, and even highly acidic solfataric fields, extremophiles can call any place on this planet their home. Extremophiles live life on the edge of what is considered the norm, having adaptations to extreme pressure, pH, salinity, and temperature (2). There are wide varieties of extremophilic organisms inhabiting virtually every point on the globe; however, the majority of these are Archean (2). Extremophiles can be broken down into groups based on which particular biotype they thrive in. Thermophiles thrive at very high temperatures, while psychrophiles live in very cold environments (2). Alkaliphiles can live at high pH, while their acidophile counterparts have an optimum pH at levels much lower than neutral (2). Also, there are halophiles who live within extremely high salt concentrations, and piezophiles who can live at pressures much higher than one atmosphere (2).
Thermophiles are perhaps one of the most interesting varieties of the extremophilic organisms. These microorganisms are those that can thrive at temperatures over 50°C (9). Based on their optimal temperature, thermophiles can be subdivided into three groups: slight thermophiles with an optimal temperature between 50°C and 64°C and a maximum at 70°C, extreme thermophiles with an optimal temperature between 65°C and 85°C, and finally hyperthermophiles with an optimal temperature above 85°C and a maximum above 90°C (9). (For the purposes of this paper, the term “thermophile” will refer all organisms with the ability to thrive at temperatures above 50°C, unless otherwise noted.) It was previously believed that life could not thrive at temperatures above 113°C, however recent discoveries have found a microbe called strain 121 that is able to grow at 121°C and can survive at 130°C (1). This changed the way many scientists look at the temperature dependence of life.
As of 2001, over sixty species of Bacteria and Archea have been isolated and grown between 80°C and 110°C (9). Of the thermophiles, there are a much higher number of anaerobes than aerobes. This is most likely due to the fact that oxygen is much less soluble at higher temperatures and therefore is not available for organisms to use in metabolic processes (9). Thermophiles can grow in both terrestrial and marine environments, including: solfataric fields, geothermal soils, volcanically heated surface waters, hot fumaroles, deep-sea vents, and even black smokers (2). These can also thrive in biotopes created by man, such as smoldering coal refuse and geothermal powerplants (2).
Due to the hazards of living at such extreme temperatures, thermophiles have evolved a variety of mechanisms that allow them to survive at temperatures no other organisms can thrive at. These traits include unique membrane lipid composition, thermostable membrane proteins, and higher turnover rates for various protein enzymes (9). One of the most important attributes to the maintenance of homeostasis within the organism is that of the plasma membrane surrounding the organism. Archaean thermophiles, and also acidophiles, have membranes containing unique ether lipids (2). These tetraether lipids span the entire membrane forming a rigid monolayer that is impermeable to both ions and protons (2). Ether-type lipids, such as these, are much stronger than the ester-type lipids found in non-thermophilic Bacteria and Eukarya (2).
Also, the lipid composition in the membranes of the thermophiles consists of more branched and saturated fatty acids than other organisms (9). Having a stronger lipid complex within the membrane helps the Archaean thermophiles to withstand higher temperatures better than other organisms. Aside from having to stabilize the plasma membrane at high temperatures, thermophiles must also stabilize their proteins, DNA, RNA, and ATP. As of now, the process of heat stabilization for DNA, RNA, and ATP is unknown (2).
Thermophiles have developed distinct ways of heat stabilizing the proteins that are required for the maintenance of life. For one, the surface energy of the protein, along with the hydration of the non-polar groups that are exposed, are minimized (2). Also, hydrophobic regions are packed into a very dense core of the protein by charge-charge interactions between amino acids (2). There is also an increase in salt bridges and other networks, which help to stabilize the structures at higher temperatures (2). Finally, it has been shown that there is a distinct increase in the synthesis of chaperonin proteins after a heat shock (2). Chaperonins are proteins that unfold and help refold proteins that are not folded properly enough to perform their required function (4). Increasing the number of these during high temperatures, most likely allows the cells to have second chance at folding proteins that misfolded due to high heat (2).
Another group of extremophiles that have adapted themselves to an extreme environment are the halophiles. These organisms have the ability to grow at very high salt concentrations (2). In this case, the salt concentrations can be anywhere from 3% to 35% (2). Commonly, this group of extremophiles can be found in such environments as sea water, hypersaline lakes (the Dead Sea, the Great Salt Lake), and saline souls (2). Halophilic organisms can also be found in man-made saline environments such as salted foods and tanned hides (2). Much like thermophiles, halophiles can also be broken down into three different groups; instead of optimum growth temperature, the groups are based on optimum salinity. There are slight halophiles that grow at an optimum salinity 2% to 5%, moderate halophiles that grow at an optimum salinity of 5% to 20%, and finally extreme halophiles that grow at an optimum salinity of 20% to 30% (6). Also, some organisms are referred to as “halotolerant,” meaning that the organism has the ability to grow in bother hypersaline environments and non-saline environments, but saline is not required for optimum growth (6).
An interesting feature of hypersaline environments is the formation of gradients in concentration with respect to time. As small bodies of hypersaline waters evaporate, the salinity gradually increases. The salinity of water can start at 1M NaCl, but as times goes by the salinity can increase to over 5M NaCl (6). This causes natural fluctuations in the halophilic species that inhabit that particular body of water. For example, when water is around 1M to 3M NaCl, the environment tends to be filled with algae, protests, and yeasts (6). However, when evaporation occurs, and the salinity increases 5M, those organisms die off because they cannot survive at such high salt concentrations. Organisms that can survive at these higher salt concentrations, such as red-orange halobacteria, drastically increase in numbers until the body is completely dried up or diluted back to a lower concentration (6). The increases in red-orange halobacteria populations are very dramatic and blooms can be as dense as 10^8 cells per mL (6). However, no matter what level of salinity the organism can thrive at, all halophiles face difficulties in surviving.
One of the biggest problems faced by halophiles in maintaining homeostasis is the balance of osmotic pressure. Since these organisms are in hypertonic solution, water diffuses out of the cells and into the surrounding environment. This even would cause non-halophilic organisms to plasmolyze or, if the organism does not have a cell wall, the organism would shrivel. Both of these reactions would be lethal to the organism (4). Usually, the organism would take up sodium ions to create equilibrium between the interior and the exterior cellular environments. However, since sodium ions at such high concentrations would be potentially lethal within a cell, most halophiles accumulate potassium ions while actively expelling sodium ions to create osmotic equilibrium (2). Aside from potassium ions, halophiles also accumulate other non-disruptive solutes to maintain equilibrium. These can include amino acids, glycine, betaine, ecotine, and sucrose (6).
Other problems faced by halophiles include: protein structure and membrane structure. In order to combat denaturation, aggregation, and precipitation of proteins at high salt concentrations, halophiles proteins often contain a high ration of acidic to basic amino acids, thus giving the surface of the proteins a negative charge (6). It is believed that this negative charge allows the proteins to be solvated in a high salt environment (6). Halophile membranes are unique in their composition, just as thermophiles are. Some halophiles make use of the protein bacteriorhodopsin (6). This compound of bacterioopsin and retinal is found in the membranes of some halophiles in lattice shaped areas, giving the membrane a purple color and sometimes covering more than 50% of the entire membrane surface (6). The function of this protein is to act as a light dependant proton pump (6). When induced by a drop in oxygen levels or a high intensity of light, this protein can help support phototropic growth (6). Halophiles also have novel gas vesicles to allow flotation of the organisms in liquid and into higher depths where more oxygen may be available, or where the salt concentration is at optimum range (6).
Another form of extremophilic living is the ability to live in pH levels lower than neutral. Organisms that inhabit the niche between pH 0 and pH 4 are termed acidophiles (2). These organisms often have the ability to grow at high temperatures as well; organisms that can do as such are called thermoacidophiles (2). Acidophiles can inhabit any niche within the bounds of low pH, however the only one genus is known to thrive at pH 0; the genus Picrophilus has the uncanny ability to grow aerobically at 60°C and at pH 0 (8).
Living at such low pH’s is not easy, so acidophiles have evolved ways to overcome the difficulties. For one, the internal pH of the cell is maintained as close to neutral as possible, usually between pH 5 and pH 7, n order to avoid the denaturation of proteins and other molecules (8). However, Picrophilus oshimae has been recorded as having an internal pH of 4.6 (8). Also, the cellular membranes have a very low protein permeability to keep stray protons from an acid out of the cytoplasm (8). In order to maintain the internal pH, acidophiles either actively excrete protons or use them in various metabolic reactions such as the reduction of oxygen in the membrane, before the acidic protons can cause internal cellular damage (8). Acidophiles also utilize non-energy processes to maintain internal pH. These include the maintenance of fixed negative charges on intracellular molecules and the upkeep of a proton diffusion potential (8). Protein enzymes must also be modified in order to keep from being denatured. Acidophilic enzymes have the charged amino acids replaced by neutral polar amino acids in their polypeptide chains (8). This reduces the electrostatic repulsion that occurs between charged groups at low pH, thus enhancing stability (8).
Living at the opposite end of the spectrum from acidophiles are alkaliphiles. These organisms thrive in environments with a pH between 10 and 12, with an optimum growth pH of about 9 (7). Alkaliphiles also have the ability to live in neutral and even acidic environments (7). Of interesting note, is the fact that when alkaliphiles are placed in a neutral or acidic environment, they have the ability to change the environmental pH to a more optimal level (7). In order to survive at these levels, alkaliphiles have novel adaptations to cell wall structure. It has been shown that the cell wall of alkaliphiles contains a variety of acidic compounds, including: phosphoric acid, aspartic acid, galacturonic acid, glutamic acid, and gluconic acid (7). Having these negatively charged amino acids in the membrane allows the cells to better absorb sodium ions and hydronium ions (due to their positive charges),while at the same time repel the hydroxide ions which are in high concentrations at high pH levels (7). Having a membrane capable of this feat allows alkaliphiles to grow at pH levels higher than any other organism.
A final type of extremophilic organism is the group called piezophiles. Piezophiles are organisms that have the ability to grow at pressures higher than normal atmospheric pressure (10). The majority of piezophiles can also be categorized by the temperatures where they thrive: thermopeizophiles grow in high temperatures and pressure while psychropeizophiles grow in low temperatures and high pressure (10). Piezophiles are found underwater, at virtually all depths. Depending on the location of their underwater home, piezophiles are subjected o different temperatures. For example, the thermopiezophiles would be found around deep-sea vents (10). As for how these organisms have the ability to survive at high pressure, it has been difficult for scientists to show how piezophiles overcome this extreme. However, scientists are beginning to look at the cell membrane to see if piezophiles have a unique membrane composition or structure that would allow them to survive at the greatly increased pressures (10).
Thermophiles, halophiles, acidophiles, alkaliphiles, and piezophiles all take life to the extreme. By studying these unique organisms, scientists can gain insight into how life arose on the Earth and even infer as to how life would be able to exist on other planets. According to professor Michael Danson, a biochemistry professor at the University of Bath in the UK, “By studying how organisms live and thrive in places like the Antarctic, if we can understand how these organisms operate, then we will have a good starting base by which to find life and study life on Europa” (3). As for the origins of life on Earth, some scientists are looking to the extremophilic microbe Dienococcus radiodurans. This extremophile has the unique ability to survive radiation at several thousand times the lethal dose for humans (5). Researchers in St. Petersburg attempted to induce this type of radioactive resistance in E. coli. They subjected the bacteria to gamma rays to kill 99.9% of the population. After allowing he survivors to recuperate, they repeated the cycle. After 44 cycles of gamma radiation, it took 50 times the original dose to kill 99.9% of the population (5). Using their data, they found that it would take thousands of these cycles before the E. coli were as resistant to radiation as the Dienococcus. They have calculated that it would take somewhere between one million and a hundred million years for Dienococcus to have acquired this resistance on Earth (5). However, these researchers feel that if this microbe had evolved on Mars, it would have been able to acquire this level of resistance in a much more reasonable time due to the amount of radiation that Mars’ surface is subjected to (5). However, it has not yet been shown that this organism did evolve this ability from living on Mars, as of now it is merely a story.
Stories like the previous are bound to appear. Extremophiles have found a way to fill niches on Earth that no other organism can even fathom to survive in- showing that life will find a way to survive almost any conditions it happens to find itself. The environments on Mars and Europa are not a far step from the extreme locales here on Earth. The variety of unique adaptations that extremophiles here on Earth have developed could very well translate to other worlds. By continuing to study these organisms, scientists will continue to discover just how extreme life really is.
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