Extremophiles and Extraterrestrial Life: Difference between revisions
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== | ==Introduction== | ||
<br>Vertually everywhere on earth where water is present also has some sign of life. This is why water is such an important biosignature when found in extraterrestrial systems. Recent discovery of life on earth's most extreme environments has also made more possible the idea of panspermia and the transport of microbes within meteorites (Rothschild & Mancinelli 2001). This is especially relevant with the finding of polyextremophiles which can flourish under multiple physical (temperature, radiation) or geochemical extremes (dessication, pH, salinity) (Rothschild & Mancinelli 2001). Also important to study is the increased resistance among different stages of an organism's life cycle, such as spores, seeds, or egg stages (Rothschild & Mancinelli 2001).<br> | |||
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== | ==Cold adaptation== | ||
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Cold adaptation can be achieved in a variety of ways, and has arisen in conjunction with adaptation to other environmental factors. This can be assumed due to the variety of environments that have extreme cold temperatures (such as deep sea, alpine regions, subterranean caverns, permafrost regions) and can present different in biotic and abiotic stresses on inhabiting organisms (Siddiqui et al. 2013). These circumstances have led to organisms capable of surviving in different extremes and good models for extraterrestrial life. | |||
While their are many different survival mechanisms, most psychrophiles face similar stressors associated with cold temperatures which include decreased membrane fluidity, osmotic stress, and RNA folding (Siddiqui et al. 2013). Cold temperatures cause RNA to decrease in flexibility and form secondary transcript structures (Siddiqui et al. 2013). Organisms address this problem in a variety of ways, one of the most studied being the expression of cold shock proteins (Csps) which can attach to the RNA and keep secondary structures from forming. | |||
While Csps are present in some mesophiles, they are usually transiently expressed in response to cold shock, whereas in psychrophiles the proteins are constitutively expressed (as reviewed in Siddiqui et al. 2013). Most studies done on csps have been conducted on the mesophile Escherichia coli, an easier bacteria to cultivate than most psychrophiles. In a recent study examining the effects of psychrophile csps, E. coli genetically modified with a Psychromonas artica gene encoding for a cold shock protein showed a tenfold increase in cold tolerance (Jung et al. 2010). | |||
Still, not all organisms have homologs of csps genes. Even organisms isolated from the same antartic lake can have differing csps genes, such as Methanogenium frigidum (does) and Methanococcoides burtonii (doesn't) (as reviewed in Siddiqui et al. 2013). M. burtonii, instead, upregulates Ctr (cold-responsive TRAM domain) proteins which might serve as RNA chaperones and respond to various stressful environments including increased methanol (as reviewed in Siddiqui et al. 2013). M. burtonii, an archaeon, expresses in lower temperatures a protein similar to DEAD-box RNA helicases (named for their Asp-Glu-Ala-Asp motif) found in some bacteria. The protein, which is predicted to have a postive charge, may work by attaching to the negative RNA backbone and then because of its central conserved domain bind to specific sequence targets on the RNA (Lim et al. 2000). One of the functions of DEAD box RNA helicases is to unwind RNA which has already formed the secondary structures (as reviewed in Siddiqui et al. 2013). | |||
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==Radiation Damage== | |||
<br>Radiation poses the most danger because of its damage to nucleic acids and consequently DNA. One of the most durable organisms in the presence of radiation is the bacterium Deinococcus radiodurans. This bacterium has been isolated from various dry and low-nutrient environments, suggesting an adaptation to dessication (as reviewed by Battista 1997). Indeed, resistance to dessication, which also can damage DNA, is usually correlated with increased radiation resistance. In one experiment, strains of D. radiodurans which were ionizing radiation-sensitive also showed sensitivity to dessication when left in a desiccator for six weeks (as reviewed by Battista 1997). Considering that there are not many natural environments on earth that produce extreme radiation, it is likely that D. radiodurans' radiation resistance is a byproduct of its adaptation to dessicaition. Similar connections between dessication and radiation resistance have been made in regard to tardigrades' ability to survive in space. | |||
<br> | |||
D. radiodurans can survive upwards of 17,000 gamma rays, compared to the lethal amounts of 10 gamma rays for humans (Talbott 2013). D. radiodurans, rather than passively resist radiation damage, uses various DNA repair mechanisms to repair the extensive DNA damage following radiation exposure (as reviewed by Battista 1997). Nonetheless, an important aspect of D. radiodurans' ability to repair itself is that it is multigenomic, containing four copies of its chromosome. Having multiple copies of chromosomes has been shown as being as improving radiation resistance in other organisms such as Saccharomyces cerevisiae (as reviewed by Battista 1997). Radiation-induced double-strand breaks (dsbs) in the DNA, among the more harmful of radiation effects, occurs randomly along the chromosome. By having multiple copies of the chromosome, there is a lower chance that damage occurs in the same place across each of the bacterium's copies. Through recombination of regular sections of DNA with damaged sections D. radiodurans is able to repair its genome once conditions are favorable to growth (as reviewed by Battista 1997). | |||
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==Conclusion== | ==Conclusion== | ||
Revision as of 20:26, 8 May 2015
Introduction
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Legend/credit: Electron micrograph of the Ebola Zaire virus. This was the first photo ever taken of the virus, on 10/13/1976. By Dr. F.A. Murphy, now at U.C. Davis, then at the CDC.
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Introduction
Vertually everywhere on earth where water is present also has some sign of life. This is why water is such an important biosignature when found in extraterrestrial systems. Recent discovery of life on earth's most extreme environments has also made more possible the idea of panspermia and the transport of microbes within meteorites (Rothschild & Mancinelli 2001). This is especially relevant with the finding of polyextremophiles which can flourish under multiple physical (temperature, radiation) or geochemical extremes (dessication, pH, salinity) (Rothschild & Mancinelli 2001). Also important to study is the increased resistance among different stages of an organism's life cycle, such as spores, seeds, or egg stages (Rothschild & Mancinelli 2001).
Cold adaptation
Cold adaptation can be achieved in a variety of ways, and has arisen in conjunction with adaptation to other environmental factors. This can be assumed due to the variety of environments that have extreme cold temperatures (such as deep sea, alpine regions, subterranean caverns, permafrost regions) and can present different in biotic and abiotic stresses on inhabiting organisms (Siddiqui et al. 2013). These circumstances have led to organisms capable of surviving in different extremes and good models for extraterrestrial life.
While their are many different survival mechanisms, most psychrophiles face similar stressors associated with cold temperatures which include decreased membrane fluidity, osmotic stress, and RNA folding (Siddiqui et al. 2013). Cold temperatures cause RNA to decrease in flexibility and form secondary transcript structures (Siddiqui et al. 2013). Organisms address this problem in a variety of ways, one of the most studied being the expression of cold shock proteins (Csps) which can attach to the RNA and keep secondary structures from forming.
While Csps are present in some mesophiles, they are usually transiently expressed in response to cold shock, whereas in psychrophiles the proteins are constitutively expressed (as reviewed in Siddiqui et al. 2013). Most studies done on csps have been conducted on the mesophile Escherichia coli, an easier bacteria to cultivate than most psychrophiles. In a recent study examining the effects of psychrophile csps, E. coli genetically modified with a Psychromonas artica gene encoding for a cold shock protein showed a tenfold increase in cold tolerance (Jung et al. 2010).
Still, not all organisms have homologs of csps genes. Even organisms isolated from the same antartic lake can have differing csps genes, such as Methanogenium frigidum (does) and Methanococcoides burtonii (doesn't) (as reviewed in Siddiqui et al. 2013). M. burtonii, instead, upregulates Ctr (cold-responsive TRAM domain) proteins which might serve as RNA chaperones and respond to various stressful environments including increased methanol (as reviewed in Siddiqui et al. 2013). M. burtonii, an archaeon, expresses in lower temperatures a protein similar to DEAD-box RNA helicases (named for their Asp-Glu-Ala-Asp motif) found in some bacteria. The protein, which is predicted to have a postive charge, may work by attaching to the negative RNA backbone and then because of its central conserved domain bind to specific sequence targets on the RNA (Lim et al. 2000). One of the functions of DEAD box RNA helicases is to unwind RNA which has already formed the secondary structures (as reviewed in Siddiqui et al. 2013).
Radiation Damage
Radiation poses the most danger because of its damage to nucleic acids and consequently DNA. One of the most durable organisms in the presence of radiation is the bacterium Deinococcus radiodurans. This bacterium has been isolated from various dry and low-nutrient environments, suggesting an adaptation to dessication (as reviewed by Battista 1997). Indeed, resistance to dessication, which also can damage DNA, is usually correlated with increased radiation resistance. In one experiment, strains of D. radiodurans which were ionizing radiation-sensitive also showed sensitivity to dessication when left in a desiccator for six weeks (as reviewed by Battista 1997). Considering that there are not many natural environments on earth that produce extreme radiation, it is likely that D. radiodurans' radiation resistance is a byproduct of its adaptation to dessicaition. Similar connections between dessication and radiation resistance have been made in regard to tardigrades' ability to survive in space.
D. radiodurans can survive upwards of 17,000 gamma rays, compared to the lethal amounts of 10 gamma rays for humans (Talbott 2013). D. radiodurans, rather than passively resist radiation damage, uses various DNA repair mechanisms to repair the extensive DNA damage following radiation exposure (as reviewed by Battista 1997). Nonetheless, an important aspect of D. radiodurans' ability to repair itself is that it is multigenomic, containing four copies of its chromosome. Having multiple copies of chromosomes has been shown as being as improving radiation resistance in other organisms such as Saccharomyces cerevisiae (as reviewed by Battista 1997). Radiation-induced double-strand breaks (dsbs) in the DNA, among the more harmful of radiation effects, occurs randomly along the chromosome. By having multiple copies of the chromosome, there is a lower chance that damage occurs in the same place across each of the bacterium's copies. Through recombination of regular sections of DNA with damaged sections D. radiodurans is able to repair its genome once conditions are favorable to growth (as reviewed by Battista 1997).
Conclusion
Overall paper length should be 3,000 words, with at least 3 figures.
References
Edited by student of Joan Slonczewski for BIOL 238 Microbiology, 2009, Kenyon College.
