Why Panspermia Is Even More Plausible?
July 15, 2010 17 Comments
Panspermia could emerge as a more plausible theories for origin of life if we are considering the important factor of microbial survivability in extreme hostile environments. In my prior post ‘Is Panspermia Occurring Right Now?‘ , I’ve given certain examples of microbial survivability in stratosphere and in UV radiation, and that analysis boosted the possibility of Panspermian origin of life. Microbial adaptability and survival is highly relevant to panspermia and the search for extraterrestrial life, i.e. astrobiology. In some environments, microbial survival is estimated on the order of centuries and millennia if not longer. Microorganisms have been used as models to predict conditions for life on other planets and solar systems. While our knowledge of distant planets and solar systems remains limited to distant observations, present technology and resources enables the visitation of nearby regions in the solar system. Of note, Mars (Levin 2010; McKay et al., 1996; Yung et al., 2010), Titan (Naganuma and Sekine, 2010; Sagan et al., 1992), and Europa (Marion et al., 2003; Tyler 2010) have the potential to support current or previous life. Below is table showing microbial survivability in extreme environments from research paper:
First it need a theory of microbial entry in space which I have reviewed in my earlier post ‘Is Panspermia Occurring Right Now?‘.
Bacterial Survival Evidence and Mechanism
- There are a variety of mechanisms by which microorganisms are able to survive extreme physical conditions, likely to be associated with ejection into space. Bacteria and Archaea are small and typically range in size from 0.3 – 2 μm. Also in some environments there can be considerable population density (estimates of surface and subsurface population densities typically range from 101 – >108 / g rock or soil. The small size and high numbers of microorganisms can result in the possibility of some being shielded and buffered in a protective, insulated microenvironment, during the catastrophic events and energy associated with impact-associated ejection into space. Other issues related to microbial survival during ejection is the shape and structure of the cell wall of bacteria. The spherical shape of many organisms, such as Staphylococcus sp. and spores produced byBacillus and Clostridium sp., has been associated with heightened resistance to shearing. Peptidoglycan, the main structural component of the bacterial cell wall, is very strong with estimates of its strength and elasticity surpassing steel. In many microbiology laboratories, bacteria are routinely harvested from liquid suspensions by centrifugation at a relative force sometimes approaching or exceeding 10,000 x gravity. Most bacteria, harvested in this fashion remain viable due to the strength of the peptidoglycan cell wall component. In contrast other biological entities, including human blood cells (erythrocytes) that are not protected by peptidoglycan, are broken (lysed) at much lower speeds (above 2000 x gravity).
- Once in space, organisms would lose the protection of a planetary atmosphere against solar radiation, and be exposed to vacuum, a lack of gravity and temperature extremes. Liquid water is unable to exist at high vacuum (< 611.73 Pa) and therefore any biological materials, including living cells, would need to survive in the absence of water. A very common mechanism to store bacteria involves freeze-drying, also called lyophilization. Cells in a freeze-dried state are essentially in a state of suspended animation, and will grow when reintroduced to appropriate growth conditions. Reduced water content is also a factor that can enhance heat and radiation survival in bacterial spores. During the normal processes that occur during spore formation in Bacillus subtilis, much of the heightened resistance of these structures to heat, radiation, and chemicals was attributed to dehydration-induced conformational changes in key proteins and membrane components, rather than quenching of molecular motion . These changes are reversible in the presence of adequate water. One would expect that vacuum-induced dehydration would provide even greater protection against physical stresses.
- Bacteria have a number of mechanisms by which they are able to resist and protect themselves against radiation and heat. When compared to mesophilic bacteria (that typically live between 20-40°C), thermophilic bacteria (optimum growth can exceed 100°C) have greatly enhanced protein and nucleic acid stability. These mechanisms include an increased cross-linking of proteins and altered DNA structure. When growing bacteria are subjected to temperatures approaching their upper growth range, cellular damage and death can arise from protein misfolding (denaturation), a loss of membrane integrity and DNA damage. Bacteria, including thermophiles, have a number of stress responses, including producing a variety of heat shock proteins. Early studies showed that bacteria such as Escherichia coli, expressed a number of heat shock genes when they approached their maximum growth temperature; and that bacteria defective in these genes had reduced thermal tolerance. Several heat shock genes encode the synthesis of a number of accessory proteins called chaperones.
- Microbes can easily survive from UV RADIATION and it was shown in this paper. Issues related to radiation survival have involves introducing various microorganisms, to the vacuum and radiation conditions on low Earth orbit and so will be addressed briefly here. Of interest, simulated meteorite materials protected microorganisms in one of these studies. Aside from bacterial survival, two higher organisms, the lichens Rhizocarpon geographicum and Xanthoria elegans, originally isolated from alpine and polar environments, also survived. In a ground-based experiment, several microorganisms were exposed to simulated radiation, atmospheric and environmental conditions of Mars. A number of spore-forming bacteria were able to survive, notably Bacillus pumulis, which has been identified as a species of concern with planetary protection, e.g., Earth based microorganisms, potentially contaminating spacecraft.
- Impact Survival: Atmospheric passage and impact yield many of the same stresses (heat, gravity, shear forces) as those experienced during entry into space. Through a series of planned and accidental events, there are several studies that have directly addressed microbial survival during this phase of panspermia. In one study (Horneck et al., 2001), spores from the bacterium, Bacillus subtilis, were placed between quartz plates and subjected to an explosion-induced, transient shock pressure of 32 GPa. Under these conditions, which mimic those of meteorite impact on Mars, a small number of spores survived (estimated survival rate was 10-4). In a second study (Burchell et al., 2001),Rhodococcus, a genus of non-spore forming bacteria, were attached to a projectile (bullet) that was then fired from a gun onto a target of growth media. Bacterial survival was also noted when spores were placed onto the ablative heat shield of a spacecraft, and exposed to heat during reentry.
- Microgravity: One other aspect of the space environment is the absence of gravity. There have been a number of investigations on life forms, including bacteria, during microgravity. These investigations have been conducted in flight as well as in devices that model microgravity . Under these conditions, microorganisms can actually thrive. There was a recent report that the virulence and stress response of one organism, Salmonella enterica, was enhanced during microgravity.
Now all of above experiments and various studies suggests that it is not too tough to assume survival of bacteria from a variety of Panspermiac processes. Now question is, would life evolve after passing through various stages of Panspermia?
The majority of biological investigations related to panspermia, have investigated the ability of organisms to survive the environmental and stress conditions during ejection from their origin, transport through space, and impact on their destination. A conservative estimation is that only a small fraction of organisms may survive interplanetary transit and that many of these organisms may be damaged by the stresses they encounter. Microorganisms are a remarkably resilient group, having a number of DNA and other repair mechanisms. Should the cells maintain or obtain sufficient resources and encounter a suitable environment, then repair and growth could occur. Growth may occur shortly after introduction to a new environment, or the organisms may remain in a state of dormancy until suitable conditions arise. Depending on the destination, incoming organisms could arrive in a pristine, abiotic environment, which they could colonize. Alternatively, they might arrive in an environment containing other life forms and so face competition. Microbial interactions including competition and cooperation have been studied extensively in many different situations.
Another possibility is that the organisms do not survive, but that some portion of their genetic material (DNA or RNA) does. There is a recent line of investigation into the long-term survival and recovery of ancient DNA from sources such as amber. A number of microorganisms are able to take up and incorporate DNA from the environment in a process referred to as transformation (Avery et al., 1944; Griffith, 1928). In this context, one cannot discount the possibility of genetic material, surviving transport through space, ultimately becoming incorporated into an organism on a destination planet. Foreign (potentially alien) DNA could introduce new characteristics into a life form on the destination planet. Another possibility is the potential for this new genetic material to alter the control (i.e. regulation) of genes in an organism.
Horizontal gene transfer, a process in which segments of new genetic information are transferred from one organism to another, is now recognized as a common occurrence in many bacteria (Patric J. Keeling ; Koonin and Wolf 2008). An example of this is the acquisition of disease-causing (virulence) genes from bacterial viruses (phage) in the intestinal pathogen E. coli O157:H7 . Genes are transferred not only between bacteria, but between archae and bacteria and viruses, and from viruses and prokaryotes to eukaryotes. Certainly, as outlined in this article, microorganisms and their component genetic material have the potential to survive interplanetary transport. The number of microorganisms in some environments can be considerable, approaching 108 / g in some materials. As a result, should the extreme physical conditions occurring during panspermia, result in reduction of viability of several orders of magnitude, there would still be the possibility of some organisms surviving. As well, nutrients released from and provided by dead organisms may enhance the survival of others in the microbial community. Dead organisms may also form a protective crust, shielding those within the inner colony from a variety of hazards. Thus there appears to be considerable evidence that microbes could survive an interplanetary journey through space and then colonize the planets upon which they arrive. And perhaps it is more sensible and evident concerning various profounded studies. Now we have to see, what else we can find in near future?
[Ref: Astrophysical and Biological Constraints on Radiopanspermia by Jeff Secker and The Interplanetary Transfer of Life Through Space by Robert J.C. McLean and Horizontal gene transfer in eukaryotic evolution by PJ Keeling]