National Academies Press: OpenBook

The Quarantine and Certification of Martian Samples (2002)

Chapter:Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars

« Previous: 8 Conclusions and Recommendations
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×

Appendixes

Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
This page in the original is blank.
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×

Appendix A
Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars

Michael J. Daly

Department of Pathology, Uniformed Services University of the Health Sciences, Bethesda, Maryland

The only genetic systems within which the equivalent of millions of years’ worth of background radiation accumulated as genetic damage have been studied in a living organism are those developed by the terrestrial bacterium Deinococcus radiodurans.1-7 This bacterium is capable of repairing massive genetic damage without lethality or increasing mutation frequency. As such, D. radiodurans is an excellent organism in which to consider the potential for survival and biological evolution beyond its planet of origin, as well as the ability of life to survive extremely long periods of metabolic dormancy in high-radiation environments. Ultimately, the survival of any organism in such environments will be determined by its ability to repair, and recover from, accumulated genetic damage. D. radiodurans could likely survive extended periods of metabolic dormancy near the surface of Mars, periods of time in which the equivalent of Mrads of radiation damage would be accumulated in any genetic material.

It is likely that during the first 500 million years (0.5 Ga) of its existence, Mars had a warmer and wetter climate than it has now, and an active hydrologic cycle. This may slightly predate the time at which Earth first began to support water and life; evidence of chemically evolved life on Earth dates to ~0.6 Ga after its formation.

1  

Battista, J.R., Earl, A.M., and Park, M.-J. 1999. Why is Deinococcus radiodurans so resistant to ionizing radiation? Trends Microbiol. 7:362.

2  

Minton, K.W. 1996. Repair of ionizing-radiation damage in the radiation resistant bacterium Deinococcus radiodurans. Mutat. Res. DNA Repair 362:1.

3  

Mattimore, V., and Battista, J.R. 1996. Radioresistance of Deinococcus radiodurans: Functions necessary to survive ionizing radiation are also necessary to survive prolonged desiccation. J. Bacteriol. 177:5232.

4  

Lange, C.C., Wackett, L.P., Minton, K.W., and Daly, M.J. 1998. Engineering a recombinant Deinococcus radiodurans for organopollutant degradation in radioactive mixed waste environments. Nat. Biotechnol. 16:929.

5  

Brim, H., McFarlan, S.C., Fredrickson, J.K., Minton, K.W., Zhai, M., Wackett, L.P., and Daly, M.J. 2000. Engineering Deinococcus radiodurans for metal remediation in radioactive mixed waste environments. Nat. Biotechnol. 18:85-90.

6  

Daly, M.J., Ouyang, L., Fuchs, P., and Minton, K.W. 1994. In vivo damage and recA-dependent repair of plasmid and chromosomal DNA in the radioresistant bacterium Deinococcus radiodurans. J. Bacteriol. 176:3508.

7  

White, O., Eisen, J.A., Heidelberg, J.F., Hickey, E.K., Peterson, J.D., Dodson, R.J., Haft, D.H., Gwinn, M.L., Nelson, W.C., Richardson, D.L., Moffat, K.S., Qin, H., Jiang, L., Pamphile, W., Crosby, M., Shen, M., Vamathevan, J.J., Lam, P., McDonald, L., Utterback, T., Zalewski, C., Makarova, K.S., Aravind, L., Daly, M.J., Minton, K.W., Fleischmann, R.D., Ketchum, K.A., Nelson, K.E., Salzberg, S., Smith, H.O., Venter, J.C., and Fraser, Claire M. 1999. Genome sequence of the radioresistant bacterium Deinococcus radiodurans R1. Science 286:1571-1577.

Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×

Subsequent to this watery period martian life, if it existed, may have become subterranean and/or dormant, or it may have become extinct during the transition to a hostile environment caused by Mars’s loss of atmosphere and water. The time when conditions effectively became hostile to life are not known; the loss of atmosphere and water from Mars was gradual and is still occurring.

It is not justified to assume a priori that all martian life, if it existed, has become extinct. If life existed on Mars, it likely evolved to survive the increasingly harsh conditions, and/or it found environments that are shielded from the planet’s present extreme climate. As our knowledge of Earth’s biosphere increases, extreme environments originally believed to be far too harsh to support life are regularly being found to support flourishing microbial communities. Organisms discovered in these extreme environments are called extremophiles, and collectively they demonstrate the remarkable ability of life to thrive over a wide range of extremely hostile environments, e.g., high temperature (these organisms are called thermophiles) or high pressure (barophiles). Similarly successful adaptations may have been made by microbes on Mars in environments at least equally hostile.

D. radiodurans is an excellent example of an extremophile. It can grow continuously, without induced mutation or any effect on its growth rate, in the presence of 6,000 rad of radiation per hour.8,9 Further, it demonstrates astounding survival: it can withstand an acute dose of radiation of 1.5 Mrad (at 0 ºC) or 4.0 Mrad (at –70 ºC) without lethality. Even more extraordinary is the fact that following such massive irradiation, these cells can repair their genomes within 12 to 24 hours.10 Unpublished work by J. Battista has shown that desiccated D. radiodurans can survive exposure at higher doses, as much as 15 Mrad (Figure 5.3 in Chapter 5). These qualities are key to answering the question of whether life is extinct or extant on Mars, where the commonly accepted “post-transition period” from a life-supporting to a life-challenging biosphere might subject a dormant microbe to up to 2 billion years’ worth of accumulating background radiation, along with freezing and/or desiccation. The background cosmic radiation on Mars is similar to that on Earth, ~0.5 rad/year. Thus on the order of 1,000 Mrad of accumulated radiation dose could have been delivered to a microbe locked in a cryptobiotic state at the end of the transition period. As a single acute dose, this amount of radiation damage would overwhelm D. radiodurans and any other known organism. However, fluctuations in the Mars surface environment, and other events (such as burial by crater ejecta), could present the microbial population with periodic opportunities to become active, repair damage, and replenish their numbers while the radiation occurred.

In the context of current technological and financial constraints, the question of where life on Mars is most likely to be metabolically active is distinctly separate from the question of where preserved life is most likely to be detected on Mars by missions to the planet in the near future. By analogy to the extremophiles on Earth, it is likely that metabolically active life on Mars would occur in geothermally heated subsurface water tables or spaces where it would be shielded from the harsh surface conditions. Such Mars subterranean environments will, however, be beyond human exploration for many years.

In the foreseeable future, exploration of Mars will be limited to surface and near-surface environments. Although these accessible environments are exposed to harsh atmospheric and solar stress, such conditions do not preclude dormant life. It is possible that periodic melts of subsurface water caused by geothermal events and/or orbital variations bring subsurface microorganisms into surface and near-surface environments where they become frozen and cryopreserved in ground ice. If a martian microorganism had a proficient genetic repair system, it could survive frozen in such a site for millions or even hundreds of millions of years. If intermittently, though rarely, the local surface environments also underwent thawing for the same reasons, it would allow revival of organisms, repair of accumulated genetic damage, and repopulation of their environments. D. radiodurans provides a good

8  

Lange, C.C., Wackett, L.P., Minton, K.W., and Daly, M.J. 1998. Engineering a recombinant Deinococcus radiodurans for organopollutant degradation in radioactive mixed waste environments. Nat. Biotechnol. 16:929.

9  

Brim, H., McFarlan, S.C., Fredrickson, J.K., Minton, K.W., Zhai, M., Wackett, L.P., and Daly, M.J. 2000. Engineering Deinococcus radiodurans for metal remediation in radioactive mixed waste environments. Nat. Biotechnol. 18:85-90.

10  

Daly, M.J., Ouyang, L., Fuchs, P., and Minton, K.W. 1994. In vivo damage and recA-dependent repair of plasmid and chromosomal DNA in the radioresistant bacterium Deinococcus radiodurans. J. Bacteriol. 176:3508.

Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×

model system in which to consider preservation and irradiation on the accessible surface and in near-surface environments of Mars.

The following factors are known to dramatically affect an organism’s radiation resistance:

  • Genome size. The smaller the genome, the more resistant it is.

  • Ambient nutrient conditions. The presence of nutrients supports DNA repair.

  • Temperature. Typically, the colder an organism is, the more resistant to radiation it is. Freezing increases radiation resistance.

  • Hydration. Organisms are much more resistant to radiation when desiccated.

Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
Page65
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
Page66
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
Page67
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
Page68
Suggested Citation:"Appendix A Deinococcus radiodurans as an Analogue to Extremophile Organisms That May Have Survived on Mars." National Research Council. 2002. The Quarantine and Certification of Martian Samples. Washington, DC: The National Academies Press. doi: 10.17226/10138.
×
Page69
Next: Appendix B A History of the Lunar Receiving Laboratory »
The Quarantine and Certification of Martian Samples Get This Book
×
Buy Paperback | $29.00 Buy Ebook | $23.99
MyNAP members save 10% online.
Login or Register to save!
Download Free PDF

One of the highest-priority activities in the planetary sciences identified in published reports of the Space Studies Board's Committee on Planetary and Lunar Exploration (COMPLEX) and in reports of other advisory groups is the collection and return of extraterrestrial samples to Earth for study in terrestrial laboratories. In response to recommendations made in such studies, NASA has initiated a vigorous program that will, within the next decade, collect samples from a variety of solar system environments. In particular the Mars Exploration Program is expected to launch spacecraft that are designed to collect samples of martian soil, rocks, and atmosphere and return them to Earth, perhaps as early as 2015.

International treaty obligations mandate that NASA conduct such a program in a manner that avoids the cross-contamination of both Earth and Mars. The Space Studies Board's 1997 report Mars Sample Return: Issues and Recommendations examined many of the planetary-protection issues concerning the back contamination of Earth and concluded that, although the probability that martian samples will contain dangerous biota is small, it is not zero.1 Steps must be taken to protect Earth against the remote possibility of contamination by life forms that may have evolved on Mars. Similarly, the samples, collected at great expense, must be protected against contamination by terrestrial biota and other matter. Almost certainly, meeting these requirements will entail opening the sample-return container in an appropriate facility on Earth-presumably a BSL-4 laboratory-where testing, biosafety certification, and quarantine of the samples will be carried out before aliquots are released to the scientific community for study in existing laboratory facilities. The nature of the required quarantine facility, and the decisions required for disposition of samples once they are in it, were regarded as issues of sufficient importance and complexity to warrant a study by the Committee on Planetary and Lunar Exploration (COMPLEX) in isolation from other topics. (Previous studies have been much broader, including also consideration of the mission that collects samples on Mars and brings them to Earth, atmospheric entry, sample recovery, and transport to the quarantine facility.) The charge to COMPLEX stated that the central question to be addressed in this study is the following: What are the criteria that must be satisfied before martian samples can be released from a quarantine facility?

  1. ×

    Welcome to OpenBook!

    You're looking at OpenBook, NAP.edu's online reading room since 1999. Based on feedback from you, our users, we've made some improvements that make it easier than ever to read thousands of publications on our website.

    Do you want to take a quick tour of the OpenBook's features?

    No Thanks Take a Tour »
  2. ×

    Show this book's table of contents, where you can jump to any chapter by name.

    « Back Next »
  3. ×

    ...or use these buttons to go back to the previous chapter or skip to the next one.

    « Back Next »
  4. ×

    Jump up to the previous page or down to the next one. Also, you can type in a page number and press Enter to go directly to that page in the book.

    « Back Next »
  5. ×

    Switch between the Original Pages, where you can read the report as it appeared in print, and Text Pages for the web version, where you can highlight and search the text.

    « Back Next »
  6. ×

    To search the entire text of this book, type in your search term here and press Enter.

    « Back Next »
  7. ×

    Share a link to this book page on your preferred social network or via email.

    « Back Next »
  8. ×

    View our suggested citation for this chapter.

    « Back Next »
  9. ×

    Ready to take your reading offline? Click here to buy this book in print or download it as a free PDF, if available.

    « Back Next »
Stay Connected!