Fans of the Hitchhiker’s Guide to the Galaxy, the audio series, will know of the the Haggunenons, a hyper-evolving race that can change their shape multiple times a day (1). Though Douglas Adams may have been imagining far outside of the scientific realm there is still a grain of truth in hyper-evolution. Though a bacterium, fungus, plant or animal is not drastically going to change its physical characteristics in its lifetime, the genetic material could be altered multiple times within a single individual’s lifespan. And if the genetic material is altered in a way that effects the offspring then hyper-evolution in bacteria and fungi is possible. Why do I bring this up? Well with months of news reports focusing on the Japan Nuclear Power plant, regular mention of Chernobyl, and rehashing of Cold War era discussions of what a post-nuclear world would look like I began to remember the various scientific discoveries that indicated to me life will survive even under the conditions that result in hyper-evolution - DNA destroying/altering nuclear radiation. There is a possibility that complex multicellular organisms will not survive, but the adaptable bacteria and its eukaryotic counterpart - the fungi will definitely be around to feed the plethora of other life forms that are also managing to survive. Some of the surviving life forms will be able to use the energy from nuclear decay to survive. Actually those life forms already exist, the only difference between pre and post nuclear world is the following: We are still around to observe and study them.
In basic biology, especially with respect to microbiology we are often taught how harmful radiation can be to life. As one progresses up one’s biological course work we learn how UV and nuclear radiation can damage the DNA. And even know of researchers that must wear those counters on their chests because they work with radioactive isotopes. Throughout ones scientific career we are inundated with a discussion of how harmful nuclear radiation can be. Looking at photos and reading accounts from Hiroshima or Nagasaki further ingrains the harmful effects of nuclear radiation. Discussions of increased cancer rates adds to the long term detrimental effects of nuclear radiation. It is because of this mindset that caused the scientific community to be blown away at the discovery of bacteria that can survive and thrive in extreme radioactive conditions. The organism known as Deinococcus radiodurans (strange berry that withstands radiation) was originally identified in the 1950s on tin meat containers (assumed to be sterilized) (2), however it strange properties were not fully characterized till the 90’s.
Most bacteria evolve the ability to survive bizarre conditions due to natural regions of limited resources, for example GFAJ-1, the bacteria isolated from Mono Lake, CA. GFAJ-1 is presumed to be capable of surviving without phosphorous (an essential element for most living organisms on earth). Scientist believe that due to the high concentration of arsenic in Mono Lake, GFAJ-1 may be able to use arsenic in place of phosphorous in making DNA (3, 4).
Unlike GFAJ-1 and other organisms, D. radiodurans is a strange specimen. Our radiation withstanding bacteria has no natural locations wherein to develop such a resistance. Even underground hot spots for uranium or thorium do not emit radiation levels close to the levels D. radiodurans can withstand. This lead researchers to examine other conditions wherein a great amount of DNA repair is necessary. Upon examination the adaptations necessary for surviving long term desiccation without spore formation is similar to the adaptations necessary to survive radiation. Scientists found that dehydration causes the same double stranded DNA breaks that radiation causes and if they mutated out the bacteria’s ability to withstand radiation those mutants were also incapable of withstanding dehydration. Here is a super bacteria, of great interest to researcher for bioremediation of nuclear waste dumps that started life just trying to survive severe droughts.
Next, we will look at a bacteria, that was separated from all other life forms 25 million years ago. In addition to separation from other species this bacteria is also isolated from the sun and most other energy sources. Add to that being isolated in a lake 2.8 kilometers underground. Its hot, the pH is very alkaline and oh yea, there is radioactive uranium down there. Its likely that when the geological isolation of the lake occurred there were other life forms within the waters, however when the lake was discovered in 2007-2008 only one genome was discovered. That of Desulforudis audaxviator (Descend, brave sulfurous traveller). D. audaxviator seemed to have quite the genome, and needed to. As it was alone in the water for millions of years D. audaxviator had to not only adapt to use the energy from radiolysis of water as an energy source, but also use the radiolysis of bicarbonate as a carbon source. Add to the conundrum the need for nitrogen fixation. D. audaxviator did not directly use the uranium for survival but it survived off the products of radioactive decay. Without those products an isolated organism would not have been able to survive. The D. audaxviator is the only known organism that does not need others to survive (5). This independence is only possible due to the byproducts of radiation. In addition, I speculate this bacteria was only able to adapt to isolating conditions due to theDNA altering abilities of radiation.
So far we’ve seen an organism that accidentally developed the ability to survive the ill effects of extreme radioactivity and an organism that has been surviving due to the byproducts of radioactive decay. But I have we have not yet examined an organism that can directly use energy from radioactive decay. Such an organism seems far fetched, however any nuclear scientist will tell you that radioactive decay is either high energy particles or gamma rays flying at your cellular parts.
Though it may be difficult to harness the random flow of high energy particles, harnessing electromagnetic radiation (light energy) has never been difficult for organisms. Cyanobacteria, (bacteria that harness sunlight to convert low energy carbon dioxide (CO2) to high energy hydrocarbons and O2) have been converting light energy into chemical energy on this planet for over 3 billion years. So why can’t cyanobacteria and their descendent the mighty plant kingdom convert the gamma rays into chemical energy? This basic answer goes back to the fact that there was not a need to develop that ability. The sunlight that reaches the earth’s surface is mostly in the visible spectra therefore cyanobacteria only developed molecules tuned to specific wavelengths within this region. Unlike like the visible spectra, the sunlight reaching earth consisted of almost no gamma rays. Therefore organisms that may have developed the ability to convert gamma rays into chemical energy had very little energy input and therefore the pay off was not sufficient to support the ability. This was until humans abandoned regions where gamma rays were in abundance.
Regions of concentrated gammas rays, such as Chernobyl where nuclear material had access to the natural environment has created an interesting selection of organisms. In Chernobyl scientist have actually discovered a fungus (not a bacterium! Go Eukaryotes) that not only survives, but thrives in regions of high radioactive decay. The fungus, Cladosporium sphaerospermum can survive the decay due to its high concentration of melanin (a pigmentation compound found in mammals to protect the skin/fur from excessive UV exposure). Though the melanin is often altered from the gamma rays, C. sphaerospermum is able to use the high energy altered melanin to convert low energy CO2 to high energy hydrocarbons. Here we see an organism that is directly using the energy provided in a controlled fashion to proliferate. Just like cyanobacteria and plants all around us capture light energy to build the complex carbon structures found all around us (wood, gasoline, and food) C. sphaerospermum is similarly able to use gamma rays to build complex carbon structures allowing it to thrive and proliferate (6).
After watching, reading and being exposed to so many stories related to the concept of a nuclear winter, humans worry about the bleak future that would exist if nuclear war were to ever break out. This concern is very true, but the world would not be left a sterile, uninhabitable land. The previous three examples tell us that a nuclear winter would still have life, though small and far from being able to observe and change the environment like humans, it would still persevere. Whose to say that multicellular organisms that were raised as the offspring of a nuclear winter may actually be well suited to the high radiation environment of space and therefore ideal organisms for space travel. Nuclear disasters are always a time of concern, however I find the discovery of life surviving and thriving in such situations shows me the glory of the natural world around us. Nothing but a complete lack of energy can take life down!
References
1. S. M. Rosenberg, Nature Reviews Genetics 2, 504–515 (2001).
2. J. Travis, Science News (1998).
3. F. Wolfe-Simon et al., Science (2010), doi:10.1126/science.1197258.
4. B. P. Rosen, A. A. Ajees, T. R. McDermott, Bioessays (2011), doi:10.1002/bies.201100012.
5. D. Chivian et al., Science 322, 275–278 (2008).
6. E. DADACHOVA, A. CASADEVALL, Current Opinion in Microbiology 11, 525–531 (2008).
