Tough Bacteria
Published on Sep 1, 2007 at 4:42 pm.
2 Comments.
Filed under astrobiology.
Within the last month, there have been news reports of scientists reviving eight million year old bacteria. The bacteria was frozen in Antarctic ice. Most life forms on Earth don’t do well with freezing. You get them too cold or too hot, and they die. But, these bacteria were in a deep freeze for millions of years. When this ancient ice was thawed, the bacteria in it were found to be viable, though reports are that that they grew rather slowly. This is particularly interesting because the bacteria were found in the Antarctic ice. They have been there, inert, for millions of years. During that time, they’ve been subjected to cosmic rays which would have degraded their DNA. That happens to life all over Earth, but most of the time limited damage is repairable. Enough damage, though, or enough cumulative damage can kill organisms outright. This is one method of sterilizing food, by the way. Expose the food to high levels of radiation and bacteria in the food die. Even viruses are susceptible to destruction from high radiation levels. While inert, though, the bacteria would have limited ability (if any at all) to repair radiation induced damage. Eight million years of cumulative damage would add up, too. On top of all that, the cosmic rays tend to be higher at the polar regions of Earth than at other places. All of this suggests that these little bugs should be dead. But, they are not. They are growing and surviving. There is some speculation that the slow growth may be a result of radiation damage. Or, of course, it could be just the way that they are.
But, these bacteria are not the only ones that can survive exposure to otherwise deadly levels of radiation. A number of years ago, biologists discovered a bacterial species, Deinococcus radiodurans, which can survive incredible levels of radiation exposure. There have even been bacteria found living on the fuel rods in nuclear reactors. These things are tough!
So, how can these things survive such intense levels of radiation? Radiation at that level blasts DNA apart into gene fragments. Repair of such extreme damage seems hopeless. That is why radiation is so useful in sterilization. However, for some forms of bacteria, such as the radiation tolerant strains, and perhaps these ancient ones, there exists a poorly understood mechanism whereby the bacteria are somehow able to gather together all the gene fragments and put them back together in close to their original form, or at least close enough for the organism to survive. That is really quite amazing. If humans could do that, we’d never have to worry about such things as old age, cancer, or a host of other problems.
And, it isn’t just extemophile bacteria that can perform such amazing feats. In November, 1969, Apollo 12 landed on the Moon near Surveyor 3. The astronauts brought back not only moon rocks, but also instruments from Surveyor 3, which had been sitting on the surface of the Moon for three years. During that time, the unmanned Surveyor 3 spacecraft had been exposed to the vacuum of space, extreme solar heating, extreme cold during the lunar night, and continual exposure to hard cosmic and solar radiation. The general feeling among pretty much every scientist around was that nothing could survive under those conditions. The astronauts got back to Earth, and the instruments from Surveyor 3 were carefully studied. NASA scientists were absolutely amazed to find viable microbial spores on the Surveyor 3 instruments! The microbes were grown and studied. At this time, NASA was still taking precautions with the astronauts and the lunar samples, fearing that perhaps some unknown lunar microbe might hitch a ride back to Earth along with the Apollo astronauts. However, the Surveyor 3 microbes turned out to be no lunar superbacteria. Rather, they were just common Earth Streptococcus mitus. The spacecraft had become contaminated upon assembly and the bacteria went along for a ride to the Moon. No one is still really clear on just how the Streptococcus spores survived the extremely hostile environment.
You may have been wondering why I was talking about bacteria on an astronomy blog site. The previous paragraph tied the bacteria to my site, but there are even more ties. For example, we know that the planet Mars at one time had liquid water in abundance. It does not have liquid water at its surface at present. However, the presence of liquid water in the past, and the possibility of liquid water beneath the surface, raises questions about whether life ever developed on that world. Certainly, Mars is quite inhospitable to life at present. And, even if there are small isolated places on Mars today where conditions are such that life is possible, those conditions are likely marginal and temporary. The standard argument goes that there is not likely life on Mars because conditions do not lend themselves to life except sporadically, and and lifeforms would be unable to move from one habitable region to another as the conditions change. But, this new discovery of eight million year old viable Antarctic bacteria may call that argument into doubt. If Martian life could go dormant for millions of years at a time, then there is the possibility that it could survive even such an inhospitable place as Mars is today. Conditions may be such that life flourishes for a while, and then things change. The local climate becomes incapable of sustaining the lifeforms. Perhaps millions of years later, though, conditions change temporarily to permit life to exist again. Then, the climate once again becomes hostile to life until a brief period millions of years later. Since we now know that these bacteria can come back after millions of years, then this raises the possibility that life of this sort might be all over Mars, just waiting for the conditions to be right for it to revive itself, repair millions of years of genetic damage, and grow and multiply.
And, this raises yet another issue, that of panspermia. If microbes can survive the harshness of space and repair millions of years of cumulative radiation damage, then they may survive being thrown from planet to planet. In 1996, scientists affiliated with NASA’s Johnson Space Center published an analysis of a meteorite called ALH84001. This meteorite, they found, was actually a piece of the planet Mars that had been hurled into space by a titanic explosion, likely an asteroid impact on Mars. The Mars rock then orbited the Sun for a few million years before it ran into Earth, landing in Antarctica. Upon careful study, the team of scientists studying the rock discovered some interesting findings that they interpreted to mean that the rock may contain fossilized tiny bacteria-like Martian lifeforms. Most scientists today believe that the claim of fossilized Mars life was premature and that the structures found in ALH84001 are most likely geologic in origin rather than biologic; however, the debate still goes on in some circles about this Mars rock. But, if bacteria can survive such extreme environments and repair themselves when environmental conditions improve, then it suggests that they can hitch a ride from one planet to another and still be viable. Life from outer space could fall to Earth as a meteorite, much as in the movies The Blob or The Andromeda Strain.
But, you probably don’t have to worry about how an ancient bacteria come back to life (or an alien lifeform) might affect Earth or wreck havoc on our biosphere, a concern that Tris expressed over at the Homely Scientist. As Tris pointed out, the risk is quite small. And, most biologists think the risk is virtually zero. After all, if you think about it, eight million old bacteria are extinct for a reason. Most likely they could not keep up with the other more aggressive bacteria, or else they are so easy to kill (obviously by some other means than freezing and radiation exposure, of course) that they simply didn’t survive. Also, modern higher lifeforms are always under attack by microorganisms. We have developed quite adept immune systems to deal with them. It is only the rare microbe that can make it past our defenses. In fact, our immune systems are so broad acting that we tend to fight off any pathogen that we haven’t seen before unless it has a special adaption specifically designed to attack us. That means, that a microbe must have evolved specifically to attack humans or some other species or else it would not be able to do so. After all, with all the scare about bird flu, that really horrible virus still can’t seem to get a hold on humans. It still needs to evolve a tiny bit to be able to overcome our immune system (though if it did, we’d be in trouble). Dogs are not susceptible to feline leukemia. I am not susceptible to dutch elm disease, and elm trees are immune to the common cold. So, the general feeling is that any alien microbe delivered to Earth would either lack the proper mechanism to attack Earth organisms, or else it would lack defense to the rather aggressive Earth bacteria already here ready to eat it.
Still, you can’t be too careful. So, when the first Apollo astronauts returned from the Moon, they and the lunar samples that they brought back were put into quarantine (both Apollo 11 and Apollo 12 were quarantined). Later missions, though, did not have to go into quarantine once NASA figured out that there were no lunar microbes to bring back to Earth. Today, we are talking about a future Mars sample return mission. When those samples are returned, they will be treated as carefully as the Moon rocks were, and they will be strictly quarantined to avoid any cross contamination with Earth. However, that is likely non necessary, for as I have already said, life may have already been able to make the trip between the two planets aboard meteorites.
-Astroprof
Gordon R. Vaughan on September 1, 2007 at 9:57 pm: 1
Fascinating. Antarctica is a huge time capsule, and I guess Mars is, too.
I don’t recall reading about the Surveyor microbes. I wonder if that’s what prompted NASA to start sterilizing interplanetary spacecraft?
BTW, I noticed on your earlier post, you’d like to visit Colorado. Me, too. … In fact, WHO in Texas DOESN’T think about going to Colorado in August?!!
A Ler…-- Rastos de Luz on September 3, 2007 at 3:10 pm: 2
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