Dec. 2, 2010: NASA-supported researchers announced that they had discovered the first known microorganism on Earth able to thrive and reproduce using the toxic chemical arsenic. The microorganism, which lives in 740,000 year old California's Mono Lake, substitutes arsenic for phosphorus in the backbone of its DNA and other cellular components.
"The definition of life has just expanded," said Ed Weiler, NASA's associate administrator for the Science Mission Directorate at the agency's Headquarters in Washington. "As we pursue our efforts to seek signs of life in the solar system, we have to think more broadly, more diversely and consider life as we do not know it."
"Biological dependence on the six major nutrient elements carbon, hydrogen, nitrogen, oxygen, sulfur, and phosphorus is complemented by a selected array of other elements, usually metal(loid)s present in trace quantities that serve critical cellular functions, such as enzyme co-factors. There are many cases of these trace elements substituting for one another," says the study. "However, there are no prior reports of substitutions for any of the six major elements essential for life. Here we present evidence that arsenic can substitute for phosphorus in the biomolecules of a naturally-occurring bacterium."
The NASA scientists trained a bacterium taken from the bottom of Mono Lake in California to eat and grow on a diet of arsenic, in place of phosphorus — one of six elements considered essential for life — opening up the possibility that organisms could exist elsewhere in the universe (or even on Earth) using unknown alien biochemical powers.
The bacterium, grown for months in a lab mixture containing arsenic, gradually replaced atoms of phosphorus as its energy source for atoms of arsenic. Phosphorus is one of six chemical elements that have long been thought to be essential for all life on Earth until now. Phosphorus chains are the backbone building block of DNA and its chemical bonds, particularly in a molecule known as adenosine triphosphate, the principal means by which biological creatures store energy.
While nature has been able to engineer substitutes for some of the other elements that exist in trace amounts for specialized purposes — like iron to carry oxygen — until now, reported The New York Times "there has been no substitute for the basic six elements. Now, scientists say, these results will stimulate a lot of work on what other chemical replacements might be possible. The most fabled, much loved by science fiction authors but not ever established, is the substitution of silicon for carbon."
“There is basic mystery, when you look at life,” said Dimitar Sasselov, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of the Institute on the Origins of Life , who was not involved in the work in an interview with The New York Times. “Nature only uses a restrictive set of molecules and chemical reactions out of many thousands available. This is our first glimmer that maybe there are other options.”
Felisa Wolfe-Simon told the New York Times: “This is a microbe that has solved the problem of how to live in a different way.” This story is not about Mono Lake or arsenic, she added, but about “cracking open the door and finding that what we think are fixed constants of life are not.”
Arsenic sits right beneath phosphorus in the periodic table of the elements and shares many of its chemical properties -a chemical closeness is what makes it toxic, Dr. Wolfe-Simon said, "allowing it to slip easily into a cell’s machinery where it then gums things up, like bad oil in a car engine."
The Viking landers that failed to find life on Mars in 1976, Dr. Wolfe-Simon pointed out, were designed before the discovery the discovery of Earth's extremophiles such as tube worms and other weird life at deep undersea vents and the ancient dry valleys and buried lakes of Antarctica.
Evidence of possible life on Mars sent back from by two Mars Viking Landers in 1976 and 1977 was inconclusive, at least according the then primitive knowledge of both extreme life that we now know exists on Earth as well as the abundant existence of water and methane on Mars past and present.
On Mars, as on Earth, methane is extremely unstable because it's continually being broken up by ultraviolet rays from the Sun and chemical reactions with other gases. The average life of a methane molecule on Mars is 400 years, which means the gas must be continually replenished or it will disappear. Something is producing methane on Mars today -the big question is: What?
A new study indicates that methane in the atmosphere of Mars lasts less than a year. Methane is replenished from localized sources that show seasonal and annual variations. This pattern of methane production raises questions as to whether the methane comes from geological activity - or biological processes. The atmosphere on Mars consists of 95% carbon dioxide, 3% nitrogen, 1.6% argon, and contains traces of oxygen and water, as well as methane.
Sergio Fonti (Universita del Salento) and Giuseppe Marzo (NASA Ames) have used observations from NASA’s Mars Global Surveyor spacecraft to track the evolution of the gas over three martian years.
“Only small amounts of methane are present in the martian atmosphere, coming from very localized sources. We’ve looked at changes in concentrations of the gas and found that there are seasonal and also annual variations. The source of the methane could be geological activity or it could be biological -- we can’t tell at this point. However, it appears that the upper limit for methane lifetime is less than a year in the martian atmosphere,” said Fonti.
Levels of methane are highest in autumn in the northern hemisphere, with localized peaks of 70 parts per billion, although methane can be detected across most of the planet at this time of year. There is a sharp decrease in winter, with only a faint band between 40 and 50 degrees north. Concentrations start to build again in spring and rise more rapidly in summer, spreading across the planet.
“One of the interesting things that we’ve found is that in summer, although the general distribution pattern is much the same as in autumn, there are actually higher levels of methane in the southern hemisphere. This could be because of the natural circulation occurring in the atmosphere, but has to be confirmed by appropriate computer simulations,” said Fonti.
“It’s evident that the highest concentrations are associated with the warmest seasons and locations where there are favorable geological -- and hence biological -- conditions such as geothermal activity and strong hydration. The higher energy available in summer could trigger the release of gases from geological processes or outbreaks of biological activity,” said Fonti.
“Our study is the first time that data from an orbiting spectrometer has been used to monitor methane over an extended period. The huge TES dataset has allowed us to follow the methane cycle in the martian atmosphere with unprecedented accuracy and completeness. Our observations will be very useful in constraining the origins and significance of martian methane,” said Fonti.
There is potentially a vast biosphere a few meters below Mars' surface, which the 1976 Viking mission may not have been able to access since it was only scratching the surface of the uppermost layer of soil.
NASA's first press release about the Viking tests announced that the results were positive. The "labeled Release" (LR) experiments had given positive results. But after lengthy discussions in which Carl Sagan participated, NASA reversed its position, mainly because another experiment detected no organics in the soil.
Yet to this day, Gilbert Levin, the principal designer of the LR experiment, believes the tests pointed to life. When the same two experiments were run on soil from Antarctica, the same conflicting results were obtained (LR - positive; organics - negative.)
Soil and ice from Antarctica's Dry Valley certainly contains extreme life forms. The test for organics was negative because it is far less sensitive than the LR experiment. The same problem could have caused the organics test on to give a false negative.
Before oxygen could accumulate in Earth's atmosphere, all the exposed iron had to rust. During that process, lasting hundreds of millions of years, Earth was also a red planet. In the journal Nature,
Corinna Wu asked: Could the oxygen that rusted the iron on have been produced biologically? Could life on Mars have simply "run out of steam" after that stage of its development? In a paper in The International Journal of Astrobiology, Felisa Wolfe-Simon and Ariel Anbar and Paul Davies, both of Arizona State University, predicted the existence of arsenic-loving life forms.
According to Science, a bacterium known as strain GFAJ-1 of the Halomonadaceae family of Gammaproteobacteria, proved to grow the best of the microbes from the lake, although not without changes from their normal development. The cells grown in the arsenic came out about 60 percent larger than cells grown with phosphorus along with large, empty internal spaces.
By labeling the arsenic with radioactivity, the NASA Astrobiology researchers were able to conclude that arsenic atoms had taken up position in the microbe’s DNA as well as in other molecules within it. It was inconclusive, however, that there was arsenic in the backbone of working DNA.