"Not only is phosphorus the active component of ATP, it also forms the backbone of DNA and is important in the structure of cell walls. But despite its importance to life, it is not fully understood how phosphorus first appeared in our atmosphere. One theory is that it was contained within the many meteorites that collided with the Earth billions of years ago."
Dr Terry Kee of the University of Leeds
(Image above is Mono Lake, site of the recent and controversial NASA discovery that ansenic may have replaced phosphorus position in microbial DNA).
Researchers at the University of Leeds may have uncovered new clues to the origins of life on Earth with the discovery of a compound known as pyrophosphite, which may have been an important energy source for primitive lifeforms.
"It's a chicken and egg question," said Dr Terry Kee of the University of Leeds, who led the research. "Scientists are in disagreement over what came first -- replication, or metabolism. But there is a third part to the equation, and that is energy."
All living things require a continual supply of energy in order to function. This energy is carried around our bodies within certain molecules, one of the best known being ATP*, which converts heat from the sun into a useable form for animals and plants.
At any one time, the human body contains just 250g of ATP. This provides roughly the same amount of energy as a single AA battery. This ATP store is being constantly used and regenerated in cells via a process known as respiration, which is driven by natural catalysts called enzymes.
"You need enzymes to make ATP and you need ATP to make enzymes," explained Dr Kee. "The question is: where did energy come from before either of these two things existed? We think that the answer may lie in simple molecules such as pyrophosphite which is chemically very similar to ATP, but has the potential to transfer energy without enzymes."
The key to the battery-like properties of both ATP and pyrophosphite is an element called phosphorus, which is essential for all living things. Not only is phosphorus the active component of ATP, it also forms the backbone of DNA and is important in the structure of cell walls.
But despite its importance to life, it is not fully understood how phosphorus first appeared in our atmosphere. One theory is that it was contained within the many meteorites that collided with the Earth billions of years ago.
"Phosphorus is present within several meteoritic minerals and it is possible that this reacted to form pyrophosphite under the acidic, volcanic conditions of early Earth," added Dr Kee.
The findings, published in the journal Chemical Communications, are the first to suggest that pyrophosphite may have been relevant in the shift from basic chemistry to complex biology when life on earth began. Since completing this research, Dr. Kee and his team have found even further evidence for the importance of this molecule and now hope to team up with collaborators from NASA to investigate its role in abiogenesis.
Ironically, bacterium that replaced phosphorus with arsenic offers hope for finding life living on alien worlds, according to a controversial NASA study led by Felisa Wolfe-Simon of the NASA Astrobiology Institute in Menlo Park, Calif., who has been conducting research at Mono Lake California for years and led the experiment.
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 scientists said that they had 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 here 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. The others are carbon, oxygen, nitrogen, hydrogen and sulfur.
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, reports 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."
Scientists said the results, if confirmed, would dramatically extend our understanding of what life could be and where it could be.
“There is basic mystery, when you look at life,” said Dimitar Sasselov, an astronomer at the Harvard-Smithsonian Center for Astrophysics and director of an institute on the origins of life there, 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 of Earth's extremophiles such as of tube worms and other weird life at deep undersea vents and the ancient dry valleys and buried lakes of Antarctica.
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 the article in 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.
Casey Kazan via University of Leeds, The New York Times, and Science
More information: The findings, entitled: 'On the prebiotic potential of reduced oxidation state phosphorus: the H-phosphinate-pyruvate system', are published in Chemical Communications - DOI:10.1039/c002689a