Life, as we don’t know it
WASHINGTON: All life on Earth – from microbes to elephants to us – is based on a single genetic model that requires the element phosphorus as one of its six essential components.
But now researchers have uncovered a bacterium that has five of those essential elements but has, in effect, replaced phosphorus with its look-alike but toxic cousin, arsenic.
News of the discovery caused a scientific commotion, including calls to Nasa from the White House and Congress asking if a second line of Earthly life had been found.
A Nasa press conference on Thursday and an accompanying article in the journal Science, gave the answer – no, the discovery does not prove the existence of a so-called “second genesis” on Earth. But the discovery very much opens the door to that possibility, and to the related existence of a theorised “shadow biosphere” on Earth – life evolved from a different common ancestor from all that we’ve known so far.
“Our findings are a reminder that life as we know it could be much more flexible than we generally assume or can imagine,” said Felisa WolfeSimon, the young biochemist who led the effort after being selected as a Nasa Astrobiology Research Fellow and as a member of the National Astrobiology Institute team at Arizona State University (ASU).
“If something here on Earth can do something so unexpected – that breaks the unity of biochemistry – what else can life do that we haven’t seen yet?” she said.
The research, funded through Nasa and conducted with samples from California’s Mono Lake, found some of the bacteria not only used arsenic to live, but had arsenic embedded into their DNA, RNA and other basic underpinnings.
“This is different from anything we’ve seen before,” said Mary Voytek, senior scientist for Nasa’s programme in astrobiology , the arm of the agency involved specifically in the search for life beyond Earth and for how life began here.
“These bugs haven’t just replaced one useful element with another, they have the arsenic in the basic building blocks of their make-up,” she said. “We don’t know if the arsenic replaced phosphorus or if it was there from the very beginning – in which case it would strongly suggest the existence of a shadow biosphere.”
Theoretical physicist and cosmologist Paul Davies, director of the Beyond Centre at Arizona State University and a prolific writer, is a co-author. He had been thinking about the idea for a decade, and had written a paper in 2005. So had University of Colorado, Boulder philosopher and astrobiologist Carol Cleland. Both asked why nobody was looking for life with different origins on Earth, and Cleland coined the phrase “shadow biosphere”.
At a Beyond Centre conference four years ago, WolfeSimon, then in her late 20s, proposed a way to search for a possible shadow biosphere, and it involved Mono Lake and its arsenic.
“We were kicking vague ideas around, but she had a very specific proposal and went out and executed it,” Davies said. “It defies logic to think she found the only example of this kind of unusual life. Quite clearly, this is the tip of a huge iceberg.”
All life as we know it contains six essential elements – carbon, oxygen, hydrogen, nitrogen, sulfur and phosphorus – that have qualities that make them seemingly ideal for their tasks. A form of phosphorus, for instance, is near perfect for building the framework for the DNA molecule, and another form is crucial to the transfer of energy within cells.
These forms of phosphorus are well suited for their job because they are especially stable in the presence of water. Arsenic is not, and that fact is one that raises concerns for some researchers familiar with the Mono Lake bugs.
Chemist Steven Benner of the Foundation for Applied Molecular Evolution in Florida has been involved in “shadow biosphere” research for several years, and will speak at the Nasa unveiling of Wolfe-Simon’s work.
He says the Mono Lake results are intriguing.
“I do not see any simple explanation for the reported results that is broadly consistent with other information well known to chemistry” but he says they are not yet proven. And a primary reason why, is that arsenic compounds break down quickly in water, while phosphorus compounds do not.
His conclusion: “It remains to be established that this bacterium uses arsenate as a replacement for phosphate in its DNA or in any other biomolecule.”
The Mono Lake discovery highlights one of the central challenges of astrobiology – knowing what to look for in terms of extraterrestrial life. While it remains uncertain whether the lake’s microbes represent another line of life, they show organisms can have a chemical architecture different from what is currently understood to be possible.
“One of the guiding principles in the search for life on other planets, and of our astrobiology programme, is that we should ‘follow the elements’,” said Ariel Anbar, an ASU professor and biogeochemist. “Felisa’s study teaches us that we ought to think harder about which elements to follow.”
Mono Lake was selected as a work site by Wolfe-Simon because it is highly unusual and already had been well studied by scientists trying to answer other questions.
The lake receives run-off from the Sierra Nevada mountains, which have relatively high concentrations of arsenic. When the water arrives at Mono Lake, it has nowhere to go because there are no rivers carrying water further downstream.
That means the arsenic, and other elements and compounds, can concentrate to unusually high levels.
Wolfe-Simon said she hoped to further test her findings in norther n Argentina where, she’s been told, some microbes can not only tolerate arsenic, but absolutely need arsenic to survive. – Washington Post