A16 From Page One life from A1
chemistry—what else can life do that we haven’t seen yet?” said Wolfe-Simon, a NASA astrobiolo- gy researchfellowandmember of the National Astrobiology Insti- tute team at Arizona State Uni- versity. “This is different from any-
thing we’ve seen before,” said Mary Voytek, senior scientist for NASA’s program 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 re-
placed one useful element with another; they have the arsenic in the basic building blocks of their makeup,” 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 cos-
mologist Paul Davies, director of the Beyond Center at Arizona State and a prolific writer, is a co-author on the new Science paper. He had been thinking about the idea of a shadow bio- sphere for a decade and had written a paper on it in 2005. Two years laterUniversity ofColorado at Boulder philosopher and as- trobiologist Carol Cleland also published on the subject. Both asked why nobody was looking for life with different origins on Earth, and Cleland coined the phrase “shadow biosphere.” At a Beyond Center conference
four years ago, Wolfe-Simon, then in her late 20s, proposed a way to search for a possible shadow biosphere, and it in- volved Mono Lake and its arse- nic. “We were kicking vague ideas
around, but she had a very specif- ic proposal and then 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, sul- fur and phosphorus — that have qualities that make them seem- ingly ideal for their tasks. A form of phosphorus, for instance, is
EZ SU
KLMNO Life on Earth — with arsenic
FRIDAY, DECEMBER 3, 2010
near perfect for building the framework for the DNA mole- cule, 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 Molecu- lar Evolution in Florida has been involved in shadow biosphere research for several years, and spoke at the NASA unveiling of Wolfe-Simon’s work.He says that the Mono Lake results are in- triguing — “I do not see any simple explanation for the re- ported results that is broadly consistent with other informa- tion well known to chemistry” — but he says they are not yet proved. A primary reason is that arsenic compounds break down quickly in water while phospho- rus compounds do not. His conclusion: “It remains to
be established that this bacteri- um uses arsenate as a replace- ment for phosphate in itsDNA or in any other biomolecule.” The tests to make a more final deter- mination, he said, are complex but available. The paper and its results have
created an excitement reminis- cent of the 1995 announcement at NASA headquarters of the dis- covery of apparent signs of an- cient life in a meteorite from Mars found in Antarctica. That finding was central to establish- ing the field of astrobiology, but was also broadly challenged, and a scientific consensus evolved that the case for signs of life in the meteorite had not been proved. The Mono Lake discovery
highlights one of the central chal- lenges of astrobiology—knowing what to look for in terms of extraterrestrial life. While it re- mains uncertain whether the lake’s microbes represent anoth- er line of life, they show that organisms can have a chemical architecture different from what has until nowbeen understood to
MARK WILSON/GETTY IMAGES
FelisaWolfe-Simon, aNASAastrobiology research fellow, says, “Our findings are a reminder that life- as-we-know-it could be much more flexible than we generally assume or can imagine.’’
be possible. “One of the guiding principles
in the search for life on other planets, and of our astrobiology program, is that we should ‘fol- low the elements,’ ” said Ariel Anbar, an ASU professor and biogeochemist. “Felisa’s study teaches us thatwe ought to think harder about which elements to follow.” Mono Lake was selected as a
work site by Wolfe-Simon be- cause it ishighlyunusual andhad been well studied by other scien- tists trying to answer different questions. The lake receives runoff from
the Sierra Nevada mountains, which have relatively high con- centrations of arsenic. When the water arrives atMonoLake, ithas nowhere to go because there are no rivers carrying water farther downstream. That means the ar- senic, and other elements and compounds, can concentrate to unusally high levels. Arsenic is
present in Mono Lake at a con- centration700 times greater than what the Enivironmental Protec- tion Agency considers safe. Wolfe-Simon was invited to
use the Menlo Park, Calif., lab of the U.S. Geological Survey and was aided in her work by senior research scientistRonOremland, who has studied arsenic inMono Lake for decades. The bugs she worked with, an
otherwise common bacteria in the Halomonadaceae family, thrived without phosphates and with lots of arsenic. She then used cutting-edge instruments to determine that the arsenic was embeddedinthe core genetic and energy-transfer systems of the bacteria — that it appeared to have replaced (or preceded) the phosphorus. As she explained, replacing
phosphorus with arsenic may seem suicidal, but the two are very similar in their makeup. Arsenic is considered toxic be-
causemost living things take it in and treat it like phosphorus, only to be destroyed by the small differences. She said that while small
amounts of the phosphorus re- mained in the arsenic-based bugs, she was able to determine that it was definitely not enough to supply the presumed phospho- rus needs of the cell. That, she said, was being done with the arsenic. “Sometimes I’m asked why
something like this has never been found before, and the an- swer is that nobody has run the experiment before,”Wolfe-Simon said. “There was nothing really complicated about it — I asked a simple question thatwas testable and got an answer.” Wolfe-Simon said she hopes to
test her findings in northern Argentina, where there’s an arse- nic-heavy ecosystem that also seems to supportmicrobial life.
kaufmanm@washpost.com
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