The first organism able to substitute one of the six chemical elements crucial to life has been found.
The bacterium, found in a California lake, uses the usually poisonous element arsenic in place of phosphorus.
The find, described in Science, gives weight to the long-standing idea that life on other planets may have a radically different chemical makeup.
It also has implications for the way life arose on Earth - and how many times it may have done so.
The "extremophile" bacteria were found in a briny lake in eastern California in the US.
While bacteria have been found in inhospitable environments
and can consume what other life finds poisonous, this bacterial strain
has actually taken arsenic on board in its cellular machinery.
Until now, the idea has been that life on Earth must be
composed of at least the six elements carbon, hydrogen, oxygen,
nitrogen, sulphur and phosphorus - no example had ever been found that
violates this golden rule of biochemistry.
The bacteria were found as part of a hunt for life forms radically different from those we know.
"At the moment we have no idea if life is just a freak,
bizarre accident which is confined to Earth or whether it is a natural
part of a fundamentally biofriendly universe in which life pops up
wherever there are Earth-like conditions," explained Paul Davies, the
Arizona State University and Nasa Astrobiology Institute researcher who
co-authored the research.
"Although it is fashionable to support the latter view, we have zero evidence in favour of it," he told BBC News.
"If that is the case then life should've started many times
on Earth - so perhaps there's a 'shadow biosphere' all around us and
we've overlooked it because it doesn't look terribly remarkable."
Proof of that idea could come in the form of organisms on
Earth that break the "golden rules" of biochemistry - in effect, finding
life that evolved separately from our own lineage.
Study lead author Felisa Wolfe-Simon and her colleagues Professor Davies and Ariel Anbar of Arizona State University initially suggested in a paper an alternative scheme to life as we know it.
Their idea was that there might be life in which the normally
poisonous element arsenic (in particular as chemical groups known as
arsenates) could work in place of phosphorus and phosphates.
Putting it to the test, the three authors teamed up with a
number of collaborators and began to study the bacteria that live in
Mono Lake in California, home to arsenic-rich waters.
The researchers began to grow the bacteria in a laboratory on
a diet of increasing levels of arsenic, finding to their surprise that
the microbes eventually fully took up the element, even incorporating it
into the phosphate groups that cling to the bacteria's DNA.
Notably, the research found that the bacteria thrived best in a phosphorus environment.
That probably means that the bacteria, while a striking first
for science, are not a sign of a "second genesis" of life on Earth,
adapted specifically to work best with arsenic in place of phosphorus.
'Weird branch'
However, Professor Davies said, the fact that an organism that
breaks such a perceived cardinal rule of life makes it is a promising
step forward.
"This is just a weird branch on the known tree of life," said
Professor Davies. "We're interested ultimately in finding a different
tree of life... that will be the thing that will have massive
implications in the search for life in the Universe.
"The take-home message is: who knows what else is there? We've only scratched the surface of the microbial realm."
John Elliott, a Leeds Metropolitan University researcher who
is a veteran of the UK's search for extraterestrial life, called the
find a "major discovery".
The bacteria were found in the salty Mono Lake
"It starts to show life can survive outside the traditional
truths and universals that we thought you have to use... this is
knocking one brick out of that wall," he said.
"The general consensus is that this really could still be an
evolutionary adapatation rather than a second genesis. But it's early
days, within about the first year of this project; it's certainly one to
think on and keep looking for that second genesis, because you've
almost immediately found an example of something that's new."
Simon Conway Morris of the University of Cambridge agreed
that, whatever its implications for extraterrestrial life, the find was
significant for what we understand about life on Earth.
"The bacteria is effectively painted by the investigators
into an 'arsenic corner', so what it certainly shows is the astonishing
and perhaps under-appreciated versatility of life," he told BBC News.
"It opens some really exciting prospects as to both
un-appreciated metabolic versatility... and prompting the questions as
to the possible element inventory of remote Earth-like planets".
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