[meteorite-list] Tough Earth Bug May Be From Mars

From: Ron Baalke <baalke_at_meteoritecentral.com>
Date: Thu Apr 22 10:08:32 2004
Message-ID: <200209252342.QAA19201_at_zagami.jpl.nasa.gov>

http://www.newscientist.com/news/news.jsp?id=ns99992844

Tough Earth bug may be from Mars
Stuart Clark
New Scientist
September 25, 2002

A hardy microbe that can withstand huge doses of radiation could have
evolved this ability on Mars.

That is the conclusion of Russian scientists who say it would take far
longer than life has existed here for the bug to evolve that ability in
Earth's clement conditions. They suggest the harsher environment of Mars
makes it a more likely birthplace.

The hardy bugs could have travelled to Earth on pieces of rock that were
blasted into space by an impacting asteroid and fell to Earth as meteorites.

Deinococcus radiodurans is renowned for its resistance to radiation - it can
survive several thousand times the lethal dose for humans. To investigate
how the trait might have evolved, Anatoli Pavlov and his colleagues from the
Ioffe Physico-Technical Institute in St Petersburg tried to induce it in E.
coli.

99.9 per cent deadly

They blasted the bugs with enough gamma rays to kill 99.9 per cent of them,
let the survivors recover, and then repeated the process. During the first
cycle just a hundredth of the lethal human dose was enough to wipe out 99.9
per cent of the bacteria, but after 44 cycles it took 50 times that initial
level to kill the same proportion.

However, the researchers calculate that it would take thousands of such
cycles before the E. coli were as hardy as Deinococcus. And on Earth it
would take between a million and a hundred million years to accumulate each
dose, during which time the bugs would have to be dormant.

Since life originated on Earth about 3.8 billion years ago, Pavlov does not
believe that there has been enough time for this resistance to evolve.

Dormant bugs

On Mars, however, the researchers calculate that dormant bugs could receive
the necessary dose in just a few hundred thousand years, because radiation
levels there are much higher.

What is more, they point out that the Red Planet wobbles on its rotation
axis, producing a regular cycle of climate swings that would drive bacteria
into dormancy for long enough to accumulate such doses, before higher
temperatures enabled the survivors to recover and multiply. Pavlov reported
the results last week at the Second European Workshop on Astrobiology in
Graz, Austria.

David Morrison of NASA's Astrobiology Institute is sceptical that
Deinococcus came from Mars, pointing out that its genome looks similar to
those of other Earthly bacteria. But he admits that there's still no obvious
explanation for the bug's resistance to radiation.

"It is certainly a mystery how this trait has developed and why it
persists," he says.
Received on Wed 25 Sep 2002 07:42:27 PM PDT