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Posted by baalke on November 28, 2006, 3:24 pm
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Caltech News Release
For Immediate Release
November 28, 2006
Geobiologists Solve "Catch-22 Problem" Concerning the Rise of
Atmospheric Oxygen
PASADENA, Calif.--Two and a half billion years ago, when our
evolutionary ancestors were little more than a twinkle in a
bacterium's plasma membrane, the process known as photosynthesis
suddenly gained the ability to release molecular oxygen into Earth's
atmosphere, causing one of the largest environmental changes in the
history of our planet. The organisms assumed responsible were the
cyanobacteria, which are known to have evolved the ability to turn
water, carbon dioxide, and sunlight into oxygen and sugar, and are
still around today as the blue-green algae and the chloroplasts in
all green plants.
But researchers have long been puzzled as to how the cyanobacteria
could make all that oxygen without poisoning themselves. To avoid
their DNA getting wrecked by a hydroxyl radical that naturally occurs
in the production of oxygen, the cyanobacteria would have had to
evolve protective enzymes. But how could natural selection have led
the cyanobacteria to evolve these enzymes if the need for them didn't
even exist yet?
Now, two groups of researchers at the California Institute of
Technology offer an explanation of how cyanobacteria could have
avoided this seemingly hopeless contradiction. Reporting in the
December 12 Proceedings of the National Academy of Sciences (PNAS)
and available online this week, the groups demonstrate that
ultraviolet light striking the surface of glacial ice can lead to the
accumulation of frozen oxidants and the eventual release of molecular
oxygen into the oceans and atmosphere. This trickle of poison could
then drive the evolution of oxygen-protecting enzymes in a variety of
microbes, including the cyanobacteria. According to Yuk Yung, a
professor of planetary science, and Joe Kirschvink, the Van Wingen
Professor of Geobiology, the UV-peroxide solution is "rather simple
and elegant."
"Before oxygen appeared in the atmosphere, there was no ozone screen
to block ultraviolet light from hitting the surface," Kirschvink
explains. "When UV light hits water vapor, it converts some of this
into hydrogen peroxide, like the stuff you buy at the supermarket for
bleaching hair, plus a bit of hydrogen gas.
"Normally this peroxide would not last very long due to
back-reactions, but during a glaciation, the hydrogen peroxide
freezes out at one degree below the freezing point of water. If UV
light were to have penetrated down to the surface of a glacier, small
amounts of peroxide would have been trapped in the glacial ice."
This process actually happens today in Antarctica when the ozone hole
forms, allowing strong UV light to hit the ice.
Before there was any oxygen in Earth's atmosphere or any UV screen,
the glacial ice would have flowed downhill to the ocean, melted, and
released trace amounts of peroxide directly into the sea water, where
another type of chemical reaction converted the peroxide back into
water and oxygen. This happened far away from the UV light that would
kill organisms, but the oxygen was at such low levels that the
cyanobacteria would have avoided oxygen poisoning.
"The ocean was a beautiful place for oxygen-protecting enzymes to
evolve," Kirschvink says. "And once those protective enzymes were in
place, it paved the way for both oxygenic photosynthesis to evolve,
and for aerobic respiration so that cells could actually breath
oxygen like we do."
The evidence for the theory comes from the calculations of lead
author Danie Liang, a recent graduate in planetary science at Caltech
who is now at the Research Center for Environmental Changes at the
Academia Sinica in Taipei, Taiwan.
According to Liang, a serious freeze-over known as the Makganyene
Snowball Earth occurred 2.3 billion years ago, at roughly the time
cyanobacteria evolved their oxygen-producing capabilities. During the
Snowball Earth episode, enough peroxide could have been stored to
produce nearly as much oxygen as is in the atmosphere now.
As an additional piece of evidence, this estimated oxygen level is
also sufficient to explain the deposition of the Kalahari manganese
field in South Africa, which has 80 percent of the economic reserves
of manganese in the entire world. This deposit lies immediately on
top of the last geological trace of the Makganyene Snowball.
"We used to think it was a cyanobacterial bloom after this glaciation
that dumped the manganese out of the seawater," says Liang. "But it
may have simply been the oxygen from peroxide decomposition after the
Snowball that did it."
In addition to Kirschvink, Yung, and Liang, the other authors are
Hyman Hartman of the Center for Biomedical Engineering at MIT, and
Robert Kopp, a graduate student in geobiology at Caltech. Hartman,
along with Chris McKay of the NASA Ames Research Center, were early
advocates for the role that hydrogen peroxide played in the origin
and evolution of oxygenic photosynthesis, but they could not identify
a good inorganic source for it in Earth's precambrian environment.
The paper will soon be available online at the following Web address:
http://www.pnas.org/papbyrecent.shtml
Contact: Robert Tindol
(626) 395-3631
tindol@caltech.edu
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