News: Billions of years ago, microbes were key in developing modern nitrogen cycle

Billions of years ago, microbes were key in developing modern nitrogen cycle
February 19th, 2009 in General Science / Biology


(PhysOrg.com) -- As the world marks the 200th anniversary of Charles
Darwin's birth, there is much focus on evolution in animals and plants. But
new research shows that for the countless billions of tiniest creatures -
microbes - large-scale evolution was completed 2.5 billion years ago.

"For microbes, it appears that almost all of their major evolution took
place before we have any record of them, way back in the dark mists of
prehistory," said Roger Buick, a University of Washington paleontologist and
astrobiologist.

All living organisms need nitrogen, a basic component of amino acids and
proteins. But for atmospheric nitrogen to be usable, it must be "fixed," or
converted to a biologically useful form. Some microbes turn atmospheric
nitrogen into ammonia, a form in which the nitrogen can be easily absorbed
by other organisms.

But the new research shows that about 2.5 billion years ago some microbes
evolved that could carry the process a step further, adding oxygen to the
ammonia to produce nitrate, which also can be used by organisms. That was
the beginning of what today is known as the aerobic nitrogen cycle.

The microbes that accomplished that feat are on the last, or terminal,
branches of the bacteria and archaea domains of the so-called tree of life,
and they are the only microbes capable of carrying out the step of adding
oxygen to ammonia.

The fact that they are on those terminal branches indicates that large-scale
evolution of bacteria and archaea was complete about 2.5 billion years ago,
Buick said.

"Countless bacteria and archaea species have evolved since then, but the
major branches have held," said Buick, a UW professor of Earth and space
sciences.

He is the corresponding author of the research, which appears in the Feb. 20
edition of Science. Lead author is Jessica Garvin, a UW Earth and space
sciences graduate student. Other authors are Ariel Anbar and Gail Arnold of
Arizona State University and Alan Jay Kaufman of the University of Maryland.
The work was funded by NASA and the National Science Foundation.

The scientists examined material from a half-mile-deep core drilled in the
Pilbara region of northwest Australia. They looked specifically at a section
of shale from 300 to 650 feet deep, deposited 2.5 billion years ago, and
found telltale isotope signatures created in the process of denitrification,
the removal of oxygen from nitrate.

If denitrification was occurring, then nitrification - the addition of
oxygen to ammonia to form nitrate - also must have been taking place, Buick
said. That makes the find the earliest solid evidence for the beginning of
the aerobic nitrogen cycle.

"What this shale deposit has done is record the onset of the modern nitrogen
cycle," he said. "This was a life-giving nutrient then and remains so today.
That's why you put nitrogen fertilizer on your tomato plants, for example."

The discovery gives clues about when and how the Earth's atmosphere became
oxygen rich, Buick believes.

Geochemical examination of stratigraphic samples from the core indicates
that atmospheric oxygen rose in a temporary "whiff" about 2.5 billion years
ago. The whiff lasted long enough to be recorded in the nitrogen isotope
record, then oxygen dropped back to very low levels before the atmosphere
became permanently oxygenated about 2.3 billion years ago.

It is unclear why the oxygen level declined following the temporary
increase. It could have been that the oxygen was depleted rapidly as it
reacted with chemicals and minerals that had not been exposed to oxygen
previously, Buick said. Or something could have halted the photosynthesis
that produced the oxygen in the first place.

But it seems clear, he said, that the tiniest creatures played a crucial
role in completing the nitrogen cycle that life depends on today.

"All microbes are amazing chemists compared to us. We're really very boring,
metabolically," Buick said.

"To understand early evolution of life, we have to know how organisms were
nourished and how they evolved. This work helps us on both of those counts,"
he said.

Provided by University of Washington
http://www.physorg.com/news154275830.html

Posted by
Robert Karl Stonjek


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