Geologists Uncover New Evidence About the Rise of Oxygen

http://pr.caltech.edu/media/Press_Releases/PR12758.html

PASADENA, Calif.--Scientists believe that oxygen first showed up in the
atmosphere about 2.7 billion years ago. They think it was put there by a
one-celled organism called "cyanobacteria," which had recently become the
first living thing on Earth to make oxygen from water and sunlight.

The rock record provides a good bit of evidence that this is so. But one of
these rocks has just gotten a great deal more slippery, so to speak.

In an article appearing in the Geological Society of America's journal
Geology, investigators from the California Institute of Technology, the
University of Tübingen in Germany, and the University of Alberta describe
their new findings about the origin of the mineral deposits known as
banded-iron formations, or "BIFs." A rather attractive mineral that is
often cut and polished for paperweights and other decorative items, a BIF
typically has alternating bands of iron oxide and silica. How the iron got
into the BIFs to begin with is thought to be a key to knowing when
molecular oxygen first was produced on Earth.

The researchers show that purple bacteria--primitive organisms that have
thrived on Earth without producing oxygen since before cyanobacteria first
evolved--could also have laid down the iron oxide deposits that make up
BIFs. Further, the research shows that the newer cyanobacteria, which
suddenly evolved the ability to make oxygen through photosynthesis, could
have even been floating around when the purple bacteria were making the
iron oxides in the BIFs.

"The question is what made the BIFs," says Dianne Newman, who is associate
professor of geobiology and environmental science and engineering at
Caltech and an investigator with the Howard Hughes Medical Institute. "BIFs
are thought to record the history of the rise of oxygen on Earth, but this
may not be true for all of them."

The classical view of how the BIFs were made is that cyanobacteria began
putting oxygen in the atmosphere about 2.7 billion years ago. At the same
time, hydrothermal sources beneath the ocean floors caused ferrous iron
(that is, "nonrusted" iron) to rise in the water. This iron then reacted
with the new oxygen in the atmosphere, which caused the iron to change into
ferric iron. In other words, the iron literally "rusted" at the surface of
the ocean waters, and then ultimately settled on the ocean floor as
sediments of hematite (Fe2O3) and magnetite (Fe3O4).

The problem with this scenario was that scientists in Germany about 10
years ago discovered a way that the more ancient purple bacteria could
oxidize iron without oxygen. Instead, these anaerobic bacteria could have
used a photosynthetic process in which light and carbon dioxide are used to
turn the ferrous iron into ferric iron, throwing the mechanism of BIF
formation into question.

Newman's postdoctoral researcher Andreas Kappler (now an assistant
professor at the University of Tübingen) expanded on this discovery by
doing some lab experiments to measure the rate at which purple bacteria
could form ferric iron under light conditions relevant for different depths
within the ocean.

Kappler's results showed that iron could indeed have been oxidized by these
bacteria, in amounts matching what would have been necessary to form one of
the Precambrian iron deposits in Australia.

Another of the paper's Caltech authors, Claudia Pasquero, determined the
thickness of the purple bacterial layer that would have been needed for
complete iron oxidation. Her results showed that the thickness of the
bacterial layer could have been on the order of 17 meters, below wave base,
which compares favorably to what is seen today in stratified water bodies
such as the Black Sea.

Also, the results show that, in principle, the purple bacteria could have
oxidized all the iron seen in the BIFs, even if the cyanobacteria had been
present in overlying waters.

However, Newman says that the rock record contains various other kinds of
evidence that oxygen was indeed absent in the atmosphere earlier than 2.7
billion years ago. Therefore, the goal of better understanding the history
of the rise of oxygen could come down to finding out if there are subtle
differences between BIFs that could have been produced by cyanobacteria
and/or purple bacteria. And to do this, it's best to look at the biology of
the organisms.

"The hope is that we'll be able to find out whether some organic compound
is absolutely necessary for anaerobic anoxygenic photosynthesis to occur,"
Newman says. "If we can know how they work in detail, then maybe we'll be
fortunate enough to find one molecule really necessary."

A good candidate is an organic molecule with high geological preservation
potential that would have existed in the purple bacteria three billion
years ago and still exists today. If the Newman team could find such a
molecule that is definitely involved in the changing of iron to iron oxide,
and is not present in cyanobacteria, then some of the enigmas of oxygen on
the ancient earth would be solved.

"The goals are to get at the types of biomolecules essential for different
types of photosynthesis-hopefully, one that is preservable," Newman says.

"I guess one interesting thing from our findings is that you can get rust
without oxygen, but this is also about the history of metabolic evolution,
and the ability to use ancient rock to investigate the history of life."

Better understanding microbial metabolism could also be of use in NASA's
ambitious goal of looking for life on other worlds. The question of which
organisms made the BIFs on Earth, therefore, could be useful for
astrobiologists who may someday find evidence in rock records elsewhere.


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