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Iron and manganese compounds, in addition to sulfate, may play an
important role in converting methane to carbon dioxide and eventually
carbonates in the Earth's oceans, according to a team of researchers
looking at anaerobic sediments. These same compounds may have been key
to methane reduction in the early, oxygenless days of the planet's
"We used to believe that microbes only consumed methane in marine
anaerobic sediment if sulfate was present," said Emily Beal, graduate
student in geoscience, Penn State. "But other electron acceptors, such
as iron and manganese, are more energetically favorable than sulfate."
Microbes or groups of microbes -- consortia -- that use sulfates to
convert methane for energy exist in marine sediments. Recently other
researchers have identified microbes that use forms of nitrogen in fresh
water environments to convert methane.
"People had speculated that iron and manganese could be used, but no
one had shown that it occurred by incubating live organisms," said Beal.
Beal, working with Christopher H. House, associate professor of
geoscience, Penn State, and Victoria J. Orphan, assistant professor of
geobiology, California Institute of Technology, incubated a variety of
marine sediments to determine if there were microbes that could convert
methane to carbon dioxide without using any sulfur compounds. They
report their results in the July 10 issue of Science.
Using samples of marine sediment taken 20 miles off the California
coast and about 1,800 feet deep near methane seeps in the Pacific, Beal
incubated a variety of sediment systems including as controls, an
autoclaved sterile sample, a sample with sulfate as a control and a
sample that was sulfate, iron oxide and manganese oxide free, but live.
She also incubated samples that were sulfate free but contained iron
oxide or manganese oxide. She placed methane gas that contained the
non-radioactive carbon-13 isotope in the empty space in the flasks above
the sediment and tested any resulting carbon dioxide produced by the
samples. All the carbon dioxide had the carbon-13 isotope and so came
from the methane samples.
The sterile control showed no activity, while the live control
without sulfate showed minute activity. The sulfate control showed the
most activity as expected, but both the iron and manganese oxide-laced
samples showed activity, although less activity than the sulfate.
"We do not think that iron and manganese are more important than
sulfate reduction today, but they are not trivial components," said
House, who is director of Penn State's Astrobiology Research Center.
"They are probably a big part of the carbon cycle today."
One reason they are important is that some of the carbon dioxide
produced reacts with both the manganese and iron to form carbonates that
precipitate and sequester carbon in the oceans. Even if the carbon
dioxide escaped into the atmosphere, it is a less problematic greenhouse
gas than methane.
On the early Earth, where oxygen was absent from the atmosphere,
sulfates were scarce. Without sulfates, iron and manganese oxides may
have been essential in converting methane to carbon dioxide.
"Sulfate comes mostly from oxidative weathering of rocks," said Beal.
"Oxygen is needed for this to occur."
While manganese and iron oxides are made in today's oxygen
atmosphere, they where also formed by photochemical reactions in a low
oxygen atmosphere. These oxides were probably more abundant in the early
Earth's oceans than sulfates.
While Beal has categorized the more than a dozen microorganisms
living in the sediments she used, she does not know which of these
microbes is responsible for consuming methane. It might be one bacteria
or archaea species, or it may be a consortium of microbes. She is trying
to identify the organisms responsible.
The National Science Foundation and the NASA Astrobiology Institute
supported this work.
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