Marine methane consumed by consortia of bacteria

Methane consuming archaeobacteria and sulfate-reducing
bacteria, acting together, are responsible for consuming most of the methane in the
world’s oceans, according to a team of microbiologists and geoscientists.

“Past research had shown that there is a consortia of these two very different
single-celled organisms, and indirect tests indicated they might be the source of
methane consumption,” said Dr. Christopher H. House, assistant professor of
geosciences at Penn State. “We decided to directly test if these organisms are
responsible.”

The research team explains their approach in today’s (July 20) issue of the journal
Science. Research team members Victoria J. Orphan, graduate student, and Dr. Edward
F. Delong, chair of the science department at Monterey Bay Aquarium Research Institute,
were responsible for identifying the organisms in the consortia. The sulfate-reducing
bacteria, Desulfosarcina, and the methane-consuming archaeobacteria,
Methanosarcinales, were tagged with flouresent dyes that attach only to individually
specific genetic material. In this way, one dye attached to the Desulfosarcina and
another, of a different color, attached to the Methanosarcinales. Investigation under
flourescent light showed the sulfate-eaters surrounding the methane eaters in an
aggregated clump about the diameter of the width of a human hair.

“To determine that these aggregates were responsible for the methane consumption,
we had to test the organisms to see which carbon isotopes were incorporated into the
cells,” says House at Penn State.

House and Orphan, working with Dr. Kevin D. McKeegan, professor of Earth and space
sciences, University of California, Los Angeles, employed an ion probe that uses cesium
ions focused to a very small spot to slowly erode the cells for study. The probe allowed
samples of the carbon from the consortia to be tested, beginning with the outer cells and
then tunnelling toward the middle of the clump.

Carbon isotopes were the component of interest because the percentages of different
carbon isotopes found in living tissue relates directly to what the organism eats. If a
bacteria eats food depleted in carbon 13, then the bacteria will be depleted in carbon 13.
Methane is very depleted in carbon 13 so a carbon signature low in carbon 13 would
indicate the bacteria ate methane.

“The carbon isotope profile that we obtained with the ion probe shows that the sulfate
eating bacteria near the outside of the clump are slightly depleted in carbon 13, but that
the archeobacteria in the center of the clump are very depleted in carbon 13. “The
isotopic signature seen in the consortia of microorganisms indicates that the
archaeobacteria in the center are consuming methane,” says House.

To confirm their findings, Kai-Uwe Hinrichs, assistant scientist at Woods Hole
Oceanographic Institute, tested the lipids in the bacteria and obtained a similar carbon
isotope signature. The researchers also tested other microorganisms from the same
environment to determine their carbon isotope signitures and found that they were all in
the normal carbon 13 range.

Marine methane is produced by archaeobacteria in the abscence of oxygen, usually in
the marine sediments. About 80 percent of this methane is consumed in the ocean and
never enters the atmosphere.

“If all the methane entered the atmosphere, the Earth would be much hotter because
methane is a greenhouse gas,” says House. “We knew that did not happen, but we did
not know who was consuming the methane. Now we do.”

The Penn State researcher notes that this is the first time that phylogenetic staining and
isotope analysis have been coupled to show that a specific organism has a specific
carbon signature.

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EDITORS: Dr. House is at 814-865-8802 or at chouse@geosc.psu.edu by e-mail or
between July 23 and 26 at 310-825-4328.

Contact: A’ndrea Elyse Messer

aem1@psu.edu

814-865-9481

Penn State

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