"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.