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 atmosphere.
"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 today's (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 isoto
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