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.
|Contact: A'ndrea Elyse Messer|