Bacteria that live in rocks a kilometer or more underground have to eat creatively, since organic molecules are thought to be scarce. Now researchers have found that a possible deep dietary staple may not exist in abundance after all. The findings, reported in tomorrow's Science, suggest that there may be enough organic molecules deep below Earth's surface to support life after all.
In 1995, Todd Stevens and James McKinley, now both at Pacific Northwest National Laboratory, showed that mixing basalt with water gives off a steady supply of hydrogen gas, which certain bacteria could mix with carbon dioxide to make energy. "This is something that's in all the microbiology textbooks," says Derek Lovley, a microbiologist at the University of Massachusetts. The finding also explained how life might survive on other planets such as Mars, where organic molecules are scarce.
But Lovley and his colleagues were suspicious, mainly because the water in the previous experiments was more acidic than normal groundwater. "You never find acidic water with basalt," Lovley says. His team repeated the experiment, once using acidic (pH 6) water, and again with slightly basic groundwater (pH 8) pumped from beneath the Snake River in Washington, part of the Columbia River watershed. The acidic water liberated hydrogen from the basalt, but the groundwater did not.
The new finding suggests that the deep bacteria may instead be feeding on organic molecules. Although Lovley says that hydrogen could still be a fuel source, "I think it's necessary now to show another mechanism for hydrogen gas production," he says. Stevens declined to comment on the study, but mentioned that unpublished data from his lab shows that groundwater can generate almost as much hydrogen from rock as can acidic water. Says Texas A&M University geochemist Ethan Grossman: "I don't think we should stop looking for this type of ecosystem."