It may sound like a masochistic activity, but for certain microbes, eating volcanic glass is an entirely natural thing to do. In fact, according to a report in the current edition of Geochemistry, Geophysics, Geosystems, such snacking abounds below the seafloor. This, researchers say, suggests that a significant proportion of the oceanic crust alterations long attributed to a chemical-physical process may stem from a biological one.

Alterations in the oceanic crust impact ocean chemistry and other key chemical cycles on our planet. Because this crust contains considerable amounts of volcanic glass, understanding what changes the glass is important. Scientists have traditionally considered glass alteration the product of seawater corrosion. Earlier work had suggested that microbes might facilitate this dissolution, but the extent of their activity remained largely unknown. To address that, Hubert Staudigel of the Scripps Institution of Oceanography at the University of California at San Diego and his colleagues studied cores extracted from oceanic crust of various ages and settings.

"We've documented how [extensively] these microscopic organisms are eating into volcanic rock, leaving wormlike tracks that look like someone has drilled their way in," Staudigel reports. "Our study has confirmed that there's no place in the oceans that doesn't have these features." The findings indicate that 20 to 90 percent of the alteration evident in the upper 300 meters of crust may result from munching microbes. And even at depths of 500 meters, the microbes may account for up to 10 percent of the changes.

Staudigel notes that the microbes may bore into the volcanic glass (see image) not only to graze but to evade predators. Calling the glass-eaters the rock bottom of the food chain, he remarks, "We've basically determined the depth of the biosphere."