A frigid, sulfate-rich pool of anoxic, hypersaline water that hasn’t seen the sun since it was capped by a 400-meter-thick Antarctic glacier at least 1.5 million years ago is host to microbes that derive energy by breathing iron scraped from bedrock.

The briny samples include no dissolved oxygen and little organic matter, but plenty of dissolved iron — an element that quickly oxidizes to form blood-red rust when the seepage hits the air. The microbes in the seepage are similar to some species found in modern-day marine environments, the researchers note. And the microbes gain their energy by metabolically transferring electrons from the water’s dissolved sulfates to its iron, which is liberated from the bedrock by the scraping of the glacier, says Mikucki.

“This is a great example of electron shuttling sustaining an ecosystem,” says Dianne K. Newman, a geobiologist at MIT.