The discovery could offer clues in the search for life on Mars and beyond, researchers said in October at a meeting of the Geological Society of America.
"We're finding that you need to look at things you might write off as not being biological—they might be biological," said Penelope Boston, a cave scientist at the New Mexico Institute of Mining and Technology in Socorro.
The microbes were found on the walls of lava tubes in Hawaii, New Mexico, and the Portuguese Azores islands, a volcanic archipelago in the Atlantic Ocean (see map).
The finds include "a lovely blue-green ooze dripping out of the [cave] ceiling in Hawaii; a vein of what looks like a gold, crunchy mineral in New Mexico; and, in the Azores, amazing pink hexagons," said Diana Northup, a geomicrobiologist at the University of New Mexico.
"That's the waste—the bug poop, if you will."
Clues to Life on Mars?
Lava tubes form when molten lava seeps out beneath a solidifying flow from an active volcano, leaving long caves in its wake.
Since 1994 Northup and colleagues have been seeking out unusual deposits in caves, including lava tubes, and putting them under a microscope or testing them for DNA.
Her team's discoveries add to a growing body of evidence that lava tubes on other planets might be the best places to look for signs of extraterrestrial life, said Saugata Datta, a geochemist from Kansas State University who was not involved in the work.
(Related: "Could Jupiter Moon Harbor Fish-Size Life?")
In 2007, pictures from a Mars orbiter showed dark holes that appear to be places where lava-tube roofs have collapsed.
"Caves [are] a unique environment where we think that [minerals precipitating out of liquids] and microbial growth are enhanced by stable physical and chemical conditions," Datta said.
On Mars, water could have percolated into subterranean caves long ago, possibly bringing with it a banquet of minerals that could have fed ancient microbes.
Also, the insides of such caves would have remained sheltered from harsh surface conditions, giving any possible Martian fossils a better shot at long-term survival.
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