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The surface of Yellowstone National Park's Norris Geyser Basin wouldn't seem a terribly appealing place to live. Poisonous hydrogen sulfide gas rises from the always-hot ground. Dissolved in water, it undergoes reactions to become sulfuric acid. The liquid that percolates through the porous rock has a pH of 1.0 -- acidic enough to dissolve nails.

But it turns out the rocky surface of the basin, which stretches for miles like pavement, is full of life, Norman R. Pace, a biochemist at the University of Colorado in Boulder, says in the journal Nature.

Embedded about a quarter-inch below the surface is a world occupied by about 40 different species of bacteria and algae. "It is a very brilliant green layer, emerald-like and quite remarkable," Pace said by phone.

The overlying rock protects the organisms from drying out and from being damaged by the sun's ultraviolet rays. But enough light gets through to allow photosynthesis.

Genetic analysis of the microbes reveals that about 40 percent of them are Mycobacterium genus -- the category whose many members include the microbes that cause tuberculosis and leprosy -- while about 25 percent are algae of the Cyanidium genus. Pace speculates that the mycobacteria, which form long filaments, may provide structure to the community and, in turn, is paid in energy captured by the photosynthetic algae.

One of the unusual features of this world is that minerals crystallize around many of the filaments, creating permanent casts of their structure. The result are microfossils.

The surface of Mars is much like that of Yellowstone -- harsh, volcanic, once percolated by water. Seeking out the remains of "endolithic" (or inside-the-rock) communities might be "the best hope for finding evidence of past life on the Martian surface," Pace wrote.

-- Washington Post