The smallest form of life known to science just got smaller.

Four million of the newly discovered microbe -- assuming the discovery, reported Friday in the journal Science, is confirmed -- could fit into the period at the end of this sentence.

Scientists found the microbes living in a remarkably inhospitable environment, drainage water as caustic as battery acid from a mine in Northern California. The microbes, members of an ancient family of organisms known as archaea, formed a pink scum on green pools of hot mine water laden with toxic metals, including arsenic.

"It was amazing," said Jillian Banfield of UC Berkeley, a member of the discovery team. "These were totally new." In their paper, the scientists call the microbes "smaller than any other known cellular life form."

Scientists say the discovery could bear on estimates of the pervasiveness of exotic microbial life, which some experts suspect forms a hidden biosphere extending miles underground whose total mass may exceed that of all surface life.

It may also influence the search for microscopic life forms elsewhere in the solar system, a discovery that would prove that life in the universe is not unique to Earth but an inherent property of matter.

The tiny microbes came from an abandoned mine at Iron Mountain in Shasta County that produced gold, silver, iron and copper, closing in 1963.

Today, rain and surface water run over exposed minerals, producing sulfuric acid. The mine is one of the largest Superfund cleanup sites.

Starting in 2002, the scientists obtained drops of the acidic slime and searched for genetic signs of novel microbes. "We were essentially looking for new stuff," Brett Baker said in a statement, "and we found it."

The microbes are about 200 nanometers wide -- the size of large viruses, which scientists consider lifeless because they cannot reproduce on their own. Bacteria average about five times that size.

The scientists must do further tests to confirm that the organisms are the smallest ever found, and that they can reproduce. If those analyses hold up, they said in their Science paper, "it may be necessary to reconsider existing paradigms for the minimum requirements for life."