The world's most alkaline lifeforms are living in contaminated water in the US.
The microbes under a phase-contrast microscope
Scientists found microbial communities thriving in the slag dumps of the Lake Calumet region of southeast Chicago where the water can reach a pH of 12.8.
Living in this extreme environment is comparable to swimming in caustic soda or floor stripper, the researchers say.
They found the microbes while studying contaminated groundwater created by more than a century of industrial iron slag tipping in Illinois and Indiana.
It was an unexpected discovery, said George Roadcap, hydrogeologist at the Illinois State Water Survey.
"It came as a surprise to find these extreme bacteria living in the ground water environment of an American city rather than in some exotic place like East Africa," he told BBC News Online.
Genetic analyses show some of the bacteria are related to Clostridium and Bacillus species.
These are found in highly alkaline waters of Mono Lake in California, tufa columns in Greenland, and cement-contaminated groundwater in a deep gold mine in Africa.
The microbes survive in alkaline pools
At some sites, the dominant microbes belonged to the Comamonadacea family of the beta-Proteobacteria.
"In high-pH microcosm experiments, one of these microbes is closely related to a hydrogen oxidiser," said Roadcap. This means the bacteria exploit the hydrogen given off from the corrosion of metallic iron slag in water.
Just how the unusual bacteria got to the slag dumps is a mystery.
One possibility is that local bacteria adapted to the extreme environment over the last century. Another possibility is that they somehow got imported.
Alkaline groundwater in the Lake Calumet region was created when steel slag was dumped and used to fill in wetlands and lakes.
Water and air reacts with the slag to create lime (calcium hydroxide), driving up the pH.
The Calumet River flows through an industrial area
An estimated 600 trillion litres (21 trillion cubic feet) of contaminated industrial fill was dumped in southeast Chicago and north-eastern Indiana, about half of which is thought to be slag, Roadcap said.
The slag dumps where the microbial communities were found resembled filled wetlands and were often devoid of surface vegetation, he explained.
Roadcap presented details of his team's find to the recent annual meeting of the Geological Society of America in Seattle.
Scientists are discovering more and more so-called extremophiles - microbes that can thrive in super-hostile environments.
These include locations with very high temperatures, acidity, radiation, and heavy metal contamination.
They have shown scientists just how robust life actually is and raise the possibility that life could exist on other planets and moons that look, on the surface, to be uninhabitable.
Images courtesy of George Roadcap.