Extreme Home for Simple Organisms

Liz is a staff writer for Science.

This bug is extreme, even for a class of organisms known as "extremophiles." While surveying the depths of an abandoned copper mine, a team of scientists has detected a microbe that survives in some of the most acidic waters on Earth, at a seemingly impossible pH near 0. That makes this critter, a member of the microbial kingdom Archaea, one of a few record-setting bugs that can survive in conditions usually toxic to life as we know it.

Not only does Ferroplasma acidarmanus survive, but it positively thrives. Indeed, it accounts for the overwhelming majority of life-forms found deep inside the inhospitable Iron Mountain mine near Redding, California, report Katrina Edwards of the Woods Hole Oceanographic Institution in Massachusetts and her colleagues in the 10 March issue of Science.

During the past 2 years, the team periodically sampled the mine's waters, isolating and growing the single species that is predominant to check out its lifestyle. They found that F. acidarmanus grows best at a pH of 1.2 but can survive from pH 0 to 2.5. Most other organisms recovered from acid mine drainage will prosper at a pH of 2.5 and make do anywhere from a pH of 1 to 4. Other researchers have also come across traces of microbial life in highly acidic settings, such as reactors used to leach precious metals from low-quality ore. But Edwards and her co-workers are the first to collect and quantify these Archaea from a more natural environment. They have since found the same or closely related species at other sites throughout the mine.

The bug's hardiness is even more surprising considering its architecture. Most microorganisms have cell walls for protection, but F. acidarmanus is encased in what appears to be just a simple cell membrane--a seemingly flimsy way to guard against sulfuric acid and the high amounts of copper, arsenic, cadmium, and zinc also present in the drainage. Yet their cell membrane "seems totally capable of maintaining them in environments that would destroy most other organisms," marvels William Ghiorse, a geomicrobiologist at Cornell University. Equally puzzling, the high acidity appears essential for keeping the membrane intact. Researchers are trying to figure out how it works.

The discovery of this archaeal species suggests that yet more bizarre microbes may exist out there, perhaps bugs that survive at negative pHs. "If you keep looking, you will find them," predicts Princeton University geochemist Tullis Onstott.

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