Novel bacteria have a taste for aluminum

Although aluminum is the most abundant metal in Earth's crust, no living organism has been known to utilize it. In fact, this ubiquitous element is often considered toxic: Aluminum-contaminated soil can dramatically reduce yields of many crops, for example.

 Now, as a result of a chance encounter on a hike in Yellowstone National Park, a research group believes it has found a bacterium that requires aluminum to thrive. 

 Some scientists are less convinced, drawing caution from a history of unconfirmed claims of a biological role for the metal. "Skepticism is the best place to start," says microbiologist Simon D. Silver of the University of Illinois at Chicago. "Anyone with sense or experience, or both, will think this is exceedingly unlikely to lead to a biology of aluminum."

 The bacterium stirring this debate came to light when Judy E. Brown of the University of Maryland Biotechnology Institute in Baltimore was hiking near a geyser. She noticed bright flecks in water running over a greenish microbial mat and later identified them as the mineral aluminum silicate.

 Brown collected a sample from the mat and, working with her Maryland colleagues, eventually isolated a bacterium whose growth seems to depend upon having access to sufficient aluminum. Like many bacteria found in or near Yellowstone's geysers, this one is a thermophile, which lives and prospers at high temperatures. In fact, the microbe grows fastest at 70@ Celsius, Brown and her coworkers reported last week at a meeting of the American Society for Microbiology in Atlanta.

 Curiously, the bacterium seems to need more aluminum when grown at higher temperatures, notes Brown's colleague Frank T. Robb.

 The bacterium's apparent demand for aluminum has its limits, however. If exposed to high concentrations of the metal, the microorganism will not grow.

 The scientists have noticed dense inclusions inside the bacteria and are investigating whether they contain aluminum. Meanwhile, the role of the aluminum in the microbe remains mysterious. One possibility, says Robb, is that the metal is incorporated into enzymes used by the bacterium, a function performed by other metals, such as magnesium. If the bacterium instead relies on aluminum as a source of energy, it might someday be used to clean up soil contaminated by the metal, he further speculates.

 Silver doubts that the bacterium studied by Robb and his colleagues employs aluminum directly. He suggests that the metal may alter the biochemistry or environment of the microbes in a way that affects growth. Aluminum "could be freeing up something that is a nutrient," he says.

 Still, not all microbiologists are so willing to dismiss the idea of aluminum-dependent bacteria. "The finding would be, in principle, quite interesting for basic biology. It would bring to life, literally, a previously almost ignored element," says Carlos Cervantes of the University of Michoacana in Mexico.