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A biological orientation.

Almost 800 years ago, Arabian sailors used a fish-shaped iron leaf suspended in water to guide their journeys northward on cloudy nights. But nature first harnessed Earth's magnetic field for navigation long before that. For millions of years, "magnetotactic" bacteria have made their own internal compasses.

Richard P. Blakemore, a microbiologist at the University of New Hampshire in Durham, discovered these organisms in mud samples almost 20 years ago, when he noticed that they tend to gather at the north end of water droplets. Since then, researchers have sought to understand how the bacteria make and use their microscopic compass needles. Recent interest in mimicking nature's handiwork has made some researchers wonder about recruiting these microbes to make magnetic particles for commercial applications.

"The bacteria are able to control the [particle's] size, shape and placement in the cells. This is submicron technology. We would like to know how they do it," says Richard B. Frankel, a biophysicist at California Polytechnic State University in San Luis Obispo.

Many kinds of bacteria make magnetic particles. Some make an iron sulfide crystal, such as agreigite; others make magnetite, an iron oxide. Scientists are evaluating the phylogenetic relationship betwen these two types to understand better how -- and how often -- magnetic biomineralization evolved.

He thinks the magnetic makers collect iron and convert it to a stable iron oxide to create the magnetic crystal. But at least three species that have poor access to oxygen combine iron with sulfur instead, Frankel reported at the March 1991 meeting of the American Physical Society in Indianapolis. Iron accounts for 2 percent of the organism's total weight, making these bacteria "the most prodigious iron accumulators in the world," he says.

But each kind of bacterium customizes its particle, says Frankel. Some adopt cubic-octahedral arrangements, while others build six-sided or rectangular prisms. "This tells us that the mineralization part in these bacteria is very highly controlled," says Dennis A. Bazylinski at Virginia Polytechnic Institute and State University in Blacksburg.

"It's the equivalent of our producing bones and teeth. For a logn time, people thought only higher organisms could do this [biomineralization], says Frankel. But the closer researchers look, the more the microbial process seems to resemble that occurring in vertebrates. A membrane encases each particle, indicating that an organic component plays a key role in forming the inorganic component. That membrane probably contains a protein that lures iron compounds out of solution and concentrates them inside the membrane.

"What the membrane seems to do is not only regulate the deposition of the particle, but also control its position relative to the cell's other particles," says Frankel.

A microbe makes about 20 particles, each some 50 nanometers long--just big enough to have the internal polarization needed to orient in a magnetic field, but not so big that the particle has multiple regions of polarization, which might weaken the magnetic response. Then the particles line up north to south, and this chain polarizes the cell. "It creates a hierarchical structure," says Frankel. While each particle will orient itself, the series of 15 or 20 makes the magnetic orientation 20 times a strong. Thus, the Earth orients the cell along a north-south axis as the organisms swims.

Although a shovelful of mud can yield thousands of kinds of magnetobacteria, it took 15 years for microbiologists to grow one of these microbes in the lab, says Bazylinski, a microbiologist. Most bacteria either thrive in air or require an absence of oxygen. But most magnetic types are more finicky: Too much or too little oxygen can kill them. They won't grown in a petri dish in air.

Thus, in nature, most of these fussy bacteria thrive where the water forms stable layers and where each layer contains a certain oxygen content. In fact, they may use magnetism to help them find the perfect layer. As the microbes propel themselves along with their whip-like flagella, Earth's magnetic field turns them toward the poles. Southern-hemisphere bacteria head south; their northern counterparts head north. The closer they are to the poles, the more steeply Earth's magnetic field orients them downward as well as poleward, Bazylinski explains. That downward tilt pulls them away from oxygen-rich surface water. Bazylinski and others suspect that when the bacteria sense favorable oxygen levels, they stop swimming.

Any bacterium with a genetic mutation that causes it to head the wrong way -- such as south when it lives north of the equator -- winds up in a hostile environment and dies. Experiments in which researchers placed bacteria in containers where the magnetic field was artificially reversed seem to bear out this idea. Within six weeks, the descendants switch the direction in which they swim, says Bazylinski. Thus, natural selection seems to segregate northseekers in one hemisphere and south-seekers in the other.

But orientation may be a side benefit of particle formation, Bazylinski suggests. He believes some bacteria may oxidize the iron to get energy. In addition, iron sulfide particles may be important in the cycling of sulfur through the environment, he says.

Growing these bacteria in the lab has proved quite a challenge. To create the right environment, Bazylinski puts hydrogen sulfide in the bottom of a test tube, then seals it and lets the sulfide gas and the air spread out. Inside, an oxygen-sensitive strip truns pink, letting him know the location of the air-gas boundary. That's where he starts his colony of magnetotactic bacteria. "I've got six strains now," he says.

Most of the interest in these microbes stems from a fundamental amazement that such simple organisms can accomplish something as complex as magnetic orientation. So even though some scientists envision creating microbial factories to produce biomaterials, neither Frankel nor Bazylinski thinks bacterially grown magnetic partiles will ever coat cassette or computer tapes. "You've got to grow hundreds of gallons of cells to make a little magnetite," Bazylinski says. "It's not worth it."
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Title Annotation:magnetotactic bacteria
Author:Pennisi, Elizabeth
Publication:Science News
Date:May 16, 1992
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