Crystal growers seek bacterial know-how....

Crystal growers seek bacterial know-how . . .

Three species of sulfur-absorbing bacteria discovered last year hold a magnetic attraction for Brigid R. Heywood, a crystal engineer at the University of Bath, England. Inside these creatures, minuscule chains of crystal particles called magnetosomes serve as compasses that point the way to relatively safe and nutrient-rich habitats.

Such biological innovation would fascinate virtually any nature lover. But for Heywood and her co-workers, the microbes' crystal-making powers prove even more enchanting. For one thing, the newly identified bacteria--which live in salty, sulfur-ridden waters and sediments -- make their magnetosomes out of iron sulfide, a magnetic mineral known as greigite ([Fe.sub.3.S.sub.4]). Previously, only magnetite ([Fe.sub.3.O.sub.4]), or iron oxide, had been seen in magnetosome-making microbes.

Further surprises have emerged from studies of the greigite crystals under high-resolution microscopes. All of these crystals fall within a remarkably small range of sizes -- 50 to 90 nanometers -- and each bacterial species grows its crystals in a specific shape, report Heywood and her colleagues in Bath and at California Polytechnic State University in San Luis Obispo. For example, the smallest rod-shaped species produces crisp-edged, cubo-octohedral crystals, which feature both hexagonal and square faces. A larger rod-shaped species has rectangular crystals with flat ends, truncated corners and irregular sides. The third species, which congregates in mulberry-like colonies, uses a puzzling combination of two mineral types -- greigite and nonmagnetic pyrite ([FeS.sub.2]).

All three species somehow control the precipitation of crystals at the molecular level, Heywood says. Moreover, they do this at ambient temperatures and under normal biological conditions. Scientists, in contrast, must use rugged chemical conditions and searing temperatures to make greigite crystals. Heywood hopes to uncover the cellular crystal-making processes of these organisms, and then mimic those processes in the lab. In the long run, she suggests, such work might help materials scientists to literally grow minerals and ceramics into lenses, engine blocks or components with specific magnetic, electrical or optical properties.