Microbial Masons

ATLANTA--Bacteria may play an important role in helping to harden some desert rock formations, according to research presented here today at the American Society of Microbiology's annual meeting. The findings suggest that microbes may be even more versatile landscapers than researchers had previously realized, and further study of these processes may even aid the search for evidence of life on Mars and other planets.

Bacterial colonies are known to help cement coastal dunes and desert soil, but up to now their role in building rock formations has been unknown. While on a field trip to southeastern Utah, microbiologist Harry Kurtz Jr. of Sam Houston State University in Huntsville, Texas, was intrigued by striped stains on eolian--or wind-formed--sandstone. To him, they looked like bacterial biofilms--well-organized colonies that coat structures ranging from pipelines to ear canals.

Kurtz and his geologist colleague Dennis Netoff collected samples from the rock and cultured cyanobacteria, common organisms whose colonies are blue-green. The cyanobacteria, which live less than 1 millimeter below the rock surface, tend to turn sand into rock in two ways, says Kurtz: They grow filaments that attach to the sand grains, and they produce acidic polysaccharides--sticky starches with a consistency resembling pasty glue. The polysaccharides trap metals and other nutrients from water that seeps through the rock.

To check out the bacteria's rock-building ability, Kurtz and Netoff inoculated samples of sterilized sand with the cyanobacterial cultures. Then they mimicked desert conditions by feeding the cultured samples a salt solution, illuminating them with a 100-watt lamp, and keeping them at a constant temperature of 33 degrees Celsius. After 4 months, they tested the samples for stiffness using a compression gauge. The bacteria-encrusted samples withstood a force of about 50 grams per square millimeter--the equivalent of placing a brick on your foot. Plain sterilized sand has zero stiffness.

"This helps us begin to understand how microbiological processes have helped to shape the landscape," Kurtz says. The microbes protect otherwise unconsolidated sand from the effects of wind and rain. The research could become useful in the search for microbes on Mars by suggesting rock formations that may have been assembled by bacteria, says Blythe Hoyle, a geologist with Iowa State University in Ames.