Since the 1950s, a government storage site in Idaho has been contaminated with radionuclides, heavy metals, organic solvents, and other hazardous wastes. Deep below the surface, microbes with no handy source of nourishment live happily in crevices in the basalt rock, dormant until water trickles down after rain or snow, bringing a burst of nutrients.

One of the things that trickles down is hexavalent chromium, a metal ion stripped of six electrons: carcinogenic, mutagenic, and highly toxic, it forms soluble compounds that readily cross cell membranes. But trivalent chromium, lacking three electrons, is much less toxic because its insoluble compounds can't easily get inside cells. Somewhere in the basalt, the "bad" hexavalent form was somehow being reduced to the "good" trivalent form.

Hoi-Ying Holman and her colleagues found 85 strains of microbes in the basalt, many of which could reduce hexavalent chromium, doing it even faster in the presence of a hydrocarbon pollutant called toluene, common around leaky fuel tanks. The champion reducers were bacteria named Arthrobacter oxydans.

To prove that the microbes were responsible for the reduction, not chemical reactions with the rock, Holman and her colleagues turned to synchrotron infrared spectrometry at the Advanced Light Source (ALS). For over four months they watched colonies of A. oxydans in thin disks of basalt, microscopically observing chromium being reduced, step by step, exactly where the microorganisms lived — a process speeded up by the extra nutrients in toluene.

The discovery promises to help in designing new and better bioremediation techniques for mixed waste sites.

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