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"
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
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.