By Meredith McGroarty

Some of the most innovative tools in environmental remediation have, in fact, been around for eons, quietly cleaning up our messes more efficiently than we ever could.

Microbes—single-cell organisms found throughout the ecosystem—have been degrading naturally occurring gases and compounds that are toxic to humans and other animals for millions of years, but scientists are now hoping these microorganisms will help curb the ever-growing problem of man-made pollution.

“Over the last ten or twenty years, it became very apparent that nature produces a large amount of chlorinated chemicals, and halogenated compounds in general,” explains Frank Loeffler, Governor’s Chair for microbiology and civil and environmental engineering at UT Knoxville. “Microbes have been degrading these materials for billions of years; we just hadn’t noticed. Now, we’re challenging microbes with much higher concentrations of these chemicals—and maybe with compounds they’ve never seen before—and we’re taking advantage of them for bioremediation.”

Loeffler’s research focuses on the role of microbes in the degradation of toxic materials, such as chlorinated solvents or heavy metals. Certain microbes are able to use organic toxic materials as a source of energy, much as humans use oxygen. In this metabolic process, the dangerous material is broken down and transformed into harmless compounds.

“In the absence of oxygen (that is, anoxic conditions generally prevailing in contaminated subsurface environments, including aquifers), some microorganisms use the chlorinated solvents as electron receptors for respiration,” Loeffler says. “These organisms don’t use oxygen; they use chlorinated solvents in place of oxygen and generate their energy. That’s a unique lifestyle that was only recently discovered.”

One of the major areas of Loeffler’s work pertains to the presence of chlorinated solvents in groundwater. For many decades, people disposed of these solvents, such as the dry cleaning fluid tetrachloroethene (PCE), by simply dumping them into the ground.

Depending on its volatility, some of the solvent might be released into the atmosphere and be destroyed in the atmosphere, while the rest might penetrate into the soil and migrate downward and contaminate groundwater. Once in the water, the solvent dissolves and a toxic plume forms, which can reach drinking water wells or is discharged into rivers and lakes.

Chlorinated solvents like PCE have a density higher than that of water migrate below the groundwater to pool in the subsurface environment and remain there as free-phase solvent. Exposure to humans can come through well water or vapors that penetrate into the home. The commonly encountered PCE has been linked to cancer and other health problems.

“This free-phase PCE can sit in the water for decades, for centuries. This problem will be with us for many more years unless we find more efficient ways of cleaning it up,” Loeffler says. “We’re working on developing technologies using microbes that will help clean up the plume and also accelerate the cleanup of the source where free-phase PCE is located.”

In addition to studying chlorinated solvents, Loeffler is also looking at an issue of special interest to East Tennessee residents: the presence of heavy metals in the environment. In December 2008, a ruptured wall at a Tennessee Valley Authority facility released 5.4 million cubic yards of coal ash—which contains several heavy metals—into the surrounding area. This research will capitalize on an important part of Loeffler’s role as a governor’s chair: collaboration with scientists at Oak Ridge National Laboratory, which performs a good deal of work involving heavy metals.

“Unlike organic compounds, metals can’t be mineralized, so they can’t be destroyed by microbes. What microbes can do is change the properties of these metals. For example, microbes can affect the solubility of toxic metals so they don’t move in water anymore. When the toxic metal stops moving, you can protect downstream recipients from being exposed,” Loeffler explains.

There are several types of bioremediation efforts that Loeffler addresses in his research. One strategy, called bioaugmentation, involves transferring microbes to polluted areas where they were previously absent.

“I have been involved in some efforts to take organisms and bring them back to a contaminated site. It worked very well,” Loeffler says. “These organisms saw the contaminant as food, and they helped detoxify it.”

Another area of study focuses on the microbes’ surroundings. If microorganisms exist in a polluted area but are performing poorly, it is possible the site has conditions detrimental to the desired microbial activity.

“If certain microbes are present and not active, we have to check out what the bottleneck is,” he says. “There are different reasons for the holdup; for example, there’s too much sulfate or too much oxygen in the area, or the soil pH is wrong. We could change the condition of the environment, like removing the oxygen, and remove the bottleneck.”

Regardless of the bioremediation strategy, it is clear that nature is far more adept than humans at producing microbes and other organisms to clean up pollution.

“For now, the idea of scientists creating microbes that will be able to quickly detoxify, for example, a large oil spill, is a fairy tale. I don’t think it’s feasible or practical to use synthetic biology and take different genes and things from different organisms, combine them, and expect that organism to perform better than microbes that have evolved naturally,” Loeffler says. “Nature is doing a pretty good job, but we have to understand Mother Nature better so that we can take full advantage of what she has to offer for bioremediation.”

Tags: Arts and Sciences • Bioremediation • Environment • Environmental Engineering • Frank Loeffler • Microbiology