The growth potential of the field is unquestionably high, says Ralph Portier, associate professor of environmental toxicology at Louisiana State University in Baton Rouge. "As we continue to look at Superfund [toxic waste cleanup] sites, and as corporations continue to dismantle old facilities and rebuild on the same sites, we'll keep finding new roles and applications for bioremediation that we hadn't anticipated," says Portier, one of the many investigators participating in this research area.
The idea of using various strains of bacteria, fungi, and other microbes to break down waste products, while now gaining in popularity, is by no means new. For centuries, farmers and gardeners have relied on composting--perhaps the oldest form of biodegradation--to decompose solid waste products from either plants or animals. In many cases (as with composting), the waste material is not only degraded, but also reclaimed; that is, some useful resource (for example, fertilizer or methane gas) is recovered from the waste.
`Munching' Microbes There are other methods of biodegradation, as well: in situ biodegradation, slurry-phase treatment, and solid-phase treatment. In situ biodegradation involves the introduction of microorganisms directly into the contaminated soils or water table. The other two techniques require the treatment of contaminated soils or sludges in a device known as a bioreactor. All of these methods depend on the ability of particular microorganisms to synthesize the enzymes needed to break down complex toxic compounds into intermediate compounds or individual elements. The waste material can be bioconverted in two ways: it can be broken down into inorganic compounds, such as carbon dioxide, methane, or water (this is known as mineralization), or the waste can be converted into a less toxic variety (this is known as detoxification).
Not all organisms can degrade all types of toxins and solid wastes, however. And just how efficiently a particular microbe will break down a particular substance depends on a number of environmental conditions, such as the available nutrients (like carbon, nitrogen, and trace elements), temperature, pH, amount of oxygen present (aerobic versus anaerobic conditions), moisture content, and type of substrate (for example, soil type).
Determining which organisms will "eat" which toxins and under what conditions is where the microbiologist comes in. Putting together a bioremediation method that is best suited to clean up a particular site, however, requires microbiologists to work in concert with chemists, civil engineers, hydrologists, and soil scientists.
Portier's group at LSU exemplifies this diversity. "We have chemists, engineers, and micro- biologists--and legal people," he says. "That's the only way you can make it [bioremediation] work."
Portier likens his role in bioremediation to that of a computer programmer. "If you think of the microbes as the software," he says, "then the engineering support--the bioreactor, the pumps, and so on--are the hardware." For example, says Portier, who is also the director of the Aquatic and Industrial Toxicology Laboratories at LSU, a microbiologist and a chemist might review a site and conclude that in order to clean it up, a bioreactor of a given capacity would have to be constructed. It's then the engineer's job, he says, to determine the feasibility of building such a facility on, say, Louisiana swampland. "If it's too big, it'll sink right down," he says.
Portier's role as a microbiologist has been to determine which organisms are best suited for various remediation jobs. Since 1982, he has successfully identified more than 400 different microorganisms and the waste products they'll degrade. He has managed to do this by creating microcosms, or tiny environments, for the naturally occurring microbes. To each of these environments, he adds a specific pollutant. Then, one by one, he removes different components of the environment until all that remains is the toxin and the microorganisms. The microbes that can live off of the pollutant will thrive, whereas those that can't will die. Some of the microbes Portier has identified through this method have been used to degrade pollutants such as chlorinated phenols, pesticides, hydrocarbons, and heavy metals found in contaminated soils, industry waste water, and groundwater.
The flurry of research in bioremediation during the latter half of the 1980s is basically a sign of the times. Says Portier, "Bioremediation as a technology has been implemented in the U.S. on a regional basis. Most of the initial work had been done in Louisiana and Tennessee. In the early 1980s it slowly worked its way to the East Coast." The field didn't really take off, he says, until it was realized that the methods used in bioremediation did not violate any environmental restrictions.
Another reason for the increased interest in the use of bioremediation, says Portier, is the simple fact that the government, industry, and various grass-roots organizations have finally decided that it's time to clean up the environment. "Bioremediation took so long [to come of age] because of the decision-making process," he says. Once the decision to reclaim a site is made, says Portier, "then it becomes an engineering and economic consideration. The engineers evaluate the existing technologies, and part of that evaluation includes a price tag."
Bioremediation, according to Portier, can cost considerably less than the other existing technologies, such as incineration and burial--and that, he says, has been an important factor in its implementation.
Bolstering Mother Nature Success is another enriching element in the bioremediation boom. "We can't fail in bioremediation," says Portier. "The whole process begins in the lab, and then you do a careful scale-up. If it fails in the lab, then it won't work in the field." But the method is not a panacea, he says. "Bioremediation as a technique," Portier cautions, "is not a total alternative answer to all sites. It does have a role in all sites, though." What that role is and which microorganisms will play the part is fodder for current and future investigations.
One area of research that has received attention in the last few years is that of genetically engineered organisms. Microbiologists have turned to the techniques developed in genetic research in their efforts to produce "super" microorganisms--microbes that can be tailor-made to most efficiently degrade particular toxins. Portier emphasizes, however, that the microorganisms he and his colleagues work with are naturally occurring creatures. "The work in my lab has been done without genetically engineering anything," he says. In essence, his research is simply a way of speeding up Mother Nature. Adds Portier, "The jury is still out as to whether or not genetically engineered organisms are functional in the real world."
The fact that no decision has been reached with respect to the viability of genetic organisms hasn't deterred the scientists who are currently working on these organisms. One group of researchers engaged in these studies is headquartered at the Center for Environmental Biotechnology (CEB) at the University of Tennessee, Knoxville. Founded in 1986, CEB supports more than 50 faculty members, graduate students, and postdoctoral fellows from numerous fields, including microbiology, organic chemistry, environmental studies, ecology, molecular biology, civil engineering, hydrogeology, and plant and soil science.
"Using classic microbiological techniques," says Alec Breen, a graduate student at CEB, "you can enrich an organism. With genetic engineering, you're accelerating the evolutionary process." Regulatory hurdles aside, Breen sees a place for these man-made organisms in the future of bioremediation. "For in situ work," he says, "we'll probably continue to use the organisms that are already out there. But there will be opportunities for genetically engineered organisms in bioreactors," where the environment is carefully controlled.
A controlled environment is what it may take in order for genetically engineered organisms to survive long enough to do the job they were made for. "The initial results [with genetically engineered organisms] have been disappointing," says Breen.
"Although the techniques are out there, we find that the engineered organisms don't compete well against naturally occurring organisms. It's also difficult to determine if the degradation you see is due to the organism you added."
Biosensors--specially designed microbes that can be used to detect certain toxins--are genetically engineered organisms that may be of use in the future. A team of researchers at CEB, led by Gary S. Sayler, a professor of microbiology and ecology and the director of the center, recently reported on an engineered organism that may serve as a biosensor for the contaminant naphthalene (J.M.H. King, et al., "Rapid, sensitive bioluminescent reporter technology for naphthalene exposure and biodegradation," Science, 249:778-81, 1990).
Sayler and his team inserted the bacterial luciferase genes (lux), which cause bioluminescence (such as in a firefly's tail) into the gene that produces the enzyme needed by the bacteria to break down naphthalene. This way, whenever the bacteria encounter the toxin and begin to degrade it, the bacteria give off light. The researchers found that the amount of light emitted seems to be proportional to the rate at which the microbes degrade the toxin. Biosensors, says Breen, are a "nondestructive, easy way to know when the catabolic genes [those needed for degradation] have been turned on. These biosensors might have some application in field studies, but certainly in bioreactors. It's still a genetically engineered organism, so there still would be regulatory problems."
Breen adds, however, that a organism engineered to glow may not be as discomforting to the public as is an organism designed to eat petroleum. "Some people are afraid that these organisms will get into their carburetor and eat their oil," he says. Less public opposition could help speed along the use of genetically engineered organisms in the field.
Even with regulatory restrictions and initial disappointments, environmental biotechnology as a field seems to be here to stay. A glance through current issues of journals such as Microbial Ecology, Environmental Technology, Enzyme and Microbial Technology, and Applied Microbiology and Biotechnology reveals a selection of articles from researchers in countries as diverse as the United States, the Netherlands, Japan, Turkey, and England. Topics include the viability of a genetically engineered microorganism in a disturbed microcosm, anaerobic biodegradability of paper mill waste water, effects of composting on the concentration of heavy metals in sewage sludge, improved bioreactor designs, and the use of dried green alga to absorb heavy metals (such as copper, zinc, lead, chromium, and uranium) from mining operations waste waters.
LSU's Portier and his associates are currently working on innovative bioreactor designs, in particular, a bioreactor that can handle volatile compounds. "We're also working on a bioreactor for use in zero gravity," says Portier. "That way we can recycle carbon in an atmosphere not of this planet. We're excited about this project."
Adds Breen, "I think--I'm hoping--that people like us will have a lot of work to do in the future."