UC TSR&TP

UC Toxics News Fall 1998



Bacteria to the Rescue: Marine Microbes Take on Heavy Metals by Joe Hlebica Reprinted with permission by Scripps Instition of Oceanography, UCSD, Explorations Magazine
		Bacteria tend to get a bad rap. In the popular view, they are usually associated with infections, bad breath, and dirty drains. In the laboratory of Scripps microbiologist Brad Tebo, however, they may be considered heroes. "We’ve begun to think that some bacteria might be useful as a way to clean up metal pollution. In the last five years, we’ve really been studying this, and it looks as if bacteria can be used for removing toxic metals from the environment, and even recovering those that are valuable," said Tebo. Processes carried out by some bacteria lead to the precipitation of metals from solution into a solid form. By isolating and then removing these bacteria from a variety of environments and investigating them in the laboratory, Tebo and his associates are studying how and why these reactions take place.
	Marine microbiologist & TSR&TP Researcher Brad Tebo. Photo courtesy of Explorations Magazine.
The key appears to be the complementary chemical processes of oxidation and reduction. Oxidation is a chemical reaction in which a compound loses electrons, thereby increasing its ability to bond with other elements. Tebo thinks that specific bacteria may be able to acquire energy by causing these types of changes in the forms of various metals. Although bacteria may more typically acquire energy by oxidizing organic matter to form carbon dioxide—the process of decay—some may be able to use metals as inorganic energy sources. The economically important metal manganese, which is relatively abundant in the marine environment, is susceptible to bacterial change. The bacteria that oxidize manganese are a focus of Tebo’s research. "You can’t destroy pure metals; you can only change their forms," Tebo pointed out. "And it’s the soluble forms that are usually toxic. Once precipitated, they’re usually nontoxic." While iron and manganese are not particularly toxic, during the process of their oxidation by bacteria, incidental toxic metals also can be removed from solution. For this reason, manganese-oxidizing bacteria can be exploited to enhance bioremediation—the use of organisms to detoxify and clean up pollution. Tebo elaborated, "Metal bioremediation might mean changing the metal to another form that’s not toxic or not bioavailable. For organic pollutants, the ultimate desirable phase is carbon dioxide; for metals, it’s some form that can be precipitated or recovered from the environment." In the Tebo lab, graduate student Chris Francis is studying the genes and enzymes involved in bacterial manganese oxidation. "In our model bacterium, we have a candidate manganese-oxidizing protein; we also have the gene sequence for this protein. We’ve made antibodies to the protein, and now we’re doing comparative studies with other types of organisms." Using this approach, Francis recently identified an active, manganese-oxidizing protein from a bacterium isolated from sediments collected at Point Loma in San Diego, California. "We’re very excited about having a bacterial protein with which we can do more biochemistry. Having the genes allows us to do genetic manipulations," commented Tebo. During these manipulations, engineered genes can be reintroduced into the same bacterium in an effort to enhance manganese oxidation. In addition, the metal affinity or the specificity of the genes potentially can be altered so other metals also may be oxidized. Tebo explained that even the rates at which the proteins work and the quantities of metals oxidized can be engineered. "It’s opened up a whole new avenue of research that we didn’t think would be possible five years ago," he said. In addition to the bacterial oxidation of metals, Tebo’s group utilizes metal reduction by bacterial processes for bioremediation. Bacteria acquire a metal that is soluble in its oxidized form and reduce it, making it less soluble and therefore less of a threat to the environment. One example is the oxidized form of chromium (chromium VI), a very toxic and highly soluble by-product of industrial electroplating processes. The reduced form of chromium (chromium III) is less toxic and largely insoluble. Tebo states that microorganisms can reduce chromium VI to chromium III and thereby detoxify it. The same is true of uranium VI, used in the nuclear industry. It too is very soluble, but can be reduced by bacteria to uranium IV, which readily precipitates. According to Tebo, there are two aspects to bioremediation of metals in the environment. One is biotechnology, in which bacteria transform metals from a soluble form to a precipitated form, selectively removing targeted toxic metals while leaving nontoxic metals in solution. The other is separation and recovery of the precipitated metals. The ultimate goal is not just the removal of toxic metals from a particular environment, but their separation and recovery in more or less pure quantities. "Obviously, if the metal can be recovered then you have the potential to recycle it, and that benefits us all the way around," observed Tebo. Researchers are exploring various technologies for recovering metal precipitates. One common recovery method uses the same type of filtration technology as in waste-water treatment. Another relies upon the magnetic properties of the targeted metals to selectively remove them from the environment. In the marine environment, many metals occur in anaerobic (oxygen-free) environments such as the sediment layers of bays and estuaries. These coastal areas often are the sites of industrial development, a common source of toxic substances. In studying bacterial reactions that lead to the formation of nonsoluble metals, the Tebo group is examining the processes of metal precipitation by sulfate-reducing bacteria. Unlike manganese-oxidizing bacteria, sulfate-reducing bacteria are anaerobic. Thriving in oxygen-free surroundings, sulfate-reducing bacteria enhance the reduction and precipitation, and therefore detoxification, of metals. Hydrogen sulfide, the source of that rotten-egg smell often encountered in tidelands, is a product of sulfate reduction. Hydrogen sulfide is highly reactive with metals, forming insoluble compounds like the iron sulfide that blackens submerged surfaces in bays and estuaries. Tebo pointed out, "If you can form a metal sulfide from the hydrogen sulfide, that effectively takes the metal out of solution. Iron sulfide tends to be one of the more soluble metal sulfides. Most of the others, such as copper sulfide and zinc sulfide, are even less soluble. Those metals can be removed very effectively through sulfate reduction." The group’s investigations have shown that some bacteria may actually increase their metabolic rates as a result of these anaerobic processes, using certain metals just as humans use oxygen in respiration. According to Tebo, this raises an important question: Do such natural processes contribute significantly to the detoxification of metals in the environment? How and to what degree bacteria might naturally detoxify metals is now a focus of the group’s laboratory research. By observing in the laboratory the extent to which the organisms can carry out metabolic processes, the researchers are determining what to look for in the environment. "We want an accurate picture. In the lab, you cannot be absolutely sure of the situation in nature, but you can achieve results that are a very reliable estimate," assured postdoctoral researcher Anna Obraztsova. Tebo elaborated, "We want to understand how the microorganisms might naturally detoxify the metals. Although these organisms occur naturally, they wouldn’t be naturally exposed to toxic metals from pollution." He described a possible scenario. When there is a toxic metal input as a result of pollution, it impacts bacterial populations. Only those bacteria that can resist toxic metals survive in this environment and fulfill their roles as decomposers. If they not only survive, but actually use the toxic metals to generate energy—as laboratory tests have suggested—the benefits to the bacteria may be considerable. If this is the case, then not only do the bacteria benefit from the energy they acquire by detoxifying metals, but their habitat benefits because the toxic metals are reduced.