flick / eutrophication&hypoxia

Microbes are nature's ultimate garbage disposal

, devouring the dead, decomposing and inert material that litters Earth's surface. They're so good at it, in fact, that humans have taken an increasing interest in coercing them to clean up our environmental messes.

The concept is called bioremediation, and it involves using organisms that either naturally love to eat contaminants or have been genetically altered to give them the taste for toxins. Scientists are designing or deploying microbes to purge sites of contaminants such as PCBs, oil, radioactive waste, gasoline and mercury, and new bioremediation research appears regularly.

Genetic Tinkering

For one study published recently in the journal BMC Biotechnology, researchers at the Inter American University of Puerto Rico modified E. coli bacteria (a common lab bacteria) with genes that allowed the microorganisms to not only survive in mercury but to remove it from waste sites. The genes in question produce proteins called metallothionein and polyphosphate kinase that allow the bacterial cells to develop a resistance to mercury and to accumulate large amounts of the heavy metal within the organism, thereby isolating it.

"Mercury is really toxic, and there are no natural organisms that can bioremediate mercury," says Oscar Ruiz, one of the study's lead authors. However, there are a few organisms that make it more dangerous. They transform the ionic or elemental mercury, which is discharged from industrial sites such as coal-burning power plants, into the more toxic version, methylmercury. Methylmercury can accumulate in plants and animals, and is most toxic to those at the top of the food chain.

Ruiz's goal for his transgenic bacteria is for them to sequester mercury contamination before the natural bacteria have a chance to turn it into toxic methylmercury. The modified bacteria wouldn't be set loose in the wild, as there are strict government regulations about releasing genetically modified organisms into the environment. Instead, these bacteria would do their work in filters that can be brought to contaminated sites and used to filter the mercury out of water. It's possible they might even be able to recover the mercury for use in other industries. "Mercury is very important in many, many industrial applications," Ruiz says.

Giving Nature a Boost

Creating genetically modified microbes to do our dirty work is only really useful when dealing with contaminants like mercury, where there are no known natural bacteria capable of doing the job. In most other bioremediation cases, nature just needs a little bit of a helping hand.

Pollution-eating bacteria have gotten the most press at large oil spills, such as the Exxon Valdez and, more recently, the Deepwater Horizon disaster. Scientific entrepreneurs poured out of the woodwork, eager to contribute their oil-eating superbugs to the cleanup effort. But, in reality, the microbes that were already there had a head start. "Bioaugmentation in the open environment really isn't effective," says Kenneth Lee, a researcher with Canada's Department of Fisheries and Oceans who has extensive experience researching bioremediation of oils spills. Bioaugmentation is a type of bioremediation that involves adding organisms directly to the open environment. But microbes that aren't adapted to the environment die quickly, simply providing more nutrients for the indigenous bacteria to feed on. "That's an extremely expensive way to make fertilizer," says Terry Hazen, the head of the Center for Environmental Biotechnology at UC Berkeley.

Instead of adding organisms to the environment, often the best way to get rid of chemical pollutants is to help the bacteria already on the job. Take the Deepwater Horizon leak. Because of natural oil seeps on the Gulf of Mexico's seafloor, communities of microbes that can break down oil, or at least parts of it, exist there. Adding nutrients to the water near an oil spill can help grow these bacterial populations. And it works for things besides oil, too. Some of Hazen's early work showed that low doses of methane injected at contaminated sites can aid the growth of methanotrophs, organisms that love methane and produce enzymes capable of breaking down chlorinated solvents and up to 300 other compounds.

Break It Down

With a bacterial-based oil spill cleanup, there is an additional step beyond adding nutrients that give microorganisms a boost. During the Deepwater Horizon cleanup, for instance, responders dumped loads of chemical dispersants to try to break up the oil in the Gulf of Mexico. One benefit of such an action is that it breaks the oil down into pieces small enough for the bacteria to eat. "When bacteria attack oil they can only attack from the water-oil interface" Lee explains. When a spill breaks up, its surface area gets larger and the microbes have an easier time feasting upon it.

Much of the Deepwater Horizon oil disappeared faster than the earliest predictions suggested. So, though there is still cleanup to be done on the shore, and the full impact on the marine ecosystem may not be known for decades, the Gulf's microorganisms played a role in cleaning up part of the spill. And bioremediation methods, using both indigenous and genetically modified microbes, will continue to garner attention as a cost-effective way to clean up contamination.

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