[Chesapeake Quarterly masthead]
2006
Volume 5, Number 1
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Lessons Aquatic
Microbes Can Teach

Contents

What the Eye Can't See

Bacteria and PCBs

Mutual Arrangements

Medicine from Microbes

Profile of a Researcher

In Memoriam:
Ken Tenore


 

The long goodbye — gone are the bowling lanes and dry cleaners that once drew customers to this strip mall north of Baltimore, but below ground a soup of toxic chemicals lingers. Will naturally occurring bacteria break down these persistent compounds, or will this become another toxic legacy in the Chesapeake watershed? Photographs by Joel Klein, except stream, by Erica Goldman.

Of Microbes & Messes
Bacteria Hold Key to Cleaning
Up Polluted Groundwater

By Erica Goldman

chained off bowling lanes - photos by Joel Klein, except stream, by Erica Goldman

Its red letters are now faded and rusty, but the old "BOWL" sign still towers above an abandoned parking lot in Towson, Maryland, a reminder of a time when teenagers flocked to Fair Lanes and customers brought their rumpled shirts and slacks to Kings Cleaners, a dry cleaning operation that stood next door. Now the bowling alley is a roofless cement hull. Yellow and blue paint peels off the sides, rows of steel girders rust beneath the sky. In the old bowling alley's vast interior, gleaming new construction equipment rests idle — the only sign that the site will soon be redeveloped.

Deep below the paved lot surrounding the abandoned bowling alley, and well out of sight, a contaminated groundwater plume slowly creeps toward Mine Bank Run, a tributary of Gunpowder Falls. Kings Cleaners has vanished entirely — except for a square gravel stain in the center of the parking lot. But its chemical signature lingers strong and clear. Twenty feet beneath the former dry cleaning operation, tetrachloroethene (PCE), a widely used cleaning solvent, persists in the groundwater at high concentrations, measured at 16,400 parts per billion (ppb) in 2004, more than 3,000 times the legal limit of 5 ppb set by the U.S. Environmental Protection Agency.

PCE is a suspected human carcinogen and difficult to clean up. It can contaminate groundwater, a grave concern in rural areas that rely on wells for their drinking water supply. When this compound percolates close to the surface along with groundwater, it can intrude into the soil, releasing volatile, noxious gases. Short-term PCE exposure can cause dizziness, headaches, and problems with balance. Over the long term, PCE exposure has been linked to cancers of the esophagus, bladder, and blood.

Kings Cleaners closed in 2000, after continuous operation for 32 years. In 1998, the Maryland Department of the Environment (MDE) began a series of environmental assessments that lasted for years — testing contaminant levels in the groundwater, in soil gases, and in nearby residential wells — to determine whether remediation would be necessary prior to any future development.

In October 2005, MDE decided to allow limited industrial and commercial development at the site, as long as the groundwater remains untouched. Tests had deemed the soil gases free of PCE. So the chemical poses no immediate threat to future occupants of the site. The agency issued what is known as a "No Further Requirements Determination" to the current property owner, meaning that no remediation steps would be mandated. But no residences can ever be constructed on the site. No wells can ever be dug.

Sites like the former Kings Cleaners exist all over the United States. According to a study by the State Coalition for Remediation of Dry Cleaners, 75 percent of all active dry cleaning facilities are contaminated (some 17,000 sites nationwide), and that doesn't include sites that have been abandoned.

Are there tools to clean up these toxic messes? Scientists know that there is one species of bacteria — only one — that can break down PCE and its chemical cousin TCE (trichloroethylene), a compound with a similar chemical structure used in metal degreasing and paint stripping. To this "bug" (Dehalococcoides ethenogenes), chlorinated solvents such as PCE and TCE are the air that they breathe. So shouldn't cleaning up PCE-contaminated sites be as simple as providing this toxin-breathing bug unfettered access?

It's not that easy. In nature, a single species of bacteria rarely acts alone. Microbes live in intricate consortia and interact with other organisms — in the marine environment these include algae, corals, and sponges. Different bugs may play complementary roles, or they may compete fiercely. Scientists have long worked to harness the power of microbes to clean up human messes. Now, more and more, they are looking to the ecology of bacteria for clues about how to tackle tough challenges — like erasing the environmental signatures of industrial pollution.

Fierce Competitors

Jennifer Becker leaves her office in the Biological Resources Engineering Department at the University of Maryland in College Park and walks briskly across campus to the Plant Sciences Building to give an invited lunchtime seminar. She sets up her computer and plugs into the room's audio-visual system as the previous class files out.

Her first Power Point slide features two potato-sized shapes, thousands, maybe millions of times the size of the bacteria they represent. Their stick figure arms wear boxing gloves. Facing each other, they square off, ready to duke it out.

In fierce competition for hydrogen, the bug Dehalococcoides ethenogenes squares off against Dehalobacter restrictus, another chlorine-breathing microbe. Scientist Jennifer Becker uses this drawing to explain her work to students and colleagues. Both bugs need hydrogen for cellular respiration. But fans should root for D. ethenogenes (red boxer), the only bug that can use hydrogen to render PCE, a toxic chemical used by the dry cleaning industry, completely harmless as ethene and chlorine (shown below in another of Becker's drawings).
cartoon of two bacteria fighting over hydrogen

sketch of D. etheogenes and D. restrictus competing for hydrogen


Microbes operate in complex environments, she explains as she clicks the remote. If, she says, our goal is to clean up human-made messes, like chlorinated solvents from the dry cleaning industry (PCE), we need to understand not only how these bugs function metabolically, but how they interact with each other.

Becker's schematic duel depicts a simulated competition between two different species of bacteria. One bug (D. ethenogenes) can render toxic chemicals totally harmless. The other bug can only go halfway, leaving a still nasty chemical compound (dichloroethene) in its respiratory wake.

This microbial duel showcases a classic battle for a limited resource. In this case the bacteria fight over hydrogen, a microscopic version of two carnivores fighting over a ravaged wildebeest in the African Serengeti. Hydrogen, explains Becker, is especially important because it acts as an electron donor, providing the engine for cellular respiration. But other bacteria also rely on hydrogen to power their metabolic needs and some can use it faster than D. ethenogenes, beating it to the punch. In these cases, competition may not favor the complete breakdown of the toxic compounds if the faster microbe only does half the job.

The trick, she explains, is to engineer the situation so competitors do not overwhelm the more thorough bug, the one that can take the chemical completely out of commission.

Becker's pictorial representation frames a conceptual model she's developed to simulate the effects of competition among bacteria in the environment — a model that revamps traditional thinking. The current paradigm in the bioremediation industry for PCE cleanup is simply to determine whether the best bug (D. ethenogenes) is present in the subsurface aquifer, explains Becker. If it is not, consultants will recommend that you add it. According to one count, such projects to clean up groundwater contamination related to industrial dry cleaning operations have already been implemented at 17 sites in ten states.

"But I've shown through modeling that competition in the environment could cause the added bug to be lost," she says. If we know that different bugs are competing for a limited amount of hydrogen, she explains, we may have to develop new strategies to ensure that both organisms get the hydrogen and other resources that they need. Becker describes her modeling work in the July 15 issue of the journal Environmental Science & Technology.

As Becker continues her talk, the audience listens intently, trying to absorb detailed information about how D. ethenogenes breaks down chlorinated compounds like PCE and TCE. As she concludes, she's got time to take a few questions from the audience before the next class takes over the lecture hall. One colleague asks whether she knows if D. ethenogenes is widely distributed (or ubiquitous) in nature. This is one of the big unknowns, Becker responds, and something that she is working to find out.

In the past few years, molecular biology tools have made it easier to determine whether these bacteria are present and active in the environment, she says. But these tools often cannot provide information about how the bacteria function in the environment. In order to validate this model of competition in the environment, we must grow the bacteria in pure culture, effectively staging the competition, and see which one wins out, she explains. One of her graduate students is also working on painstaking kinetic rate measurements, to determine how fast these bacteria carry out certain reactions, which will also help to ground-truth Becker's theory.

"Eventually, we want to be able to go back to the environment and make predictions about whether we would expect to find one bug or the other at a particular location," she says. In the future, Becker plans to test her model predictions at a site where there's a contaminated groundwater plume.

Jennifer Becker and her student Yen-jung - photo by Micheal Fincham culture bottle  with D. ethenogenes - photo by Micheal Fincham
D. ethenogenes micograph - photo by Electron Microsope Laboratory, Berkeley, CA
Looking for Mr. Goodbug — Jennifer Becker and her student Yen-jung Lai check out a bacteria called D. ethenogenes (shown in micrograph, above, and glass bottle, above right. This microbe breathes a chemical called PCE, short for tetrachloroethene, a widely used, long-lived cleaning solvent which fouls groundwater under thousands of industrial sites. Top two photos by Michael Fincham and micrograph by Electron Microsope Laboratory, Berkeley, CA.

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