MIT scientists discover arsenic-eating organism

MIT scientists have discovered the first organism known to thrive on arsenic.

The microscopic bacterium, dubbed MIT-13 for now, is aiding scientists' understanding of how arsenic moves through the environment. It also has potential for cleaning up the arsenic-contaminated areas where it lives because it converts particle-bound arsenic to a water-soluble form that might then be filtered from the water.

The organism, which was found in eastern Massachusetts, is a biological curiosity.

"Arsenic is a legendary poison, renowned in its effectiveness against heroines, gentlemen, rats, and wood rot," write the scientists in the October 27 issue of Nature. It also kills most microorganisms. While some such microbes are known to tolerate arsenic, until the discovery of MIT-13 none has been known to actually thrive on it.

Specifically, MIT-13, which lives in muck that contains no oxygen, "uses arsenic to break down its food just as we use oxygen," said Dianne M. Ahmann, a graduate student in biology and lead author of the Nature article. Other authors are A. Lynn Roberts of Johns Hopkins University, Lee R. Krumholz of the University of Oklahoma, and Francois M.M. Morel of Princeton (Drs. Roberts, Krumholz and Morel worked on the MIT-13 studies when all three had appointments in MIT's Department of Civil and Environmental Engineering, or CEE).

The discovery of MIT-13 began with a mystery.

For about 10 years a large team of MIT scientists has been studying pollutants in the Aberjona Watershed, a 25-square-mile drainage basin close to Boston that includes parts of seven communities (Woburn, Winchester, Wilmington, Lexington, Burlington, Reading and Stoneham). In one study, a group led by Professor Harold F. Hemond of CEE found arsenic in the water.

While the group wasn't surprised by the actual presence of arsenic-for about 80 years from the mid-1800s to the 1930s, chemical companies dumped it into Aberjona waters as a waste product-they found more of an especially potent form of the compound than expected.

Arsenic exists in the environment in two main forms: arsenic (III) and arsenic (V). "Arsenic (V) poses much less of an environmental hazard than arsenic (III)," said Ms. Ahmann, "because it binds strongly to sediments and in that form is almost insoluble in water." Arsenic (III) however, is very toxic and much more mobile because it does not bind to sediments and dissolves in water. Given the years since the Aberjona arsenic was originally dumped and conditions in the watershed, the MIT scientists found much more of the potent arsenic (III) than they expected.

Enter Ms. Ahmann. "When I began my research," she said, "the question was, `Is there a biological cause for the unexpectedly high levels of dissolved arsenic (III)?' "

To find out, she donned hip boots and waded into a shallow reservoir at the northern end of the watershed to collect samples of the thick, orangy sediments there. That particular reservoir receives all the runoff from the Industri-Plex Superfund site, an area so polluted as the result of past industrial practices that the US government has placed it on a special list for cleanup. "I took my samples from an area barely 10 meters from the chain-link fence that keeps people out of the Superfund site," Ms. Ahmann said.

Back in the lab, she and her colleagues incubated the samples in bottles. Each bottle contained the sample (mixed with arsenic in its insoluble form and various substances known to aid bacterial growth) topped by a layer of water.

And "lo and behold," Ms. Ahmann said, in five days the scientists found "that the concentration of arsenic (III) dissolved in the water had increased tremendously." Because bottles that were autoclaved or treated with formaldehyde showed no change, "we knew that a living organism was responsible," Ms. Ahmann said (autoclaving kills all living things; formaldehyde kills most bacteria).

From there the scientists isolated the organism responsible: MIT-13. (The "13" stands for the number of the test tube in which the scientists finally isolated a pure colony of the bacterium; eventually the organism will receive a formal name, complete with genus and species.)

Further studies showed that MIT-13 is using the insoluble form of arsenic to break down its food, and in the process converting the arsenic into its more toxic form and releasing it into the environment.

Nevertheless, Ms. Ahmann notes that MIT-13 could potentially be manipulated to actually clean up the environment. "If the growth of these organisms were stimulated in sediments, then perhaps they could be used to dissolve arsenic into the water. A filter could then be used to get that arsenic out of the water," she said. The filtered arsenic could then be stored at a hazardous-waste site.

Ms. Ahmann emphasized that while MIT-13 is certainly contributing to the amount of dissolved arsenic (III) in Aberjona waters, "it's probably not exclusively responsible."We know that many other microorganisms live in Aberjona sediments. MIT-13 is just one of them."

The work was funded by the Howard Hughes Medical Institute through a doctoral fellowship to Ms. Ahmann and by the National Institute of Environmental Health Sciences.

A version of this
article appeared in the
November 2, 1994

issue of MIT Tech Talk (Volume
39, Number

Topics: Biology

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