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Arsenic: Toxic to people, vital for some microbes

Monday, May 19, 2003

By Byron Spice, Post-Gazette Science Editor

Say the word "arsenic" and most people think"poison. "

"Arsenic is far more prevalent in the environment than we give it credit for," says John Stolz, a Duquesne University biologist who has found arsenic-consuming microbes in Pittsburgh's three rivers. (Martha Rial, Post-Gazette)

Arsenic's toxicity is so legendary that it was once known as "inheritance powder" for its real or imagined role in bumping off unwanted relatives. Just last month at a church in Maine, a man was killed and 15 others sickened when someone slipped arsenic into their coffee.

So the idea that some organisms might actually thrive on it might seem preposterous.

Yet scientists have uncovered a surprisingly large community of bacteria that actually breathe arsenic, using it to break down food and release energy in much the same way as humans use oxygen.

The first arsenic-breathing microbe was found in the muck of a polluted Massachusetts watershed. Others likewise have been "extremophiles," bacteria that are adapted to hostile environments, such as hot springs and soda lakes.

But early suspicions that these microbes would dwell only in strange niches have been proved to be mistaken, said Ronald Oremland, a geochemist at the U.S. Geological Survey in Menlo Park, Calif.

"We've pretty much found these guys all over the map," he said, including rivers and ground water aquifers.

John Stolz, a Duquesne University biologist, recently isolated arsenic-metabolizing microbes from water and sediments samples gathered from Pittsburgh's Three Rivers and is now analyzing them in his laboratory.

"It's not that there's significant amounts of arsenic in the rivers," Stolz said. Rather, microbes that breathe arsenic appear to be versatile, often capable of using selenium, nitrate, or other molecules for respiration if arsenic isn't present.

Scientists still have much to learn about these microbes, as Stolz and Oremland pointed out in a recent article in the journal Science on the "ecology of arsenic," but it's possible that they are environmentally significant. For instance, they may play a role in arsenic contamination of water wells by converting arsenic from a largely inert form into a toxic, water-soluble form.

And researchers, including Stolz, are trying to figure out when and how these bacteria might have developed their arsenic-breathing capability. It's possible that they date back billions of years, to a time when the Earth didn't have an oxygen-rich atmosphere and life forms were all "anaerobic," that is, they did not use oxygen for respiration.

It's everywhere

Arsenic isn't an abundant element -- it comprises just 0.0001 percent of the Earth's crust -- but it is widespread. A silvery-white element that has some of the characteristics of a metal, it rarely is found in its pure, elemental form. Usually, it is part of a larger molecule. For instance, the white, tasteless powder that is used as an insecticide -- or to poison people -- is arsenic trioxide.

"Arsenic is far more prevalent in the environment than we give it credit for," Stolz said. Arsenic trioxide, for instance, is used to treat some forms of leukemia. Chromated copper arsenate, or CCA, has long been used as a preservative in pressure-treated wood used for decks, fences and playground equipment, though its use is being phased out because of health concerns. Gallium arsenide is a semiconductor used in electronic devices.

Poultry producers mix roxarsone, which contains arsenic, into chicken feed to prevent diseases and make chickens plumper, Stolz noted. The arsenic doesn't accumulate in the chickens, but passes in their excrement, making chicken manure a source of arsenic.

Arsenic usually is found either as arsenate, a form that tends to bind to metal ores and other minerals, or as arsenite, a more toxic soluble form.

Just as people inhale oxygen and, after using it to release energy in food, exhale carbon dioxide as a waste product, microbes that use arsenic for respiration take in arsenate and subsequently release arsenite as a waste product.

This difference in forms of arsenic is one of the hints that led to the discovery of the first arsenic-respiring microbe a decade ago. Dianne Ahmann, then a graduate student, was a member of a research group at the Massachusetts Institute of Technology that was studying Aberjona Watershed, a 25-square-mile drainage basin near Boston.

Chemical companies had used the watershed as a dumping ground from the mid-1800s to the 1930s, so the researchers weren't surprised to find arsenic there. They anticipated most of it would be bound up in the sediments in the insoluble, arsenate form. But they found a surprising amount of arsenite dissolved in the water, leading them to suspect something was transforming the arsenic from one form to the other.

Wading in hip boots through a shallow reservoir, Ahmann gathered samples of the orangey sediments. Back at the MIT lab, the samples were placed in bottles, along with bacterial growth factors and arsenate. When the researchers checked a few days later, they saw that the arsenite levels had significantly increased. They subsequently isolated the arsenic-respiring bacterium from the samples and announced the discovery in 1994 in the journal Nature.

At the same time, Oremland had isolated a bacterium that breathes selenium from Mono Lake, a highly alkaline, highly saline 70-square-mile lake in the Eastern Sierra of California. Water drains into Mono Lake from the surrounding mountains, but doesn't drain out, so the only way water is removed from the lake is by evaporation. As water evaporates, it leaves behind salt and impurities, including arsenic, that then become concentrated, making the water poisonous.

"You can't drink Mono Lake water," Oremland said. "By the time the arsenic reaches you, you're already dead."

Oremland found that the same Mono Lake bacterium that respires selenium also can respire arsenic. At least 16 different bacteria have been identified that use arsenic in this way.

Arsenic-rich environments such as Mono Lake and the Aberjona Watershed are good places to find arsenic-breathing bacteria. But because they often can use other molecules, such as nitrate, sulfate, or selenium, they can also exist in arsenic-poor environments, where they are much more difficult to find.

Making identification easier

Stolz's lab is working to characterize arsenic-breathing organisms, perhaps leading to a test that would make it easier to identify them in the environment. A Duquesne colleague, bioinorganic chemist Partha Basu, is trying to figure out exactly how the enzymes in the bacteria use arsenic to release energy from food.

Stolz and Oremland suspect arsenic-breathing bacteria may be playing a role in the arsenic contamination of water wells in Bangladesh, where more than 10,000 people are known to have arsenic-related diseases. Drilled over the past 20 or 30 years to provide water that was safer to drink than surface water, thus preventing cholera, the wells subsequently became contaminated with arsenic.

Many scientists have studied the Bangladesh wells to figure out what has caused arsenic to be released from ancient sediments.

Arsenic-breathing bacteria might be at work in much the same way as in Aberjona Watershed, transforming arsenate that is bound to sediments into water-soluble arsenite, Oremland said. But he acknowledged that no one has yet proved that the bacteria can have the same effect in aquifers as they do in arsenic-polluted lakes and drainages. More research is needed, he added.

Arsenic contamination may increasingly become an issue in the United States, where the Environmental Protection Agency recently reduced the level of arsenic considered safe in drinking water from 50 parts per billion to 10 parts per billion. Elevated arsenic levels have been found in eastern New England, western Minnesota, parts of Michigan and parts of the Southwest.

Some bacteria, Oremland and Stolz note, metabolize arsenic in a way that transforms water-soluble arsenite into the safer, insoluble arsenate, suggesting it might someday be possible to use the bacteria to clean up or reduce arsenic contamination of aquifers.

Byron Spice can be reached at bspice@post-gazette.com or 412-263-1578.

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