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John Roachfor National Geographic News
June 30, 2004
Microscopic organisms that get their energy by inhaling metals in the ground play a key role in the arsenic poisoning of drinking water for millions of people in Bangladesh and the Indian state of West Bengal, according to a new study.
Researchers hope that the finding will shed light on how the drinking water came to be so heavily laced with arsenic—and that, in turn, it is hoped, could yield a way to reduce the level of the toxin.
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Cited by the World Health Organization as the "worst mass poisoning in human history," as many as several million wells in India and Bangladesh became contaminated with the poison in the early 1990s. The poisoning remains a grave threat to those who continue to drink and irrigate with the water today.
Exposure to high levels of arsenic can cause cancers of the skin, bladder, kidney, and lung, and diseases of blood vessels of the legs and feet. It may also contribute to diabetes, high blood pressure, and reproductive disorders.
"Some researchers have estimated that two-thirds of the population in Bangladesh are at risk for chronic arsenic poisoning," said Willard Chappell, an emeritus professor of environmental sciences at the University of Colorado at Denver and expert on arsenic.
For the past decade, research teams from around the world have tried to determine why arsenic is present in such high concentrations in the Bangladesh and West Bengal aquifers. That knowledge would help them identify areas of high risk and develop appropriate remediation strategies.
Reporting in the July 1 issue of the science journal Nature, an international team of researchers indicts bacteria for the rising arsenic levels.
"When we found maximum rates of arsenic mobilization, we found signatures for known metal-reducing bacteria including a commonly found iron-reducing bacteria," said John Lloyd, a microbiologist at U.K.'s University of Manchester. Lloyd led the research team.
Metal-reducing bacteria "breathe" metals such as iron to get energy from their food, in the same way that we humans breathe oxygen to break down our food.
The bacteria breathe by passing electrons onto metals, which changes the characteristics of the metals. Scientists refer to this as metal reduction.
Derek Lovley, a microbiologist at the University of Massachusetts, Amherst, said the University of Manchester-led study definitively shows what many scientists had suspected.
"The paper puts together components that were known individually, but nobody had done that experiment with material from the [Bengal] aquifer before," he said. "It's important in that way.
To speculate is one thing, it's another to definitively show it."
In the University of Manchester-led study, the researchers found that arsenic reduction and release took place after the microbes had reduced and released iron, not simultaneously. This is an indication the processes are decoupled.
Lloyd said that while the reasons for the decoupling are not fully understood, the decoupling is not totally unexpected.
One explanation for the decoupling could be that the bacteria feed on the substrates that give the most energy first. Since iron is abundant and preferred by several of these microorganisms, they go after it before they move in on arsenic. Another possibility is that iron reduction causes a change in the mineral structure of the sediments, so that the arsenic becomes more readily available to the metal-reducing bacteria, leading to the release of arsenic into the groundwater.
"We're looking at that in detail now," Lloyd said. "We're trying to get to grips with the details to answer those sorts of questions."
Earlier research by Lovley and his colleagues has shown that acetate—essentially vinegar—is a favorite food of metal-reducing microbes and causes populations to explode.
The University of Manchester-led team added acetate to its sample to simulate an influx of organic carbon to the sediments where the microbes live. This resulted in marked stimulation of iron reduction followed by arsenic release.
The researchers said the stimulation of iron reduction and arsenic release and reduction by acetate demonstrates that the availability of organic carbon controls the mobilization of arsenic by metal-reducing bacteria.
"These sediments are starved of organic matter and electron donors," Lloyd said. "If organic matter does get into the subsurface it will stimulate the activity of these organisms."
Influxes of organic carbon are known to occur when irrigation wells are drawn down, leading several researchers to propose that the introduction of organic carbon by irrigation pumping can be a factor in increasing arsenic mobility in shallow groundwaters in Bangladesh and West Bengal. This theory is supported by the Manchester-led study.
Now that the researchers have a better understanding of the processes that control the release of arsenic into the region's groundwater, they are looking for a way to reverse the processes so that the drinking water can be made safe to drink.
According to Chappell, the arsenic-research community has yet to reach consensus on the mechanisms that cause the arsenic poisoning. He cautions against accepting the University of Manchester-led study as the final word. "The problem itself, however, is very real and very bad," he said.
"Also, while it is much worse in Bangladesh and West Bengal than elsewhere, more and more countries are identifying the problem—these include Nepal, Cambodia, Laos, Vietnam, China, and others where tube wells were installed to prevent diarrhea-related problems associated with surface-water use."