Could bacteria be the culprit behind widespread arsenic poisoning in South Asia? That's the suggestion of new research demonstrating that certain bacteria can mobilize naturally present arsenic from soil samples in the lab.
Chronic arsenic poisoning, caused by long-term exposure to contaminated drinking water, is known to cause cancer of the skin, lungs, urinary bladder, and kidney, as well as other ailments. In what the World Health Organization has described as the "worst mass poisoning in human history," dangerous levels of arsenic in large parts of the Ganges Delta have poisoned millions of people in the last 20 years. Hundreds of millions more may be at risk. But, as yet, scientists have been unable to nail down how the arsenic gets into the water supply.
The alluvial sediments of the riverbed are naturally rich in arsenic, which usually binds up in other compounds. But due to little understood biogeochemical mechanisms, in the last 2 decades the arsenic has been mobilized and a toxic form, As(III), has slowly leached into the groundwater extensively used for drinking and irrigation.
In the new study, described in the 1 July issue of Nature, Jonathan Lloyd of the University of Manchester, U.K., and colleagues from the United Kingdom and India incubated soil samples from a contaminated aquifer in West Bengal, India, under different conditions in the lab. When the researchers incubated the samples in an oxygen-free chamber, levels of As(III) doubled, reaching those seen in contaminated regions, over the course of about a month. There was no such increase in samples incubated with oxygen, however, or in samples that were autoclaved to kill bacteria before incubation. The results suggest to the team that anaerobic bacteria are somehow responsible for releasing the arsenic.
Additional experiments cast particular suspicion on iron-reducing bacteria, which grow dramatically under the anaerobic conditions that favor mobilization of As(III). Because these bacteria need organic carbon to thrive, the experiments support earlier theories that agriculture could have contributed to the arsenic problem ever since the Green Revolution took a strong hold in the region a few decades ago. Water drawn from aquifers for irrigation in farmed areas is replaced by carbon-rich water trickling down from above.
But not everyone believes the case is airtight. "There are many factors working in the field" that might escape detection in the lab, cautions Junji Akai, a professor of mineralogy at Niigata University in Japan.
Department of Earth Sciences, the University of Manchester
West Bengal and Bangladesh Arsenic Crisis Information Center
School of Environmental Studies, Jadavpur University, India, with links to arsenic research projects