With methane as a food source, bacteria release toxic arsenic

Toxic arsenic pollutes rivers and groundwater in many Southeast Asian countries such as Bangladesh and Vietnam. It is released by the activity of microorganisms, but little was known about their food sources. A team from the Geomicrobiology of the University of Tübingen, led by Professor Andreas Kappler, recently demonstrated that the arsenic-releasing bacteria find their food in the deeper soil layers and do not get it from the water surface, for example from algae or plants. The team has now discovered that, in addition to the organic matter deposited on sediments, methane gas plays an important role as a food source in the release of arsenic by bacteria. This explains the high arsenic concentration of the water in many areas of Southeast Asia better than before.Nature Communications Earth & Environment .

Arsenic-containing iron minerals, originally from the Himalayan mountains, are now found in the sediments beneath the surface of many river deltas in Southeast Asia. These groundwater sediments house various associations of microorganisms that release the toxic arsenic by dissolving the iron minerals. “According to our previous study, we assumed that they maintain metabolism and growth through organic deposits on the sediments,” says doctoral student Martyna Glodowska.

High natural gas concentration

The test area is a groundwater system contaminated with arsenic in Van Phuc, a village 15 kilometers southeast of Hanoi in Vietnam. “We were able to observe a high concentration of methane there, which is produced by microorganisms. In some places the methane concentration is so high that the gas bubbles out of the water to the surface, ”reports Andreas Kappler. Methane forms the main component of natural gas and is widely used as biogas in industry and households to generate energy. “That gave us the idea that the arsenic-releasing microorganisms could also use it,” reports Glodowska, the first author of the new study.

With the help of experiments in the Tübingen laboratory in which methane was added to the sediment from Vietnam, the research team was able to confirm these assumptions. “We have thus discovered a mechanism by which arsenic can accumulate,” says Kappler. “Under the surface of the water, microorganisms produce methane, which supplies other microbes, so-called methane-eaters, with the energy to dissolve the iron minerals and thereby separate arsenic.” 

A comparative analysis of water systems around the world revealed that many of them contain large amounts of methane as well as a wide distribution of methane producing and consuming microorganisms. “Therefore, the mobilization of arsenic by methane-eating bacteria could be an important mechanism for the arsenic contamination of numerous areas,” says the scientist. Glodowska believes that an important step has been taken with the identification of methane as a source of food. Now it must be ascertained what extent this path of arsenic accumulation assumes under natural conditions.

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