How bacteria release arsenic into groundwater


Incubation experiment: When methane is added to the sediment, bacteria release arsenic, recognizable by its brown color. (Image: Andreas Kappler)

In many Southeast Asian countries such as Bangladesh and Vietnam, the groundwater is heavily contaminated with arsenic. It is released by microorganisms that extract the poisonous element from minerals in the sediment. Researchers have now taken a closer look at these microorganisms. They discovered that the bacteria and archaea not only metabolize organic materials, but also methane gas, which is found in high concentrations in many of the affected bodies of water.

In detective novels, arsenic is a popular substance for poisoning. However, many people are exposed to high levels of the dangerous element day in and day out. The slow arsenic poisoning leads to pathological changes to the skin and even cancer. In Southeast Asia in particular, the groundwater that people use as drinking water and to irrigate their fields is heavily contaminated with arsenic. As a result, rice is also often contaminated. It is released from arsenic iron minerals that originally come from the mountains of the Himalayas. It has long been known that microorganisms play an important role in this. How exactly the metabolic processes involved take place, however, was unclear.

Sediment samples from Vietnam

A team led by Martyna Glodowska from the University of Tübingen has now examined the starting materials and products of bacterial metabolism in more detail. The team had previously proven that the arsenic-releasing microorganisms do not feed on organic material on the surface of the water, as originally assumed, but that they find their food deeper in the sediment. “According to our previous study, we assumed that they maintain metabolism and growth through organic deposits on the sediments,” says Glodowska.

For the current study, the researchers used sediment samples from a Vietnamese village near the city of Hanoi. “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 Glodowska’s colleague Andreas Kappler. Methane forms the main component of natural gas and is widely used in industry and households to generate energy. “That gave us the idea that the arsenic-releasing microorganisms could also use it,” reports Glodowska.

Methane as a food source

In the laboratory, they added methane to the microbe-containing sediment samples from Vietnam. After 125 and 220 days, they measured the concentration of released iron and arsenic and also used genetic analyzes to determine which microorganisms were represented in which proportions. In fact, an increasing concentration of released iron and arsenic was found in the course of the experiment. In addition, those microorganisms that are known to metabolize methane increased.

After 125 days, bacteria of the species Methylogaea oryzae and closely related species dominated. It was previously assumed that these bacteria needed oxygen to oxidize the methane. In the current experiment, however, their metabolic processes took place under anaerobic conditions. The researchers conclude from this that the bacteria can transfer the electrons from the methane to the iron instead of to oxygen and in this way dissolve the iron-arsenic compounds. After 220 days, the researchers also found many archaea that also metabolize methane and thereby release iron and arsenic from the sediment.

Relevant for bodies of water worldwide

“We have thus discovered a mechanism by which arsenic can accumulate,” says Kappler. “Under the water surface, microorganisms produce methane, which supplies other microbes, so-called methane eaters, with the energy to dissolve the iron minerals and thereby separate arsenic.” Many water systems around the world contain large amounts of methane, and microorganisms that produce and consume methane are also widespread . “Therefore, the mobilization of arsenic by methane-eating bacteria could be an important mechanism for arsenic contamination in numerous areas,” says Kappler. An important step has been taken with the identification of methane as a food source, believes Glodowska. Now it must be ascertained what extent this path of arsenic accumulation assumes under natural conditions.

Source: Martyna Glodowska (University of Tübingen) et al., Nature Communications Earth & Environment, doi: 10.1038 / s43247-020-00037-y

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