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Mercury in the soil: How bacteria and fungi cope with the contamination

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Between 1930 and 1970, the chemical plant Lonza in Visp (Canton Valais) released mercury into the Grossgrund Canal via its waste water. Some of the poisonous metal accumulated in the sediment, but was not for a long time identified as a problem. Up until the 1990s, the Canton and Municipality merely dredged the Canal periodically to keep it flowing. The excavated material was used as fertilizer or filling material for the nearby fields and gardens. It was not until 2010, during preparatory work for the construction of the A9 motorway, that the soil contamination was, by chance, discovered.

Adapt and remove

Nobody has, up until now, investigated how long-term mercury contamination affects soil organisms. The microbiologist Aline Frossard and her colleagues therefore analysed soil samples from around the Grossgrund Canal. The activity and growth rates of the microorganisms in the contaminated soil were more-or-less the same as those in the uncontaminated soil – which indicates that the mercury probably did not influence the soil quality. The species diversity of the microorganisms also did not differ. What did, however, change was the composition of the bacterial and fungal communities.

These results came as no surprise to Aline: “For one thing, microorganisms adapt quickly to changes in environmental conditions. If one species, for example, disappears because the level of mercury is too high, another will take over its place and function. Moreover, microorganisms in the soil can transform toxic mercury into a non-poisonous form and thus adapt to the inhospitable conditions.” In a further experiment Aline was able to show that the soil texture influenced how the mercury affected the soil organisms. The amount of biologically available mercury was, for example, less high in limy soils than in other soils.

The contamination of the Grossgrund Canal and nearby soils had, apparently, no effect on human health, as a University of Zurich study already found in 2016. The contaminated soil is now being gradually removed and cleaned up. When peace and quiet finally return to the region, the locals will be happy. (Lisa Bose, Diagonal 1/18)