A closer look at mercury in porcini mushrooms

Porcini mushrooms are amongst the most often collected and eaten wild mushrooms. Many people know about their value in the kitchen, but hardly anyone is aware that they can take up significant amounts of mercury and selenium from the soil. While selenium is an essential element for humans and can be beneficial to our health, mercury is a toxic heavy metal, and excessive consumption can pose a health risk to mushroom lovers. Although this extraordinary accumulation of mercury and selenium is known and often observed by researchers, there have hardly been any attempts to investigate this phenomenon in more detail. The first task of this project (carried out in the group of Prof. Frank Vanhaecke Ghent University, Belgium), is the investigation of the distribution of mercury and selenium in porcini mushrooms with laser ablation coupled to inductively coupled plasma mass spectrometry (ICPMS). It will reveal whether the element is distributed evenly in the mushroom, or whether it is transported to a designated place, like the spores or the outermost layer of the cap. By removing these parts before cooking, the health risk could be minimized easily. Further, a co-occurrence of mercury and selenium could indicate interactions of the two elements and a potential detoxification of mercury by selenium. Next, the ratios of the natural stable isotopes of mercury (atoms of mercury which differ from one another in terms of mass and/or magnetic properties of their nucleus) will be determined in porcini mushrooms and the underlying soil with multi-collector ICPMS. These isotope ratios are slightly different in different soils and soil layers and are affected by (bio)chemical reactions. Thus, the results from this part of the project could be used for indicating the geographical origin of mushrooms and for identifying the soil layer from which the mushrooms are taking up the mercury. The results can help to understand (bio)chemical reactions of mercury in the mushrooms and the elements transport routes in the environment. During the return phase in the group of Prof. Walter Goessler, University of Graz, Austria, high- performance liquid chromatography coupled to ICPMS will be used to determine the chemical forms in which mercury and selenium are present in porcini mushrooms. This will give us a better insight into the interactions and transformations of the two elements. It will allow a better assessment of the health risk related to mushroom consumption. Overall, this project will use sophisticated state-of-the-art techniques to shed light on the uptake and fate of mercury in porcini mushrooms. It will significantly contribute to our understanding of the cycling of mercury in our environment. The findings will enable a better evaluation of the health risk arising from eating mercury-accumulating mushrooms.

Porcini mushrooms are amongst the most often collected and eaten wild mushrooms worldwide. Many people know about their value in the kitchen, but hardly anyone is aware that they can take up significant amounts of mercury and selenium from the soil. While selenium is an essential element for humans and can be beneficial to our health, mercury is a toxic heavy metal, and excessive consumption can pose a health risk to mushroom lovers. Although this extraordinary accumulation of mercury and selenium is known and often observed by researchers, there have been hardly any attempts to study this phenomenon and the driving factors behind it in more detail. To close this gap, this project used an array of sophisticated state-of-the-art analytical techniques, all based on "inductively coupled plasma mass spectrometry" (ICP-MS), to investigate a variety of different aspects on mercury and selenium in mushrooms. The techniques were thoroughly evaluated and optimized to obtain data of the highest quality possible. One of the main goals was the investigation of the spatial distribution of mercury and selenium in mushrooms. After optimization of the methods, it was possible to generate high resolution images of the two elements in different mushroom parts, and even quantify them directly in these images (which is still a rather tricky endeavor). The first data indicate that the outermost layer of the mushroom fruit-bodies contains far higher mercury concentrations than the inner parts, suggesting that simple peeling of the mushrooms before cooking could significantly reduce the health risk of the consumers. Further, these analyses can now be carried out in a much shorter amount of time than before, and with much better image quality. Another task was the investigation of the isotopic composition of mercury in different parts of mushroom fruit-bodies, because this can be affected by various (bio)chemical reactions. Therefore, such data can deliver information on the transport routes and the ultimate fate of mercury in the environment. The results have been inconclusive so far, but are still undergoing thorough statistical evaluation. Finally, the chemical forms of mercury in mushroom tissues were determined. This is highly relevant, because some forms (e.g., methylmercury) are considered more toxic than others (e.g., inorganic mercury). For this purpose, suitable methods were developed and optimized to extract mercury from the mushrooms and subsequently analyze it with chromatographic methods. According to these investigations, mushrooms contain mainly inorganic mercury, and only very little of the most toxic methylmercury. Another finding was that certain already published extraction methods are actually not suited for the application to biological samples, and that some existing data might have to be re-evaluated as a consequence.