Determination of Silver Accumulation in Ectomycorrhizal and Saprobic Macrofungi

Silver (Ag) has an average concentration below 1 mg kgˉ1 in soil and presence in the rhizosphere makes it susceptible to uptake by plants, bacteria, and other organisms in the soil. However, little is known about its biogeochemical cycle. This paper considers macrofungi because they are vital in the biogeochemical cycles of many elements and have the ability to accumulate Ag (Borovička et al. 2010). Silver in its non-charged state is not considered a toxin, but there are locations that have acquired an excess of silver where it might be. It is known that Ag-polluted areas have a positive correlation with Ag concentration in ectomycorrhizal and saprobic macrofungi. The research presented here combines a review of related published literature with a summary of the data on accumulation and distribution of Ag, and original research on Ag concentration in fungi in pristine and Ag-polluted areas. —Daniella Barraza
Borovička, J., Kotrba, P., Gryndler, M., Mihaljevič, M., Řanda, Z., Rohovec, J., Cajthaml, T., Stijve, T., Dunn, C., 2010. Bioaccumulation of silver in ectomycorrhizal and saprobic macrofungi from pristine and polluted areas. Science of the Total Environment 408, 2733–2744.

Borovička et al. collected samples of soil and fungi from various regions throughout the Czech Republic. For Ag-polluted areas, macrofungal samples were gathered from a forest in Lhota. The macrofungi were a mixture of ectomycorrhizal and saprobic fungi and consisted of 30 different species encompassing 22 genera. Lhota is polluted by a nearby lead smelter and mining activities. The topsoils in this area were reported to have values of up to 78 mg kg–1. The soil samples for verification were also collected from this area near a Norway spruce. The sample was a soil profile with five organic soil horizons: O1 (needles), Of (decomposing matter), Oh (well-decomposed matter), and mineral soil horizons, Ah and Bw. A representative part of each soil horizon was used for analysis. For the case study, a fungus Agaricus bernardii was chosen and was collected from the center of Prague. The soil profile from Prague was a technosol which is soil whose composition is altered by urban and industrial activity. To detect Ag concentration in the fungi, the fruit-bodies of a specific species were cleaned, cut, and dried to a constant weight. These samples were then ground and analyzed by instrumental neutron activation analysis (INAA). This method involves exposing the sample to a neutron flux so it will produce characteristic gamma rays. Silver concentration in soil was determined with the same method. For A. bernardii, Ag concentration in the caps and stipes of 19 fruit-bodies was also measured using the same technique. The authors also did a meta-analysis of related literature. They included only papers stating Ag concentration values for ectomycorrhizal and saprobic fungi from pristine places. The data were separated into eight classes based on concentration ranges and compared to the original data the authors had collected from pristine places in another, unpublished, study.
In pristine areas, the median Ag concentration was 0.79 and 2.94 mg kg–1 in ectomycorrhizal and saprobic fungi, respectively. The macrofungi have higher Ag concentrations than soil suggesting that they are very potent in absorbing Ag from the soil. The effectiveness of absorption can sometimes be intense and leads to the term hyperaccumulation. A hyperaccumulator is a species with 100 times higher concentration than the surroundings. A few fungi from the Amanita genus, an ectomycorrhizal fungi hyperaccumulators. In Ag-polluted areas with Ag concentration in the soil around 26.9 mg kg–1, the median concentration for both types of macrofungi was 24.7 mg kg–1 unlike the differences between the two types of macrofungi in pristine areas. The highest Ag concentrations were found in the ectomycorrhizal fungi. The hyperaccumulators had concentration values of 287–692 mg kg–1and Boletus edulis had a Ag concentration of 206 and 242 mg kg–1. In saprobic fungi, the highest Ag concentrations were found in Lepista nuda with a value of 84–123 mg kg–1. This is the opposite case in pristine areas where ectomycorhizzal fungi, of the genus Amanita, have the highest Ag concentrations . The reason saprobic fungi have lower values is because they absorb other metals besides Ag. The best competitors for silver for uptake by saprobic fungi are probably cadmium and mercury. In soils containing these two metals, an increase in Ag concentration does not result in an Ag concentration increase in the fungi. Ecotmycorrhizal fungi, however, do differentiate between metals. As another study showed, these fungi near a gold deposit absorbed the Ag but not the gold. The results of the case study show that caps accumulate twice as much Ag as the stipes. The caps had a range between 81–544 mg kg–1 and the stipes has values between 31–303 mg kg–1. For both parts, there was positive correlation between Ag concentration and biomass.
Other results show that there is no current risk of metal poisoning associated with consuming fungi. The most common fungi on the market are Agaricus bisporus and their Ag concentration does not surpass 1 mg kg–1. There is even less risk of bioaccumulation in humans since mushrooms comprise a small part of the consumer’s diet.
There are many unknowns still remaining about macrofungi. A few unknowns are which tissues favor Ag accumulation, how age of the fungi plays a role in the Ag accumulation, or the purpose of hyperaccumulation. It appears that the reason why fungi are able to accumulate high concentrations of Ag is because they can store excess metal safely so that it does not intervene in cellular processes.

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