In this post, Jochen Zubrod talks about his recent paper “Inorganic fungicides as routinely applied in organic and conventional agriculture can increase palatability but reduce microbial decomposition of leaf litter

The use of fungicides is considered an indispensable measure to secure global food supply. However, as an unwanted consequence of their application, these compounds also enter freshwater ecosystems, where they, due to their mode-of-action, likely affect aquatic microorganisms and thus disrupt important ecological functions. Despite these potential implications in the field, aquatic microorganisms are largely ignored during the environmental risk assessment of fungicides, which is why the European Food Safety Authority recently called for suitable test designs involving relevant representatives of this group of organisms.

Stained aquatic hyphomycete spores
Stained aquatic hyphomycete spores.

Test designs using aquatic hyphomycetes would be highly suitable as this group of fungi accomplishes two fundamental ecological functions in the course of the breakdown of terrestrial-borne leaf litter in streams: first, they contribute to leaf processing by direct decomposition and second, they condition leaf material, which means their activity transforms leaves into a more palatable and nutritious food source for leaf-shredding macroinvertebrates and thus supports the energy-flow to higher levels of the aquatic food chain.

Top view of a food-choice arena used to assess the palatability of leaf material
Top view of a food-choice arena used to assess the palatability of leaf material

Against this background, we tested a recently developed experimental design for its application to fungicide risk assessment. Leaf material that was microbially colonized under control conditions or in the presence of fungicides was used to quantify microbial leaf decomposition and assess fungicide-induced alterations in microorganism-mediated leaf palatability via the food-choice of a typical leaf decomposer, the amphipod Gammarus fossarum. To facilitate mechanistic understanding of potential functional effects, we characterized leaf-associated microbial communities.

As model substances we used four inorganic (three copper- and one sulphur-based) fungicides as these are routinely applied not only in conventional but also in organic farming. We found that all tested inorganic fungicides reduced microbial leaf decomposition by up to 30%. Moreover, Gammarus unexpectedly preferred copper-exposed leaves over unexposed ones, which was likely related to uncovered alterations in leaf-associated fungal communities.

Any effects on microorganisms involved in leaf litter breakdown may have far-reaching consequences for detritus-based food webs due to their bottom-up regulation. Thus, the uncovered structural and functional changes in leaf-associated microbial communities at inorganic fungicide concentrations realistic for surface water bodies impacted by conventional or organic farming indicate the need for a careful re-evaluation of the environmental safety of the agricultural use of these compounds. Moreover, inclusion of an experimental design similar to the one used in the present study in environmental risk assessments of fungicides or biocides may aid to safeguard the integrity of aquatic microbial communities and the essential functions they provide.

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