From culture collections to biorecovery of strategic elements: exploring fungal interactions with Germanium and Indium

Fungi in nature play crucial roles in geology and ecology, and represent valuable resources for sustainable recovery processes1. Fungi can tolerate, bioaccumulate, and transform toxic compounds and several strategic elements, e.g. germanium (Ge) and·indium (In). These are elements of fundamental interest in high-tech products, and their extraction often heavily impacts ecosystems.Their fungal-mediated recovery from electronic wastes offers an eco-friendly solution to mitigate environmental impacts and support sustainable resource management2. In this study 24 fungal isolates were selected based on available literature on fungi-element interactions, known metal-fungi mechanisms, and biological and ecological features. These fungi were chosen among those preserved in the culture collections of the Fungal Biodiversity Laboratory at Sapienza University of Rome and CNR mycology laboratories. Since siderophores are crucial in metal-fungi interactions, the selected fungi were screened for their production through the ChromeAzurol-S (CAS) assay3. Moreover, considering the importance of laccase in bioleaching processesand the fact that these elements share chemical features with those playing primary roles in fungalbiology, a screening to evaluate the production and release of laccase was carried out through theGuaiacol assay 4. Finally, fungal tolerance to Ge and In was evaluated using exposure to 5 mM Ge as GeO2 or 2.4 mM In as In2O3. The screenings were carried out on solid Raper Hoffman medium for 14 days at 25°C. The Rt:Rc (%) index, based on colony areas, and the T.I. (%) index, based on dry weights5, were calculated to evaluate fungal tolerance. The variation in medium pHwas also assessed. Most of the strains were able to produce siderophores, with 7 showing high response, whereas laccase release was only detected in 8, all basidiomycetes. Finally, all fungi were able to grow in all tested conditions, even though several evident Ge/In effects were observed.