Fungal strains and interactions with metals: studies in different contexts

Heavy metal contamination in soils and water is an emerging problem. These elements, which are toxic even at low concentrations, can interact with living organisms, showing mutagenic and carcinogenic activities. This problem affects in particular the developing countries, where the population is in direct contact with highly contaminated water and food resources and thus exposed to real risks to their health. Among the many consequences of this phenomenon, the contamination of agricultural soils is a serious problem for environmental integrity and food safety. The accumulation of heavy metals in these soils affects soil fertility, reducing crop yields and ending up in food through the food chain. The sources of contamination are numerous and include both natural and anthropogenic activities. Many natural processes contribute to the deposition of trace metals, including rock erosion, forest fires and geological activities such as volcanic activity. However, the release of metals resulting from human activities, from the excessive use of chemical fertilisers and pesticides to the extraction and exploitation of resources and the production of waste, has increased significantly in recent decades. Effective remediation strategies are needed to meet these problems. The remediation techniques developed in recent years are numerous and of various types, mainly based on the use of selective chemical-physical methods. However, they can be expensive, with a high energy consumption and a release of residual products. Emerging bioremediation methodologies are aimed at decontaminating and preserving the environment through the use of selected organisms. Promising techniques include mycoremediation, which uses fungal organisms in the treatment of contaminated substrates. Indeed, fungi are metabolically predisposed to rapid growth and can absorb and accumulate harmful substances without producing any waste. Their mycelial structure allows them to colonize large areas efficiently, making them a practical and accessible solution for soil decontamination. The thesis is structured as follows: Chapter 1 introduces the issues related to environmental contamination by metals and the factors influencing their bioavailability; the interaction between fungi and metals; the differences between conventional decontamination methods and mycoremediation; the objectives of the thesis. Chapter 2 presents the main materials and methods of different experiments. Chapters 3 and 4 focus on the study of fungal interactions. Specifically, Chapter 3 examines the tolerance and bioaccumulation of arsenic from contaminated groundwater by native fungi compared to selected non-native fungi. Chapter 4 provides an analysis of the potential mechanisms of tolerance and accumulation in fungi from soils with varying degrees of heavy metal contamination. The focus is on arsenic, cadmium, chromium and copper. An innovative method of washing the fungal biomass was also assayed. Chapter 5 focuses on the characterization of the functional traits of fungi as potential plant growth-promoting fungi (PGPF). Subsequently, the application phase of mycoremediation was designed, simulating a paddy field environment within a greenhouse. The objective was to evaluate the effects of two distinct fungal inocula on three rice varieties under two different cultivation regimes in soil with high concentrations of heavy metals. Through this work, the study and the application of fungal strains in an important agricultural context such as rice cultivation were further explored. The findings highlighted the importance of selecting fungal strains based on metabolic and functional traits to enhance the effectiveness of bioremediation strategies, enabling not only the reduction of soil pollution but also improvements in plant health and harvest quality. Chapter 6 is dedicated to the mycological characterization of three different paddy soils with varying levels of heavy metal contamination. Chapter 7 focuses on a study of the fungal community in serpentinitic soils, which are naturally rich in potentially toxic elements (PTE) such as nickel, cobalt, and chromium. The results obtained through culture-based techniques were collected and compared with metabarcoding data. Finally, Chapter 8 draws conclusions and discusses the significance of the results obtained, highlighting their implications for future developments.