# Fungi and Arsenic: Tolerance and Bioaccumulation by Soil Saprotrophic Species
## 1 Introduction  
Increasing arsenic (As) environmental concentrations are raising worldwide concern due to the impacts on human health and ecosystem functionality [1, 2]. In fact, chemical species of As can be toxic for living organisms as arsenate can compete with the essential inorganic phosphate, and arsenite can inactivate many enzymes by binding to protein thiols [3]. In humans, As can cause severe toxic effects to integumentary, cardiovascular, reproductive, and neurological systems, leading in extreme cases to the development of malignant tumors and death [4]. Arsenic is a naturally occurring metalloid that is widely distributed in the Earth's crust [3]. Primarily associated with igneous and sedimentary rocks, As is naturally found in the environment because of natural processes including weathering of As-enriched minerals, volcanic emissions, and biological activities [4, 5, 6, 7]. In several locations worldwide (e.g., Bangladesh, India, China, USA) high As concentrations in groundwater, drinking water, and soils are associated with natural geologicsources [1; 5; 6]. In many Italian areas, highly natural As concentrations in lakes, volcanic aquifers, drinking waters, rocks, soils, plants, and food have been reported to be associated with geological and environmental factors [8; 9; 10]. On the other hand, other important sources contributing to As environmental contamination are anthropogenic activities including, for instance, the use of agricultural pesticides, wood preservatives, and medicines, waste incineration, and mineral ore processing [3; 5; 11]. Indeed, in some Italian areas, former mining activities have contributed to an increase of As in the environment, leading to its high concentrations [12; 13]. In order to cope with As contamination, bioremediation and bioaugmentation using microorganisms can represent an efficient, environmentally-friendly, and cost-effective technological solution. In particular, fungi are promising bioresources in the remediation of As pollution [1; 2; 3; 11]. Fungi are important geoactive agents, playing very important geological roles in several processes including decomposition, biogeochemical cycling, element biotransformation, metal and mineral transformations, bioweathering, and soil formation [3; 14; 15; 16]. They are able to tolerate extreme and very limiting environmental conditions such as highly concentrated mixtures of toxic substances. Some species have been reported to accumulate high concentrations of arsenic and volatilize it via methylation [1; 2; 14; 17]. Several soil fungi isolated by As-contaminated soils have been successfully tested for As tolerance and removal [18; 19]. Among these, the majority belong to the genera _Aspergillus, Fusarium, Penicillin, Rhizopus,_ and _Trichoderma_[2; 19; 20; 21]. Most of the above-mentioned tested strains were isolated from contaminated sites such as agricultural soils, paddy fields, and mines mainly in China and India. In order to maximize the efficiency in mycoremediation applications, it is very important to focus on saprotrophic fungi isolated from soils with high As concentrations as they can be better equipped to survive and cope with the associated chemical stress [22; 23]. In this context, siderophores, which are low-molecular-weight organic compounds, are crucial for many of the above-mentioned roles played by fungi in several processes, especially for soil mineral weathering and biogeochemical cycles [24]. Their main role is to bind extracellular Fe(III), generally not available for organisms, and transport it inside the cell to meet the metabolic requirements [25; 26]. However, siderophores are remarkably effective in solubilizing and increasing the mobility not only of Fe, but also of a wide range of elements including Cd, Ni, As, and actinides [24; 25; 26; 27]. Consequently, in the last years, there has been an increasing interest in siderophore application for metal bioremediation [24].