# Fungi and Arsenic: Tolerance and Bioaccumulation by Soil Saprotrophic Species
## 4 Discussion  
This work evaluated the potential of four saprotrophic fungal species to tolerate high sodium arsenite concentrations, accumulate arsenic in their biomass, and release siderophores with an affinity for the complexation of As and Fe. The tested species, even though they have been previously reported for their potential in the bioremediation of other toxic elements [32; 33; 34; 35; 36; 37; 38; 39], have never been studied before for their tolerance to As. The tested species were tolerant to all of the tested As concentrations, as pointed out by the tolerance indexes in almost all experimental conditions (Table 3). In fact, only _A. spinosa_ was very sensitive to increasing As concentrations on MEA, despite it growing in all tested conditions (Table 3). Since this sensitivity was observed only on MEA, this is clearly related to the differences in nutritional conditions. In particular, it may be determined by the influence of one or more interacting factors including increased As bioavailability, reduced nutrient bioavailability, formation of toxic As organic species by interaction of arsenite with MEA components, and inhibition of fundamental enzyme activities due to As(III) reactions to thiols [61]. In fact, malt extract generally possesses a rich composition of carbohydrates, proteins, peptides, amino-acids, and vitamins [62; 63], which require an efficient extracellular enzyme system to be metabolized. On the contrary, CDA grants a greater accessibility of nutrients as it is characterized by a composition mainly based on salts including sodium nitrate as the N source and sucrose as the main C and energy source. However, the importance of the nutritional conditions in determining the tolerance to As seems to be species specific. In fact, contrary to _A. spinosa_, _C. nanum_ was not affected in tolerance to As by the differences in nutritional conditions, despite, in all conditions including the controls, it showing a greater diametric growth and an higher biomass production on CDA than on MEA (coherently with the higher accumulation of Fe, K, Mg, Na, P, and S) (Tables 1 and 2). Different patterns of growth between the different nutritional conditions were observed in _M. marquandii_, which showed a higher diametric growth on CDA than on MEA, but not a significant greater biomass production. However, it was able to tolerate As in both media. Considering the equal biomass production in _M. marquandii_ on both media, the greater development of the colony on CDA suggests a greater presence of exploration hyphae with a lower number of branches and, therefore, a lower density. It is also worth mentioning that _M. marquandii_ and _C. nanum_ were slightly stimulated in the presence of sodium arsenite, as the tolerance indexes were higher than 110% (Table 3). Given that toxic elements can influence the physiology and morphology of fungal mycelia, resulting interactions can lead to different mycelial growth strategies and hyphal aggregation [64; 65]. Adaptative fungal growth patterns, observed at high metal concentrations in different fungi, led to growth exceeding the control and tolerance index higher than 1 (100%) [20]. High fungal tolerance to sodium arsenite is even more noteworthy, if it is considered that chemical species of arsenic(III) are thought to be more toxic for these organisms than arsenic(V) ones [61; 66]. High fungal tolerance to toxic elements may be related to metabolism-dependent or -independent mechanisms of tolerance/resistance to cope with chemical stress [64; 67]. Despite there being no previous data on the As tolerance of the tested species, several fungal species have been reported as being tolerant to very high concentrations of As trivalent and pentavalent chemical species [19; 21; 44; 68; 69; 70] including As minerals [3; 11]. For instance, [PERSON] [44] reported that _Aspergillus candidus_ was very tolerant to 25 and 50 mg/L of sodium arsenite and sodium arsenate, showing a luxviant growth in all treatments. In another work, [PERSON] et al.