# Fungi and Arsenic: Tolerance and Bioaccumulation by Soil Saprotrophic Species
## 4 Discussion
[19] reported values of tolerance index around 1 for taxa belonging to different genera including _Aspergillus_, _Penicillium_, _Rhizopus_, and _Trichoderma_, which were tested to high concentrations of sodium arsenate up to 10,000 mg/L. Other mechanisms that result from interactions between fungi and toxic elements include accumulation of metals and metalloids, and the production of extracellular enzymes and other metabolites such as organic acids and siderophores [64; 71]. The dominance of As species in the environment is strictly dependent on pH and redox potential (E\({}_{\text{h}}\)). Under reducing conditions and pH less than 9.2, the arsenite species H\({}_{3}\)AsO\({}_{3}\) predominates, while under oxidizing conditions, the arsenate species H\({}_{2}\)AsO\({}_{4}\)\({}^{-}\) and HAsO\({}_{4}\)\({}^{2-}\) are dominant at low and high pH, respectively [7]. Arsenic additioncan alter the pH of the medium and its presence, influencing the fungal metabolism, which can indirectly determine pH modification during the fungal growth. In this study, none of the tested species at any concentration showed a significant pH modification compared the control, nor did the addition of sodium arsenite change the pH of the media in the chemical blanks (Table 4). Conversely, the pH variations due to the fungal growth were strongly related to the tested medium, with the pH values of all tested conditions on MEA generally more acidic than those on CDA. Once again, this may be explained by the composition of the different media. Moreover, the pH was strongly related to the metabolism of the tested species (Table 4). In fact, _A. spinosa_ acidified both media more than the other species. This may be due to the release of organic acids by this species, which can excrete gluconic acid [72]. Other species of the _Absidia_ genus can produce organic acids such as _A. coerulea_ that can release citric, tartaric, succininic, and glutaratic acids [73]. Fungi can produce and release organic acids (e.g., oxalic, citric, and lactic acids) according to the growth conditions including the C and N sources, the availability of nutrients, and the presence of trace elements [3; 67; 74; 75]. The pH is another important factor that can influence organic acid production and can play an important role in the transformations of minerals and elements by fungi, controlling hydrolysis and complexation, mineral dissolution and precipitation, medium acidification or alkalinization [3; 67; 71; 75]. However, acidification can also be due to other mechanisms including proton excretion through plasma membrane ATPase, uptake of cations in exchange for H\({}^{+}\), and CO\({}_{2}\) production for fungal respiration [64]. Unlike the situation for _P. lilacinum_, that on MEA slightly lowered the pH while on CDA it was slightly increased, showing a greater influence of the medium than any other species (Table 4). In fact, the medium pH around 6 for _P. lilacinum_, _M. marquandii_, and _C. nanum_ on CDA (Table 4) may be related to different factors such as the buffering effects of CDA components (e.g., phosphate) and ammonium and lipase production [19]. As well as the assimilation of nitrate, the N source in CDA, can result in medium alkalinization for the release of OH\({}^{-}\), as observed with _Penicillum radicum_ and _P. cyclopium_[76]. pH modulation by fungi is important in the regulation of several enzymes, as reported, for instance, in the case of pathogenic fungi [77; 78]. For example, in the entomopathogenic fungus _Metarhizium anisopliae_, the production of ammonia, increasing the medium pH, allowed the production of subtilisin proteases, which were active only at basic pH [79]. All tested species were able to accumulate high concentrations of As in their biomass during the seven days of growth (Table 5). In _C. nanum_, As concentration was the highest at 50 mg/L sodium arsenite in both media, while _A. spinosa_ showed the highest As content. This difference is mainly due to the biomass production, which was higher in _A. spinosa_ than in _C. nanum_ (Table 2).