## 4 Discussion  
Morphological changes and the production of pigments in some fungi in contact with PMs, in a solid culture medium, have been reported for other metals such as Cd, Cu, Zn, Ni and Pb, showing that some fungal species reduce their growth due to metallic stress [52; 53]. Such is the case of the fungi _Trametes versicolor_ and _Stereun hirsutum_ which, when grown in a solid medium with Cd (1 mM), showed several morphological changes in their mycelium and produced a brown pigment [52]. In the case of Cu (6 and 10 mM), the fungi _Wolfiporia cocos, Laetporus sulfureus_ and _T. versicolor_ showed significant modifications in the intensity of the color (from white to brown) and the texture (from cotton to downy) of the mycelium [54]. In this sense, pigment production has been related to the survival mechanisms used by fungi to alleviate metallic stress; it has been observed that higher concentrations of metals lead to a greater number of produced pigments [55; 56]. The present investigation found that Au was more toxic to the tested filamentous fungi than Ag and Pt. Some studies with other microorganisms have found that the presence of Au\({}^{0}\) and Au\({}^{3+}\) ions  
Figure 9: Morphological characteristics of four filamentous fungi exposed to four doses of platinum for 20 days. (**A**) _Trichoderma harzianum_ MX2, (**B**) _Aspergillus niger_ MX5, (**C**) _Hypocrea lixii_ MXPE12 and (**D**) _Fusarium oxysporum_ MX17. in trace amounts may not affect the growth of the microorganisms due to the fact that Au ions are usually deposited in the cell wall and the periplasmic membrane [57]. Therefore, this could explain why the used fungi did not show any growth inhibition for the concentration of 50 mg L\({}^{-1}\). However, [PERSON] and [PERSON] [58] indicate that the growth of _Saccharomyces cerevisiae_ is inhibited in culture media with Au at doses higher than 39.4 mg L\({}^{-1}\), suggesting that the used fungi are more tolerant than the mentioned yeast. As in the present study, [PERSON] et al. [59] found that the fungus _A. niger_ seems to be quite tolerant to five organometallic compounds of Au, which could indicate that, within the _Aspergillus_ genus, the _niger_ species are capable of tolerating a variety of Au compounds. Regarding the utility of tolerance tests for the selection of filamentous fungi in the recovery of Au from secondary or primary sources, [PERSON] et al. [38] carried out previous tolerance tests for Au (0, 50, 100, 300 mg L\({}^{-1}\)) from several species of the gender _Aspergillus_ for their Au bioleaching experiments from electronic residues, finding that the consortium of the most tolerant fungi managed to leach 87% of this metal. Another example of the usefulness of these tests is the pigmentation that _F. solani_ MXPE15 presented before 50 mg L\({}^{-1}\) of Au; in this situation, the fungus was tested in a liquid medium with 30 and 37 mg L\({}^{-1}\) Au, showing that it is capable of precipitating this metal [60]. Furthermore, [PERSON] et al. [61] reported that _A. niger_ MX5 in combination with _Sphingomonas_ sp. is capable of bioleaching approximately 1% of Au from the printed circuit boards (PCB) of cell phones. Regarding Ag, it has been reported that fungi isolated from nearby silver mine soils can tolerate concentrations above 170 mg L\({}^{-1}\) without any problem, but there has been no report on whether the fungi show morphological changes in their growth [62]. This information could only be compared with more Ag-tolerant fungi such as _T. koningiopsis_ MX11, _T._ sp. MX13 and _T. viridescens_ MX1. Concerning the usefulness of tolerance tests with silver, [PERSON] [63] reported that the fungi _H. lixii_ MXPE12 and _F. oxysporum_ MX17 are capable of bioleaching 0.02% of Ag from printed circuit boards from computers. Besides, the same work showed that _F. solani_ MXPE15 was able to leach 1% of Pd, another precious metal.