## 3 Results and Discussion
### Fungal Colonization of Solid Substrates and Impact on Their Chemical Composition  
One of the significant constraints inherent in solid-state bioconversions, compared to those in the liquid phase, is represented by the impossibility of directly determining the microbial biomass [46]. Contrary to the submerged fermentation, in which the fungal biomass is easily separated from the culture medium and determined by gravimetry, this approach is not possible in solid-state conversions on natural substrates. Therefore, it is necessary to resort to indirect approaches based on either molecular or chemical markers [46]. The most used chemical \"markers\" for fungi are ergosterol and D-glucosamine. In the first case, the solid material is subjected to saponification, followed by extraction with n-hexane and quantification of ergosterol by reverse-phase chromatography [20]. The second parameter's determination requires acid hydrolysis of the material, deacetylation, and quantification of the D-glucosamine, released by chromatography or spectrophotometry [24]. However, D-glucosamine levels vary in fungal species in a range of from 67 to 125 mg g\({}^{-1}\) of biomass, depending on the culture's species and age [14; 47]. Although aware that cultivation conditions can also influence this parameter's levels, we adopted D-glucosamine as an indirect indicator of colonization levels, bearing in mind that the comparisons took place within the same fungal species and between cultures of the same age. We also considered the possible presence of D-glucosamine residues which are sometimes associated with glycoproteins in the lignocellulosic substrates [14]. Although this parameter's levels were significantly low in the incubation controls, varying in the range of from 0.03 to 0.18 mg g\({}^{-1}\) substrate (data not shown), these were subtracted from those determined in solid-state cultures. Table 2 shows that, among the non-amended cultures, the lowest growth was observed on barley straw, while the colonization levels did not differ significantly from each other in the remaining substrates, ranging from 0.79 to 0.94 mg D-glucosamine g\({}^{-1}\) substrate. On the contrary, an element that emerged from the D-glucosamine data analysis was the stimulating effect of WB on fungal growth in the amended cultures compared to non-amended ones except those carried out on DSF. This stimulation effect was of variable magnitude depending on the substrate. In particular, in solid-state cultures on WB-amended barley straw, the colonization levels increased by a factor of more than four compared to non-amended counterparts (2.99 \(\pm\) 0.5 vs. 0.70 \(\pm\) 0.04 mg D-glucosamineamine g\({}^{-1}\)), and visual inspections corroborated the observed data. A loose mycelium with a limited extension to the most peripheral layers of the substrate characterized non-amended cultures on barley straw. Conversely, in WB-amended cultures, the mycelium appeared to be more compact, and the colonization affected the entire substrate mass. In WB-amended cultures on RTB and SCG, the factors of increase in D-glucosamine levels compared to homologous non-amended cultures were equal to 2.2 and 2.7, respectively. In _A. olivimuriae_ solid-state cultures, colonization led, compared to incubation controls, to the slight acidification of some matrices, such as in RTB (6.13 \(\pm\) 0.02 vs. 6.93 \(\pm\) 0.03) and, to a greater extent, in SCG (4.02 \(\pm\) 0.05 vs. 5.77 \(\pm\) 0.04), and this effect was also observed in the respective WB-amended cultures. In those carried out on barley straw, the pH values did not differ significantly from those of the relative incubation control while, on the WB-amended ones, slight acidification took place (6.66 \(\pm\) 0.02 vs. 6.88 \(\pm\) 0.01).