## 4 Discussion  
Over the years, many studies have focused on the selection of fungal strains capable of degrading hydrocarbons and the potential of _Trichoderma_ in this field [26; 27; 47; 48]. The aim of our work, however, was to compare the bioremediation actions of _Trichoderma_ strains from different areas of the world  
Figure 5: Degradation of used engine oil by _T. harzianum_ F26 at Time 0 (T0) and after 45 days (T45). The compounds were grouped into microclasses. A red asterisk indicates statistically significant data (two-tailed (Wilcoxon) Mann–Whitney \(U\) test). Figure 6: Colorimetric screening. (**A**) Gallic acid test for _T. aspellum_ F1020 (very high activity); (**B**) gallic acid test for _T. harzianum_ F26 (no activity); (**C**) guaiacol test for _T. asperellum_ F1020 (very high activity); (**D**) guaiacol test for _T. harzianum_ F58 (no activity); (**E**) Remazol Brilliant Blue R (RBBR) test for _T. harzianum_ F26 (very high activity); (**F**) RBBR test for _T. asperellum_ F1020 (no activity). It is possible to appreciate the dark halos around the colonies for the gallic acid and guaiacol tests and the decolorization of the medium for the RBBR test. \begin{table}
\begin{tabular}{c c c c} \hline  & **Guaiacol** & **Gallic Acid** & **RBBR** \\ \hline _T. asperellum_ F1020 & ++++ & ++++ & 0 \\ T. _harzianum_ F12 & ++++ & + & + \\ T. _harzianum_ F58 & + & 0 & + \\ T. _harzianum_ F26 & ++ & 0 & ++++ \\ \hline \end{tabular}
\begin{tabular}{c} 0 no activity; + activity; ++ high activity, ++ very high activity. \\ \end{tabular}
\end{table} 
Table 3: Results of the colorimetric testing of _Trichoderma_ strains. and isolated from contaminated and noncontaminated substrates. Moreover, while most of the works already published showed the ability of _Trichoderma_ to degrade crude oil or PAH mixtures [49; 50; 51], our investigation focused on the biodegradation of used engine oil. This pollutant is certainly very easy to find in the environment, as it is used daily all over the world, so finding appropriate bioremediation remedies for used engine oil could have a great beneficial impact. The tolerance test showed that all strains could grow on used engine oil, meaning that the pollutant does not totally inhibit their growth and that they could use it as a nutrient source. However, there were differences in the growth of the strains. _T. harzianum_ F26 was the best performing, while _T. asperellum_ F1020 was the worst. Moreover, in the GC/MS analysis of hydrocarbon degradation after 45 days, _T. asperellum_ F1020 was the only one that did not produce statistically significant changes in the used engine oil macroclasses, while _T. harzianum_ F26, F12, and F58 changed them significantly. This result can be explained by the isolation substrate origin: _T. asperellum_ F1020 was isolated from toxin-free hydroponic plant cultivation; on the contrary, the three strains of _T. harzianum_ were isolated from contaminated substrates with pesticides or hydrocarbons and could be naturally adapted to the use of recalcitrant substances for their growth. The literature reports several cases of _T. asperellum_ being able to degrade hydrocarbons efficiently. For example, [PERSON] et al. [26] reported the case of _T. asperellum_ removing large amounts of PAHs in soils contaminated with 1000 mg kg\({}^{-1}\) of a mixture of phenanthrene, pyrene, and benzo[a]pyrene, degrading 74%, 63%, and 81%, respectively, in 14 days. Similarly, [PERSON] et al. [52] showed how _T. asperellum_ TUB F-1067 (SA4), _T. asperellum_ Tt48 (SA5), and _T. asperellum_ TUB F-756 (SA6) strains can grow on a minimal substrate containing 1% (_v/v_) of used engine oil. However, these strains had been isolated from soil heavily contaminated with crude oil or used motor oil. Probably, highly tolerant organisms, having the capacity to use hydrocarbons as the only source of carbon, are more likely to adapt to a more polluted environment [53]. We also analyzed the composition of the used engine oil by grouping the compounds into microclasses.