## 3 Discussion
Another likely remarkable pathway by which VOCs are produced includes the degradation of l-phenylalanine and l-tyrosine, whose oxidative processes result in the emission of 2-phenylethanol [51] produced by all three species of _Malassezia_ only in the  
Figure 6: Different metabolic pathways can produce volatile organic compounds (VOCs) in _Malassezia_ species. Representative examples are given per class of volatiles [13]. stationary phase. It would be worth performing studies to elucidate the production of this compound in this development step. In addition, 2-phenylethanol seems to be a common compound released by several microorganisms [11; 42; 43; 44]. The identification of another VOC, 2,5-dimethylpyrazine, in all species mainly in the stationary phase (except _M. restricta_, which presents it in both phases) suggests that _Malassezia_ likely metabolize L-tryptophan [53]. This volatile draws much attention because it has been reported that during infection, bioactive indoles, such as malassezin, indirubin, indololcarbazole, and formylindolcarbazole (ligands of the aryl hydrocarbon receptor (AhR)) produced by _Malassezia_ from tryptophan, could be involved in melanogenesis. For instance, malassezin can cause the apoptosis of melanocytes [54]. This suggests that VOCs produced from tryptophan may play an important role in infection processes. To summarize, the results presented here show that VOCs are very important compounds produced in the main metabolic processes carried out by _Malassezia_. They could direct different interaction processes with the host, so it is necessary to pay more attention to these compounds. Regarding the intent to determine which VOCs released by _Malassezia_ have a possible ecological role, for other microorganisms, it has been reported that VOCs are important in interaction processes. For instance, 3-methylbut-3-en-1-ol and 2-methylpropan-1-ol are involved in biofilm formation in _Pseudomonas aeruginosa_ and methicillin-susceptible _Staphylococcus aureus_ (MSSA) [55]. Notably, 3-Methylbut-3-en-1-ol is emitted by _M. globosa_ and _M. restricta_ only in the stationary phase, and all species produce 2-methylpropan-1-ol in the same phase, even in _M. furfurfur_[12]. Moreover, 3-Methylbutan-1-ol is also implicated in the formation of bacterial biofilms [55], and it has been identified in biocontrol yeast [21] and in association with plants [56; 57]; all _Malassezia_ species produce it in two phases. It is possible that if these compounds are implicated in the interaction between _Malassezia_ and the microbiota, they must be produced in a mature growth phase or even during growth. Other compounds related to the interaction between bacteria and fungi are sulfur and pyrazines emitted by _Pseudomonas aeruginosa_. These compounds influence the growth of _Aspergillus fumigatus_[22]. _Malassezia_ releases similar compounds, such as 2,5-dimethylpyrazine. Compounds such as 2-methylbutan-1-ol produced by _Saccharomyces cerevisiae_ or 2-methylpropan-1-ol produced by _Muscodor albus_ have antifungal activity [40] and were also identified for the _Malassezia_ species. Other volatiles identified for _Malassezia_ in this study are 2-methylpropan-1-ol, hexan-1-ol, and 2-methylpropanal, and they are common compounds reported by several microorganisms and related to biological interactions [40; 45; 58; 59]. Further research is required to elucidate the biological role of VOCs in biological interactions and the metabolism of _Malassezia_. Finally, it is interesting to note that VOCs are responsible for the odor of fruits and food; for instance, octan-3-ol and octan-3-one generate a sweet odor [38], and microbial VOCs are probably responsible for the earthy smell and body odors of humans. Many mushrooms have distinctive odor profiles [17]. Moreover, the odors released by microbes influence the attraction of insects, functioning as pheromones; for example, the odors emitted by grape-born yeasts positively affect moth oviposition and help insects locate hosts [11].