## 2 Fungal Pigments Found on Paper
### Polyktide Colorants  
The important natural products encompassing a variety of toxins, antibiotics, therapeutic compounds, and colorants are represented by a group of chemical substances called polykeptides. Due to their biosynthetic complexity and importance in the pharmaceutical industry, polykeptides have drawn great attention among researchers, which has led to extensive molecular genetic studies carried out in actinomycetes and Gram-positive bacteria [57]. Polykeptides are one of the largest and most structurally diverse classes of naturally occurring compounds with a variety of biological activities [58]. These compounds are biosynthesized from acyl CoA precursors catalyzed by polykteid syntheses (PKSs) [59] that are members of a large enzyme family that are involved in the synthesis of different secondary metabolites such as pigments, toxins, antibiotics, and signaling molecules [60]. Pigments produced by fungi are mostly the result of the paucity of nutrients when mycelium produces secondary metabolites [61]. Fungal colorants are either polykteide- or carotenoid-based. Those that are polyktetide-based are composed of tetraketides and octaketides with eight C\({}_{2}\) units, creating a polykteide chain [62]. Polyketides comprise azaphilones and quinones, including anthraquinones, hydroxyanthraquinones, and naphthoquinones [55; 62]. Azaphilones can be defined as a small but structurally diverse subset of the polyketide class that are excreted by fungi as second metabolites [55; 63]. Due to their affinity for ammonia, the name azaphilones was given to these substances. In addition, azaphilones react with amines such as proteins, amino acids, and nucleic acids to construct red or purple vinylogous \(\gamma\)-pyridones due to the exchange of pyrane oxygen for nitrogen [55]. Fungi possessing these compounds show bright yellow, red, or green colors of mycelia. These polyktetide derivatives are produced by a variety of representatives of ascomycetes, including the genera _Aspergillus_, _Penicillium_, _Chaetomium_, _Talaromyces_, _Pestalotiopsis_, _Phomopsis_, _Eremicella_, and _Epicoccum_, as well as _Monascus_ and _Hypoxylo_[64], which may colonize paper and cause the appearance of colored spots [2]. Quinones are a class of quinoid compounds widely present in nature [65]. They are derived from aromatic compounds by the conversion of an even number of methane (-CH=) groups into carbonyl (C=O)groups with any necessary rearrangement of double bonds, resulting in a fully conjugated cyclic dione structure [66]. Those biosynthesized by the polykteide pathway have been isolated from fungi showing different colors, from yellow and orange to red. However, the color of the quinone isolated from a fungus does not reflect the color of that fungus. The reason for this may be that the quinone is accompanied by its reduction products, which lead to the formation of the quinhydrone complex [67]. The most produced quinones in fungi are Hydroxyanthraquinoid (HAQN) colourants. These colorants are derivatives of quinones, which are widely distributed in microorganisms. In particular, they have been produced mostly by the genera _Penicillium_ and _Aspregillus_, with a different color tone [68]. Specifically, _Aspergillus glaucus_, _Aspergillus versicolor_, and _Penicillium pururogenum_ have been conspicuous anthraquinone producers [69]. The shade-like appearance of a specific anthraquinone called chrysophanols has been reported to be a consequence of the methanolic processes of _Trichoderma_ with the species _T. harzianum_, _T. polysporum_, _T. viriidae_, and _T. aureoviride_[70]. Chrysophanols are peculiar anthraquinones giving the shade-like appearance of different colors due to the presence of chromosphere for absorption of electronic spectra at diverse wavelengths [71]. Naphthoquinones, widespread quinones with naphthalene backbones, have been shown to possess biological activities such as antibacterial, antimuoral, antiviral, and antifungal activities [69; 72; 73; 74].