Fungal Pigments and Their Prospects in Different Industries

The public’s demand for natural, eco-friendly, and safe pigments is significantly increasing in the current era. Natural pigments, especially fungal pigments, are receiving more attention and seem to be in high demand worldwide. The immense advantages of fungal pigments over other natural or synthetic pigments have opened new avenues in the market for a wide range of applications in different industries. In addition to coloring properties, other beneficial attributes of fungal pigments, such as antimicrobial, anticancer, antioxidant, and cytotoxic activity, have expanded their use in different sectors. This review deals with the study of fungal pigments and their applications and sheds light on future prospects and challenges in the field of fungal pigments. Furthermore, the possible application of fungal pigments in the textile industry is also addressed.


Introduction
Color has always played an important role in the life of all organisms on Earth. Human life has become truly "colorful" due to the use of colors in all its aspects, including clothes, food, and furniture. Much archaeological evidence has shown that the use of pigments as coloring agents has been practiced since ancient times [1]. Pigments, especially synthetic ones, have occupied the entire market due to their wide range of applications in different industries since their discovery in the 19th century. Different attributes such as low production costs, ease of production, and superior coloring properties have largely contributed to the establishment of synthetic pigments in the market. However, the use of synthetic colors has been found to be detrimental to human health and the environment because of their many adverse impacts [2][3][4][5][6][7]. Many disadvantages of synthetic pigments, such as poor degradation, longer persistence, potential to cause cancers/allergies, etc., have increased the demand for natural, organic, and eco-friendly pigments in the current era.
The global response, as well as the demand for eco-friendly natural pigments, has significantly increased in recent decades due to their advantages over hazardous synthetic pigments. They are used as colorants, color intensifiers, additives, antioxidants, etc., in many industries including the textile, pharmaceutical, cosmetic, painting, food, and beverage industries [1,8]. In recent years, fungi have emerged among the prominent, eco-friendly sources of natural pigments. Easy processing, fast growth in cheap media, and weather-independent growth make them an excellent alternative to natural pigments. The present review highlights the role of fungi as small factories in pigment production and their potential application in different industries, including the textile industry. Table 1. Updated list of pigment-producing fungi and their respective pigments [25,61].

Fungal Species Pigments References
Cryptococcus species

Neurospora intermedia
Uncharacterized (yellow-orange), a mixture of carotenoids
Also, many studies have revealed the production of polyketide pigments (N-threonine rubropunctamine) and chlorinated azaphilone pigments (chaephilone-C, chaetoviridides-A, chaetoviridides-B, chaetoviridides-C) from marine fungal isolates of Talaromyces spp. and Chaetomium sp., respectively ( Figure 11) [72,82]. A recent study has reported a novel pigment, N-GABA-PP-V (6-[(Z)-2-Carboxyvinyl]-N-GABA-PP-V), along with N-threonine-monascorubramine, N-glutarylrubropunctamine, and PP-O from the marine-derived fungus Talaromyces albobiverticillius ( Figure 11) [131]. Many antarctic fungi have also been discovered to produces pigments of different chemical classes and characteristics. A number of yeast and filamentous fungi isolated from the different samples collected from Antarctic regions have been reported to produce a variety of pigments with different colors [86].

Optimization for Enhancement of Pigment Production
Most of the investigators have focused their study on the enhancement of pigment production from different fungal strains such as Monascus, Penicillium, Talaromyces, Fusarium, etc., by optimizing various fermentation parameters such as media, media composition, pH, temperature, light intensity, orbital speed, etc. [26,[132][133][134][135]. Some studies have reported about the assessment of the pigment production potential of different fungi on natural substrates (rice, corn, wheat, cassava, whole sorghum grain, dehulled sorghum grain, and sorghum bran) and on different agro-industrial residues (feather meal, fish meal, cheese whey, grape waste, soybean protein, soybean meal, chicken feather and rice husk, orange processing waste) [134,[136][137][138]. Enhancement in xylindein production was reported in Chlorociboria aeruginascens upon addition of test woods (Acer saccharum, Populus tremuloides, spalted P. tremuloides, and Ailanthus altissima) in agar-based media [33].
Some studies have also evaluated the effect of different sugar sources such as glucose, fructose, lactose, sucrose, and maltose on pigment production by the species of Monascus. Results of these studies have shown that maximum pigment production was acheived in media with fructose as a carbon source for M. purpureus, and lactose as a carbon source for M. ruber [132,139]. Studies have also discovered that the addition of different nitrogen sources such as ammonium, peptone, sodium nitrate, glutamic acid, monosodium glutamate, 6-furturylaminopurine, and tryptophan could enhance the yield of pigment, alter the hue of the fermentation liquid, and also improve light stability of the pigments of Monascus species [132,[140][141][142][143]. NaCl has been proved to be a very good enhancer that stimulates pigment production and inhibits citrinin production in M. purpureus without affecting the growth of the fungus [144]. A study on the effect of nutrients on pigment production of C. aeruginascens shows that high biomass but no pigment production was observed in media with high nutrient concentration, whereas low biomass and high pigmentation was observed in media with low nitrogen concentration [145]. Investigators have also found variations in the yield, color characteristics (hue and chroma values), and structure of the pigments of Monascus species with respect to the type of amino acids in the media [146,147]. Beside this, the pH of the media also plays an important role in pigment production. In the case of Monascus species (M. purpureus, M. major, and M. rubiginosus), pH optimization studies have shown that a low pH of the media increases pigment production [140,146,148]. Another study has revealed that the pH of the substrate plays an important role in melanin production by X. polymorpha, T. versicolor, Cerioporus squamosus (formerly known as Polyporus squamosus), Lentinus brumalis (formerly known as Polyporus brumalis), F. fomentarius and I. hispidus. The maximum pigment production was observed in the pH range from 4.5 to 5.5 [35]. Similar studies in other fungi such as Penicillium purpurogenum, P. aculeatum, A. niger, Altemaria sp., Fusarium sp., C. aeruginascens, have shown that the optimum pH for maximum pigment production varies with the fungal species in submerged fermentation [35,[149][150][151][152].
Along with chemical parameters, physical parameters such as temperature, light intensity, color of light, agitation speed, and oxygen supply have an impact on pigment production. Studies have also been reported showing the influence of temperature on the biosynthesis of pigments by certain fungal isolates such as M. ruber, T. purpureogenus (formerly known as P. purpurogenum), C. aeruginascens, etc. [150,152,153]. Enhancement of yellow pigment production in a Monascus anka mutant strain under submerged fermentation using a two-stage agitation speed control strategy (400 rpm followed by 300 rpm) has been successfuly reported [154]. A study has also revealed that a sufficient supply of oxygen is necessary for xylindein production by C. aeruginascens [152]. The impact of darkness and different color light on the yield of extracellular and intracellular pigment and biomass has been assessed by various investigators. Most of the studies have shown that incubation in total darkness resulted in enhanced biomass and pigment production [152,155,156]. Studies have also reported that there is an enhancement in the pigment production in the case of A. alternata and M. ruber when exposed to blue and red light, respectively [156,157], and in F. oxysporum when exposed to blue and green light [158]. In contrast, reduction in biomass and pigment yield has been observed in I. farinosa, E. nidulans, F. verticillioides, P. purpurogenum (currently known as C. farinosa, A. nidulans, F. fujikuroi, T. purpureogenus, respectively), and M. purpureus when exposed to green and yellow light [155]. Light intensity has also been found to influence the growth and pigment production of M. ruber under submerged fermentation [156]. Another study on the influence of moisture content of wood substrate on fungal pigment production in spalted wood was described. Based on the results, low moisture content stimulates the pigmentation in T. versicolor and X. polymorpha, while enhanced pigment production was observed at higher moisture content in the case of I. hispidus, L. brumalis (formerly known as P. brumalis), C. squamosus (formerly known as P. squamosus), and S. cuboideum [34,159]. Optimization of pigment production by simultaneously altering the physical and chemical parameters has been explored by many investigators. Several studies have reported an enhancement of the yield of pigment and biomass from different fungal genera such as Monascus, Penicillium, Fusarium, Alternaria, etc., when the physical and chemical parameters were simultaneously altered [104,133,135,158,[160][161][162][163][164][165][166][167].
Nowadays, co-culturing has been found to be an effective method for the activation of cryptic pathways via cell-cell interactions, which ultimately results in the production of novel secondary metabolites such as pigments from the fungi [168,169]. Studies have reported that the induction or enhancement in pigment production was possible using co-culturing of fungi with bacteria or yeast, but it was species-specific. In case of Monascus and A. chevalieri, co-culturing was found to be effective, whereas in case of F. oxysporum, the results were negative [158,170]. Co-culturing of C. neoformans with Klebsiella aerogenes led to synthesis of melanin by the fungus, using dopamine synthesized by bacteria [171]. Researchers have also found that many fungi produce different types of zone lines when co-cultured with other fungi. Zone lines are narrow, dark marks composed of pigments (primarily melanin) produced in decaying wood by fungi in response to other fungi, to self-isolate from other decaying fungi and protect their resources [172]. It has been observed that many white rot fungi such as T. versicolor, Stereum gausapatum, Bjerkandera adusta, X. polymorpha, and few brown rot fungi (Poria weirii, Piptoporus betulinus) produce zone lines upon detection of another fungus in their territory [173]. T. versicolor and B. adusta were found to be the best fungal pair which produce zone lines upon co-culturing, whereas X. polymorpha produces zone lines individually in the absence of other fungi [174]. This clearly reveals that the method of co-culturing of these fungi has a significant impact on their pigment production which supplies pigments used for coloring different types of woods in order to enhance their market value.
Various modes of cultivation and various methods and techniques of pigment extraction were investigated by several researchers to enhance fungal pigment production and recovery. Different strategies such as the use of different surfactants (Tween 80, Span 20, Triton X-100, and polyethylene glycerol polymer 8000), different solvents (acetone, acetonitrile, chloroform, cyclohexane, chloramphenicol, dichloromethane, dimethyl sulfoxide, hexane, isooctane, methanol, methyl sulfoxide, pyridine, tetrahydrofuran, and water), and potential extraction techniques (pressurized liquid extraction technique) have also been assessed, compared, and confirmed by researchers for the rapid extraction and enhanced recovery of pigments from submerged fermentation [72,134,[175][176][177]. Researchers also suggested the use of shake culture methods using water as a carrier instead of using wood-based malt-agar media for pigment production from wood-degrading fungi [178].
Genetic engineering techniques for enhanced pigment production in fungi have been reported [1,20,179]. Certain genetic approaches such as alteration or modifications of genes, cloning of genes, or elimination of non-essentilal genes (mycotoxins) have been investigated for increasing pigment production and reducing mycotoxins production in fungi [180][181][182]. The manipulation of biosynthetic pathways has also been investigated by researchers for boosting fungal pigment production. A study on F. graminearum has shown that the transcription factor AurR1 has a positive regulatory effect on the aurofusarin gene cluster, enhancing the production of aurofusarin [183].
A recent study on Monascus strains, revealed that transcription factors play an important regulatory role in pigment diversity [184]. More research on this aspect may lead to enhanced pigment production.

Applications or Biological Activities of Fungal Pigments
Many fungal pigments have been reported to have a variety of biological applications because of their different properties such as antimicrobial, antioxidant, anticancer, and cytotoxic activities in addition to coloring property [1,20,25,179]; however, the degree of purity of pigments investigated in the various studies is not always known.

Fungal Pigments as Food Colorants
The majority of work done on fungal pigments is related to their use as food colorants. The possibility of the use of fungal pigments in different industries, particularly in the food industry, has been revealed long ago by many researchers [9, 25,46,48,179,[185][186][187]. The potential of fungal pigments to be used as food colorants or as food additives in different food products has been assessed by many researchers [51,188]. Some of the fungal pigments have already entered into the market as food colorants such as Monascus pigments, arpink red from P. oxalicum, riboflavin from Ashbya gossypii, and β-carotene from B. trispora [12,25,189].

Fungal Pigments as Antimicrobial Agents
Numerous microbial pigments have been reported to possess many health benefits over synthetic pigments [8,14]. Several studies have proved that the pigments or pigment extracts of certain species of fungal genera (Monascus, Fusarium, Talaromyces, Trichoderma, Penicillium, and Aspergillus) and yeast R. glutinis possess antimicrobial activity against different pathogenic bacteria as well as yeast and fungi. All these studies suggest the potential use of bioactive pigments as food preservatives or as antibacterial ingredients in the food and pharmaceutical industries [19,66,70,82,135,166,[189][190][191][192][193][194]. Similarly, the antimicrobial potential against selected pathogenic bacteria of different types of fabrics (cotton, silk, etc.) dyed with pigments of fungi (A. alternata and Thermomyces spp.) has also been evaluated, and positive results of these studies suggest their possible use in producing specific products for medical application, such as bandages, suture threads, face masks, etc. [195][196][197].

Fungal Pigments as Antioxidant Agents
It has been reported that microbial pigments such as carotenoids, violacein, and naphthoquinones have antioxidant potential. Many review articles mention the antioxidant potential of pigments from certain fungi and yeast [1,17,20,179,198,199]. Studies on assessment of the antioxidant activity of the pigments of certain fungi such as Penicillium (P. miczynskii, P. purpureogenum, P. purpuroscens, Penicillium sp.), Fusarium sp., Thermomyces sp., Chaetomium sp., Sanghuangporus baumii, Stemphylium lycopersici, and species of Trichoderma (T. afroharzianum, Trichoderma spp.) confirm the promising antioxidant potential and their possible applications in the healthcare industry [71,97,192,200,201].

Fungal Pigments as Cytotoxic Agents
The cytotoxic activity of pigments of certain fungal isolates (F. oxysporum, T. verruculosus, and Chaetomium spp.) has been assessed by many researchers using different methods such as sour orange seeds toxicity assay or yeast toxicity test (YTT) using Saccharomyces cerevisiae, brine shrimp lethality bioassay, or cell counting kit-8 (CCK-8) assay. These studies confirm the possible application of pigments in different industries, especially in health and pharmaceutical ones [47,82,106,202]. A latest study on the evaluation of dermal toxicity of pigments of Thermomyces spp. and P. purpurogenum in Wistar rats has revealed the nontoxic nature of pigments and suggested its potential application in cosmetics and dyeing [203].

Fungal Pigments in the Cosmetic Industry
As the demand for natural products is increasing in the market, cosmetic industries are also in search of new types of natural pigments to replace synthetic pigments. Among the natural pigments, the use of fungal pigments is also rapidly expanding in cosmetics because of their advantages. Fungal pigments, especially melanin, carotenoids, lycopene, etc., have been reported for their application in cosmetics, sunscreens, sun lotions, sunblocks, face creams, anti-aging facials, etc. [1,206,207]. Excitingly, some of the fungal pigments (Monascus pigments and Monascus-like pigments) have already entered the market for their application in cosmetics such as skin conditioning and skin care products, lipsticks, etc. [25].

Fungal Pigments in the Textile Industry
The textile industry is the largest industry after agriculture in terms of economic contribution and employment generation. It majorly depends on synthetic dyes for dyeing different types of fabrics (cotton, silk, and wool). Currently, natural pigments from fungi, with their many advantages (eco-friendly, non-toxic, easy degradation, high colorfastness, high staining capability, etc.) over hazardous synthetic pigments, have proven to be a good alternative to the synthetic dyes in the textile industry. Many investigations have shown that organic pigments produced by fungi have extensive applications in the textile industry [1,5,8,18,25,207].
The literature reveals that only a handful of studies have investigated the application of fungal pigments in the textile industry, especially for dyeing different types of fabrics, such as cotton, silk, and wool. Various studies on the dyeing potential of pigments of different species of fungal genera (Monascus, Fusarium, Aspergillus, Penicillium, Talaromyces, Trichoderma, Alternaria, Curvularia, Chlorociboria, Scytalidium, Cordyceps, Acrostalagmus, Bisporomyces, Cunninghamella, Thermomyces, and Phymatotrichum) for different types of fabrics such as wool, cotton yarn, silk, polyester, and nylon have been reported [37,42,47,106,108,124,195,196,[208][209][210][211]. Studies on the dyeing potential of pigments from wood spalting fungi (red pigment from S. cuboideum, yellow pigment from S. ganodermophthorum, and green pigment C. aeruginosa) have shown the possible use of these pigments for deying bleached cotton, spun polyacrylic, spun polyamide (nylon 6.6), worsted wool, spun polyester (Dacron 54), and garment fabrics, because of their high stability and good colorfastness to washing [37,212]. Another study has revealed that natural oils cannot be used in conjunction with these fungal pigments, as these fungal pigments are unstable in natural oils [42]. Results of all these studies have shown that these fungal pigments have good color stability, colorfastness properties, and dye uptake potential. Moreover, these fungal pigments do not have any adverse effects on fabric and are non-toxic to human skin. Therefore, the scope of applications of fungal pigments has the opportunity to expand into the textile and clothing industry.

Fungal Pigments in Dyeing Woods or as Color Modifiers
Pigment produced by wood-decaying fungi such as T. versicolor, X. polymorpha, I. hispidus, S. cuboideum, B. adusta, C. aeruginascens, and Arthrographis cuboidea have been used for dyeing different types of wood samples to increase their commercial importance [173,174,213]. Researchers have successfully used the red, green, and yellow pigments obtained from S. cuboideum, S. ganodermophthorum, and C. aeruginosa, respectively, to attenuate the presence of blue stain on wood samples of Pinus spp. [39].

Fungal Pigments in (Opto) Electronics
A recent study of the (opto)electronic properties of blends of the pigment xylindein extracted from C. aeruginosa has revelaed that this pigment has high photostability and electron mobility in amorphous films, which suggests its possible use for the development of sustainable, organic semiconductor materials [214,215].

Conclusions
Several advantages of fungal pigments over synthetic pigments have increased the demand for fungal pigments worldwide in recent years. This increased public awareness, eco-safety, and health concerns as well as the application of strict environmental and ecological rules and regulations, have challenged researchers to undertake both qualitative and quantitative research on pigments derived from clean, eco-friendly bio-resources, such as fungi, having minimal ecological negative impacts. Therefore, there is a necessity to explore other novel, safe pigments from the diverse taxonomic group of fungi, to meet the existing demand of eco-friendly pigments. Though several fungal strains are known as pigment producers, a large number of fungi have not been systematically explored for their pigment-producing capability. Therefore, there is a great need to explore the vast fungal diversity for rare, novel, safe pigments, using appropriate tools and techniques. A review of the literature revealed that most of the studies focused on the application of fungal pigments in the food and healthcare industries; however, fungal pigments need to pass toxicity tests and quality tests and receive many regulatory approvals before their final entry into the market as food colorants or as drugs. Therefore, the application of fungal pigments in these areas is quite difficult.
Moreover, meager studies on the applicability of fungal pigments in other areas such as textiles, paints, varnishes, and daily household utensils leave immense possibilities to explore the indigenous diversity of fungi for their pigment production potential and their applications in different sectors, including the textile industry. In addition to the coloring properties, the biological properties of fungal pigments may open new avenues for their use in the production of valuable textiles for medical use. This provides an extensive area of exploration to identify natural, eco-friendly fungal pigments and develop their diverse applications to satisfy the public interest and market demand.