Rapid industrialization in modern times has pressed the swift formulation and use of synthetic colorants in increased volumes in the food, medical, textile, and other industries regardless of their carcinogenic, immunosuppressive, and non-eco-friendly effects. Presently, many studies are focusing on finding alternatives to synthetic colorants, thereby improving the quality of the environment that affects various life forms. Nevertheless, advancements in biotechnology and the widespread awareness in ecological conservation, environmental protection, healthcare, etc. have generated a fervent interest among the public, industry, and researchers for exploring colorants from natural resources as an alternative to synthetic colorants.
Textiles is the key industry, among other industries, that uses diversified dyestuff. Lebeau et al.
[3] reports that the anguish dyestuff industry requires higher natural pigments that have led to the emergence of natural colorants from microbes
[4]. The increasing demand for pigments promotes multiplying research activities to explore filamentous fungi for pigment production. Research studies have shown that eco-friendly natural pigments from fungi are the best alternative to synthetic pigments because of their fast growth, easy processing, and important roles in transcription and intercellular communication. Fungal pigments have resistance and protect against biotic and abiotic agents (antagonistic microbes and UV radiation) and possess various biological applications
[5]. Fungal pigments also have possible opportunities for commercializing their pigmented biomolecules for their application in food, cosmetics, textile fabrics, etc., because of their versatility, structure, and ease for large scale cultivation
[6].
2. Historical Note on Pigments
The art of coloring has spread right from the development of human civilization. The use of pigments as coloring agents has been in vogue since prehistoric times. Europe and China, more than 5000 years ago, practiced dyeing with plants, leaves, barks, and insects. The Indus Valley-era, as early as 2600–1900 BCE (Before the Common Era or Before the Current Era), used madder dye to color garments at Mohenjodaro and Harappa. Roman centurions extracted red colorants from marine molluscs,
Murex sp. to dye tunics and Egyptians used natural indigo from the plant
Isatis tinctoria. Chinese Yanghai used alizarin, purpurin, and indigo in textiles in the Late Bronze Age (1700 BC). During the same Bronze Age, Phoenicians extracted Tyrian purple from the murex shellfish; though it was expensive, it was highly looked-for and so this colorant was the first initiation to global trade
[7]. Generally, natural dyes have a strong tradition in India, Turkey, Mexico, Morocco, Europe, China, and countries of West Africa. Thus, natural dyes and dyestuffs are closely associated right from human civilization and are as old as textiles.
Dyes from insects (cochineal and kermes) were common in the 15th century. Cochineal (crimson) dye from cactus insects was in use to dye clothes and as an artist’s pigment, and later in food items. France, Holland, and Germany started using plant dyes in industry in the 16th century, and England used wood to dye clothes in the 17th century. Following the gradual application of natural dyes in different parts of the world, quercitron, the pigment extracted from the inner bark of oak, was patented as a dyeing material in America in 1775
[8].
Later, in the 18th century, Swedish chemist, Scheele discovered that chlorine destroyed vegetable colors, and following that, indigo began to develop in England. A natural dye, cudbear, which was extracted from various lichen sources, was later patented. The use of saffron from plants and cochineal from animals to dye clothes have been reported, and natural colorants from plants and animals were used until the middle of the 19th century
[9]. About 95% of plants were characterized for plant pigments in America and Europe, however, due to several drawbacks to extract color from plants (stability, shelf life, etc.), researchers in the dye stuff industries looked forward to other alternate sources for colorants with better stability and shelf life.
In 1856, as an alternative to natural colorants, William Henry Perkin, a British chemist, discovered the first synthetic dye ‘mauve’ from artificial quinine. It was a favorite color of the royal family and so its importance consequently promoted the bloom of the innovative synthetic colorant industries. Thereafter, the industrial revolution rapidly propelled the production of synthetic colorants, which attracted various markets due to the easy manufacturing processes and production of superior coloring properties with less cost. Commercial dyeing industries, appreciating the undercurrent of the industrial revolution, switched over to synthetic dyes due to their production advantages and market potential, and consequently to date, synthetic colorants rule these industries, especially in the textile industry.
3. Ecology of Fungal Pigments
Fungi are diverse and abundant eukaryotic organisms on earth and their presence, even in extreme ecosystems, make it possible for them to produce novel secondary metabolites. Fungi inhabiting a plethora of ecosystems from terrestrial milieus to marine environments are omnipresent. They are spread across various eco zones, from polar to tropical regions and from aerial to deep-sea environments
[10]. Current studies have reported the production of new molecules, mainly new pigments from a marine environment. Several polyketide compounds with novel biological activity have been isolated from fungi in deep-sea environments
[11]. Marine ecological niches are still mostly unexplored and characteristics of marine ecosystems like salinity, low temperature, and dark induce microbes to produce novel metabolites. Tropical ecosystems are potential niches, and it is mainly mangroves that have the highest diversity of marine fungi because of their rich organic matter, which favors the production of valuable metabolites. Researchers have found that in extreme conditions, pigmented fungi could tolerate hydration/dehydration cycles and high radiation better than non-pigmented fungi. For instance, fungal melanin produced by many filamentous fungi has antioxidant activity, thereby protecting the structures, and provide durability to survive in aggressive environments. Hirot et al.
[12] reported a novel metabolite, a green pigment, terphenylquinone, from
A. niger isolated from Mediterranean sponge,
Axinella damicornis.
4. Fungal Pigments
Filamentous fungi produce amazing pigments like carotenoids, melanins, flavins, phenazines, quinones, and monascins from different chemical classes
[13]. Carotenoids and polyketides come under the classification of fungal pigments
[14]. Most fungi produce pigments that are water soluble and ideal for industrial production since it is easy to scale up in industrial fermenters and could be extracted easily without organic solvents (
Figure 1).
Figure 1. From Petri dish to fermenter: scale-up of pigment producing fungi. (A) Growth of pigmented fungi in agar medium; (B) Maintenance of pigmented fungi; (C) Scale-up of pigment producing fungi in fermenter.
5. Fungal Dyes for Textile Applications
The textile industry is the largest industry by economic contribution and employment generation. Fungal pigments have excellent colorfastness and staining properties and they are of great interest to textile industries; they warrant production under controlled conditions, with no seasonal fluctuations, and are biodegradable
[14]. The burden of reducing the hazards to the environment rests with the dyeing industries. In spite of the positive features of fungal dyes, it has not met the expectations of the textile industry because of irregular fixation
[76][15]. In addition, there are no standardized methods for the industrial dyeing of fungal dyes
[48,77][16][17]. Therefore, a proper method to standardize the industrial dyeing of fungal dyes and many novel fungal pigments should be taken forward for standardized industrial applications.
Fungal pigments, due to their stability and consistency, have been reported for their use as alternatives to synthetic dyes in the textile industry
[47][18]. These pigments can absorb light in the ultraviolet region and when applied as textile dyes, can protect human skin from harmful UV radiation
[78][19] (Nambela et al. 2020). The potential of fungal pigments in the textile industries has been investigated by various researchers
[79,80,81,82][20][21][22][23]. Through a biotechnological approach, anthraquinones are produced by fungi, namely
Trichoderma sp.,
Drechslera sp.,
Aspergillus sp., and
Curvularia sp. Cynodontin extracted from
Curvularia lunata successfully produced two anthraquinone dyes similar to Disperse blue 7 and Acid green 28, the characteristics of which are similar to conventional dyes
[83][24].
Anthraquinones have been reported to be produced from various fungi with antimicrobial activities. Anthraquinones from
Sclerotinia sp. produce a pink shade and dyed cotton yarns with chemical and natural mordants. The yarns showed excellent stability to heat, light, pH, and temperature
[84][25]. Osmanova et al.
[55][26] reported the existence of water-soluble red and yellow pigments from
T. australis and
P. murcianum with an affinity toward wool. Results of this study indicated that the dyes were suitable for industrial conditions as they could withstand temperature and pH. Chemical analysis reported that the red dyes were similar to
Monascus type pigments
[85][27].
Penicillium sp. produces ankaflavin (
Monascus type pigments) and the ionic nature has a strong affinity of dyes with wool
[86][28]. Nagia and El-Mohamedy
[87][29] reported that anthraquinones from
Fusarium oxysporum could be used for dyeing wool with excellent color fastness properties and high dye uptake. Anthraquinones produce bright hues with excellent fastness properties and chemical modification may be interesting if it could facilitate the synthesis of dye molecules.
Morales-Oyervides et al.
[88][30] reported the potential application of pigment from
Talaromyces sp. in the textile industry. The results confirmed the pigment to be a novel source to dye wool textiles. High pigment uptake by the fabric was observed, and the dyeing rate constant and half time dyeing with kinetic behavior well matched those of natural dyes used in the textile industry. The
Talaromyces pigment has potent antimicrobial properties coupled with the absence of toxicity, which makes this pigment a valuable alternative as a natural dye in textile dyeing
[89][31]. Further characterization of the molecule and study on the interactions between the dye and fabric are worth future research. Celestino et al.
[90][32] reported pigmented fungi
Penicillium sclerotiorum 2AV2,
Penicillium sclerotiorum 2AV6,
Aspergillus calidoustus 4BV13,
Penicillium citrinum 2AV18, and
Penicillium purpurogenum 2BV41.
P. sclerotiorum 2AV2 from soil from the Amazon produces intense color pigments, which could be used for textile applications.
Hinsch et al.
[91][33] reported that the fungal pigments isolated from rotting hardwood logs in Canada produced xylindein (green pigment from
C. aeruginosa), draconin red (red pigment from
S. cuboideum), and yellow pigment (
S. ganodermophthorum) and were able to dye multi fabric test strips. Their results indicated that all these pigments could be used to dye fabrics without the need for additional chemicals. Xylindein exhibited good potential to dye garment fabrics and draconin red for second layer garment fabrics. Spalting (wood-rotting) fungi have been the hot topic of research to extract novel pigments for textile applications. Awkwardly, dichloromethane (DCM) is used to extract colorants from spalting fungi and causes environmental problems and health issues, which is one of the major hurdles holding back spalting fungi from commercialization. Researchers have found that natural oils could be used to transfer pigments from
Chlorociboria sp.,
Scytalidum cuboideum, and
Scytalidium ganodermophthorum onto host substrates
[92][34].
Gupta et al.
[93][35] attempted to isolate pigment from
Trichoderma sp. and explored the possibility for dyeing silk and wool fabrics. The dye from
Trichoderma was non-toxic to human skin and possesses antimicrobial properties. The color value for the dyed sample was higher for wool when compared to silk fabric, and showed excellent fastness properties to washing and rubbing. Poorniammal et al.
[94][36] evaluated the yellow pigment from
Thermomyces sp. for the textile dyeing process. Natural mordants and the yellow pigment reduced the influence of pathogens. The overall color fastness properties for the dyed silk fabric was moderate. Due to these antimicrobial properties, this can be used especially in medical applications like bandages, masks, wound dressings, etc. Devi and Karuppan
[95][37] reported on reddish brown pigments from
Alternaria alternata for their efficiency to dye cotton fabrics. The pigments showed good color fastness properties to perspiration and rubbing. This was the first report to study the reddish brown pigments for
Alternaria alternata for dyeing cotton fabrics. Velmurugan et al.
[96][38] assessed different water-soluble fungal pigments from
Monascus purpureus,
Isaria farinosa,
Emericella nidulans,
Fusarium verticillioides, and
Penicillium purpurogenum for dyeing cotton yarns (
Figure 2). This study testified that pre-mordanting with alum and ferrous sulfate achieved variation in shade and color.
Figure 2. Water-soluble fungal pigments for dyeing cotton yarns. (
A) Growth of pigment producing fungi in liquid medium; (
B) Extraction of fungal pigments; (
C) Cotton yarns dyed with fungal pigments.
The reddish-brown pigment from
Phymatotrichum sp. (NRC 151) was produced using H-acid (1-naphthol-8 amino, 3,6-disulfonic acid) as a precursor in the medium that showed better fastness to washing, perspiration and light and can be used to dye various fabrics
[24][39]. Sharma et al.
[77][17] isolated
Trichoderma virens, Alternaria alternata, and
Curvularia lunata from different habitats for pigment production. The pigments applied to silk and wool showed good fastness properties. The optimum condition for maximum pigment production should be standardized for commercialization in an eco-friendly manner with a cost reduction.
Weber et al.
[48][16] isolated three eco-friendly fungal pigments from
S. cuboideum, C. aeruginosa, and
S. ganodermophthorum, which showed their textile dyeing capability without using water and thermal energy. The pigment showed no fading over one week’s time and further research is required to study the relationship between the pigments and fabrics, skin sensitivity, toxicity, and stability (UV, time, wash and wearing). Different fungal species with their active pigment for application in the textile industry are shown in
Figure 3.
Figure 3. Chemical structures of fungal pigments with potential coloring properties that could be used in textile dyeing.
6. Toxicity Testing for Fungal Pigments
Heo et al.
[105][40] studied the toxicity of fungal pigments extracted from
Penicillium miczynskii,
Sanghuangporus baumii,
Trichoderma sp. 1, and
Trichoderma afroharzianum. The pigments exhibited high radical-scavenging activity. Moderate toxicity was observed in
S. baumii by the acute toxicity test limiting the applications of this pigment in industry.
P. miczynskii,
Trichoderma sp. 1, and
T. afroharzianum were reported to be the best fungal pigment for producing strains for safe, water-soluble pigments in the industry.
The cytotoxic activity of fungal pigments from
F. oxysporum, T. verruculosus, and
Chaetomium sp. has been tested using various methods such as the yeast toxicity test (YTT), brine shrimp lethality bioassay, or cell counting kit-8 assay. This method of cytotoxicity assay warrants the application of pigments in various industries, especially in the health and pharmaceutical sectors
[87][29]. Poorniammal et al.
[106][41] studied the dermal toxicity of pigments from
Thermomyces sp. and
P. purpurogenum in Wistar rats and their results showed that the pigments were non-toxic and have broad scope in the dyeing, printing, and cosmetics industries.
The deep sea fungus
Chaetomium sp. AN-S01-R1 produces chlorinated azaphilone pigments like chaephilone C (compound
1) and chaetoviridides A-C (compounds
2–
4). Compound
2 exhibited potent cytotoxic activities toward HepG2 cells with IC
50 below 5 µM, whereas compounds
1 and
3 showed stronger cytotoxic activities against HeLa cells
[107][42]. To study the cytotoxic mechanism of pigments and their possible industrial applications, further research should be conducted.
The red pigment from
Talaromyces verruculosus was used to dye cotton fabrics with excellent dye uptake and color fastness properties. The cytotoxicity assay conducted using the brine shrimp lethality test revealed insignificant toxicity and was harmless to use. Hence, researchers should focus on obtaining pigments from this strain for industrial applications and further chemical characterization will open new avenues in the dyeing industry that will reduce the adverse effects of synthetic dyes
[104][43].
Pandiyarajan et al.
[108][44] reported the presence of water-soluble yellow pigments from
Aspergillus sp. The pigment possessed maximum dyeing ability of the fungus with the hydrothermal method for textile fabrics without mordants and showed better dye uptake in comparison with synthetic dyes. The toxicity of the pigment was tested using a zebra fish model system with an IC
50 value of 710 µg/mL. This novel pigment can be used an alternative to synthetic dyes for applications in the textile industrial sector.