Microbial melanin: Recent advances in biosynthesis, extraction, characterization, and applications

Highlights

• Microbial melanin represent complex biopolymers in medical sciences, cosmeceuticals, bioremediation, and bioelectronics.

• Higher cost is the biggest obstacle in proper utilization of melanin for various applications.

• Different approaches can be used to extract pure melanin from microbial broth

• Melanin can be characterized using various spectrophotometric techniques.

• The strain improvement can be performed in order to increase production of melanin through genetic approaches.

Abstract

Melanin is a common name for a group of biopolymers with the dominance of potential applications in medical sciences, cosmeceutical, bioremediation, and bioelectronic applications. The broad distribution of these pigments suggests their role to combat abiotic and biotic stresses in diverse life forms. Biosynthesis of melanin in fungi and bacteria occurs by oxidative polymerization of phenolic compounds predominantly by two pathways, 1,8-dihydroxynaphthalene [DHN] or 3,4-dihydroxyphenylalanine [DOPA], resulting in different kinds of melanin, i.e., eumelanin, pheomelanin, allomelanin, pyomelanin, and neuromelanin. The enzymes responsible for melanin synthesis belong mainly to tyrosinase, laccase, and polyketide synthase families. Studies have shown that manipulating culture parameters, combined with recombinant technology, can increase melanin yield for large-scale production. Despite significant efforts, its low solubility has limited the development of extraction procedures, and heterogeneous structural complexity has impaired structural elucidation, restricting effective exploitation of their biotechnological potential. Innumerable studies have been performed on melanin pigments from different taxa of life in order to advance the knowledge about melanin pigments for their efficient utilization in diverse applications. These studies prompted an urgent need for a comprehensive review on melanin pigments isolated from microorganisms, so that such review encompassing biosynthesis, bioproduction, characterization, and potential applications would help researchers from diverse background to understand the importance of microbial melanins and to utilize the information from the review for planning studies on melanin. With this aim in mind, the present report compares conventional and modern ideas for environment-friendly extraction procedures for melanin. Furthermore, the characteristic parameters to differentiate between eumelanin and pheomelanin are also mentioned, followed by their biotechnological applications forming the basis of industrial utilization. There lies a massive scope of work to circumvent the bottlenecks in their isolation and structural elucidation methodologies.

Introduction

Natural pigments are considered safe, with pronounced multifarious benefits compared to synthetic pigments, among those, ‘Melanin’ forms a ubiquitous heterogeneous polymer group with a broad range of structural and functional diversity (Gosset, 2017; Stepien et al., 2013). On tracing back the Greek history, ‘Melanin’ came from melanos, meaning dark, but scientifically the term was coined in 1840 by Berzelius, a Swedish scientist for extraction of a dark-colored pigment from eye membranes. The polymeric pigments arise through the oxidation of phenolic or indolic substrates involving the enzyme catalysis. Solano (2014) defined melanin as a heterogeneous polymer derived by the oxidation of phenols and subsequent polymerization of intermediate phenols and their resulting quinones. Categorically melanin can be divided into three main types based on their structural monomers; eumelanin, pheomelanin, and allomelanin. Eumelanin and allomelanin impart black/brown color to the cells, whereas pheomelanin (animal kingdom) provides yellow/red pigmentation (Pralea et al., 2019; Nicolaus, 1968). Precursor units for the polymer of eumelanin consist primarily of indole-type units that arise from L-tyrosine or L-DOPA (L-3,4-dihydroxyphenylalanine) oxidation. Similarly, pheomelanins are the products of oxidative polymerization of cysteinyl conjugates of DOPA via benzothiazine intermediates (Prota, 1992; Wakamatsu and Ito, 2002; Ito et al., 2019). On the other hand, allomelanins are derived by the oxidation of nitrogen-free diphenols, such as catechol, 1,8-dihydroxynaphtalene, and γ-glutaminyl-3,4-dihydroxybenzene. Two more types of melanin also exist namely, pyomelanin and neuromelanin. Pyomelanin is a product of oxidation of homogentisic acid, whereas neuromelanin comprises of both benzothiazine and indole units (Pralea et al., 2019; Haining and Achat-Mendes, 2017). Apart from providing pigmentation to cells, melanin protects cells against UV radiations (Stepien et al., 2013; Coelho et al., 2009), performs quenching of free radicals (Meredith and Sarna, 2006), are responsible for varied functions in various phyla (Bonser, 1995; Trullas et al., 2007; Stuart-Fox and Moussalli, 2008), helps with defense mechanism in insects and mollusks (Palumbo, 2003; Vavricka et al., 2014), enhances virulence mechanism in various fungi and bacteria (Nosanchuk and Casadevall, 2003), provides the advantage of antibacterial and antioxidant activities. Furthermore, melanins have applications in semiconductors (Bothma et al., 2008), as metal chelators, as optical imagers (Abbas et al., 2009), in cosmeceuticals and pharmaceuticals, MRI probes, soil bioremediations, etc. (Martinez et al., 2019). Despite such promising and diverse attributes, it is quite taxing to elucidate specific conclusions regarding their properties and structures because of their physicochemical properties and heterogeneities. A major reason for heterogeneity is the lack of genetic makeup responsible for melanin biosynthesis and sequential metabolic pathways. Further, microbes have the proficiency in utilizing multiple precursors for melanin synthesis, making the polymerization process more haphazard amongst them (Cao et al., 2021). Difficulties in unscrambling the structural features and the insolubility of melanin in most solvents potentially challenges its extraction and purification techniques obstructing its cost-effective industrial production (Borovansky and Riley, 2011; Sun et al., 2016). Conclusively, the development of environment-friendly and cost-effective melanin production from eukaryotic resources is challenging; in such conditions, microbial melanin can pave the way (Sun et al., 2016; Pavan et al., 2020). Production of melanin from microbial sources is effective because of easy fermentation procedures where the yield can be accelerated by optimizing factors affecting melanin synthesis.

According to our knowledge, within the last five years, some exceptional reviews (Roy and Rhim, 2021; Tran-Ly et al., 2020; Vasanthabharathi and Jayalakshmi, 2020; Pavan et al., 2020; Pralea et al., 2019; Martinez et al., 2019) on microbial melanin have been reported focusing on specific attributes, either biosynthesis, characterization, production perspectives or biomaterial obtained from melanin; there lacks a comprehensive report of microbial melanin from biosynthesis to its application in several fields. Moreover, the present review targets the literature of melanin obtained from microbial sources, including bacteria, fungi, and actinobacteria, which so far is among the first reviews on microbial melanin. Herein, this review is focused on biosynthetic, extraction, and purification techniques, characteristics, and applicative attributes of microbial melanin. The review aimed to address how different functional properties of microbial melanin can be endorsed in industrial applications.