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

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.