Posidonia oceanica egagropili and Magnolia grandiflora leaves as lignocellulosic substrates to enhance eumelanin production by Streptomyces nashvillensis

The melanin biotechnological production by Streptomycetes is a valid alternative to the manufacturing process by extraction from animal tissues or through chemical synthesis routes. To be economically reliable this approach might assure high yields, eventually by employing innovative, sustainable strategies. The use of lignocellulosic waste biomasses as substrates to boost the pigment production is an advanced, but low investigated approach. In this paper, Streptomyces nashvillensis DSM40314 melanin synthesis was prompted by adding different concentrations of Posidonia oceanica egagropili or Magnolia grandiflora leaf powders to a semidefined medium. Maximum pigment productions of 3.0 ± 0.2 and 4.1 ± 0.1 g/L were reached in 96 h, in shake flasks when 5.0 g/L of egagropili or 1.0 g/L of magnolia leaves were added, respectively. In these conditions the activities of the tyrosinase, the first enzyme involved in the melanin synthetic pathway, were also determined. The strain capacity to express enzymes to degrade the two substrates was evaluated by checking the genome as well as by testing its endoglucanase and lignin peroxidase activities. The produced pigments were then purified from the broth supernatants by acidic precipitation, characterized by UV-visible, FT-IR, mono and bi-dimensional NMR, and then tested for their antioxidant and antimicrobial activities. Stirred tank reactor fermentation allowed to reach a melanin production of 7.1 ± 0.1 and 7.9 ± 0.1 g/L, when 5.0 g/L Posidonia oceanica egagropili or 1.0 g/L of Magnolia grandiflora leaf were added, respectively, with an increase of about 2.4 and 1.9 folds compared to shake flasks.