Syngas (CO, CO₂, and H₂) has attracted special attention due to the double benefit of syngas fermentation for carbon sequestration (pollution reduction), while generating energy. Syngas can be either produced by gasification of biomasses or as a by-product of industrial processes. Only few microorganisms, mainly clostridia, were identified as capable of using syngas as a substrate to produce medium chain acids, or alcohols (such as butyric acid, butanol, hexanoic acid, and hexanol). Since CO plays a critical role in the availability of reducing equivalents and carbon conversion, this work assessed the effects of constant CO partial pressure (P_CO), ranging from 0.5 to 2.5 atm, on cell growth, acid production, and solvent production, using Clostridium carboxidivorans. Moreover, this work focused on the effect of the liquid to gas volume ratio (V_L/V_G) on fermentation performances; in particular, two V_L/V_G were considered (0.28 and 0.92). The main results included—(a) P_CO affected the growth kinetics of the microorganism; indeed, C. carboxidivorans growth rate was characterized by CO inhibition within the investigated range of CO concentration, and the optimal P_CO was 1.1 atm (corresponding to a dissolved CO concentration of about 25 mg/L) for both V_L/V_G used; (b) growth differences were observed when the gas-to-liquid volume ratio changed; mass transport phenomena did not control the CO uptake for V_L/V_G = 0.28; on the contrary, the experimental CO depletion rate was about equal to the transport rate in the case of V_L/V_G = 0.92.

As Streptomycetes might produce melanin to survive in stressful environmental conditions, like under metal exposure, supplementing metal ions to the growth medium could be a wise strategy for boosting the production of the pigment. The aim of this study was to test, for the first time, the possibility of boosting S. nashvillensis DSM40314 melanin biosynthesis by adding to the growth medium singularly or, at the same time, different concentrations (1.0, 1.5, and 2.0 g∙L⁻¹) of CuSO₄ or/and Fe₂(SO₄)₃. A maximum melanin production of 4.0 ± 0.1 g·L⁻¹ was obtained in shake flasks with a 2.0 g∙L⁻¹ coupled addition of the two metals, while the extracellular tyrosinase activities ranged values between 5.4 and 11.6 ± 0.1 U·L⁻¹. The pigments produced in different conditions were precipitated from the broth supernatants under acidic conditions, purified, and characterized by UV-VIS, FT-IR, and NMR analyses that determined structures like eumelanin pigments. Fermentation experiments in stirred tank reactors allowed to scale up the process in more controlled conditions, further boosting the pigment production up to 4.9 ± 0.1 g·L⁻¹, with an increase of about 22.0% compared to the results obtained in shake flasks.

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.