Statistical optimization of melanin-like pigment biosynthesis by marine Streptomyces isolate MOE3 and evaluation of its cytotoxic activity against A-431 cells

Marine actinomycetes represent a promising source of biologically active natural pigments. In the present study, isolate MOE3, recovered from mangrove sediment along the Egyptian Red Sea coast, was provisionally assigned to Streptomyces lateritius based on partial 16 S rRNA gene sequencing and phylogenetic analysis, showing approximately 99% sequence similarity to closely related Streptomyces taxa. The extracted pigment exhibited UV-Visible and FTIR characteristics consistent with a melanin-like pigment, including an absorption maximum at 255 nm. Plackett-Burman screening produced a maximum observed response approximately 1.65-fold relative to the basal response, while the highest experimentally observed response within the Box-Behnken design represented an approximately 2.3-fold increase relative to the basal response. The pigment also exhibited concentration-dependent in vitro cytotoxicity against A-431 human epidermoid carcinoma cells, with an IC₅₀ of 45.11 µg/mL after 24 h. These findings identify MOE3 as a promising microbial source of a melanin-like pigment and provide a basis for further structural, biological, and process-development studies.