Mangrove-derived Exiguobacterium himgiriensis produces a bioactive carotenoid-associated pigment with antioxidant, antimicrobial, and plant growth-promoting activities

Abstract

Abstract

Mangrove ecosystems harbor metabolically versatile microorganisms that represent a rich yet underexplored source of structurally diverse bioactive metabolites with considerable biotechnological potential. In this study, a yellow pigment-producing bacterium was isolated from Sundarbans mangrove sediment and comprehensively characterized to evaluate its taxonomic identity, physiological characteristics, pigment production, and biological activities. Morphological, biochemical, and 16 S rRNA gene sequence analyses identified the isolate as Exiguobacterium himgiriensis strain DUMN008, exhibiting 99.44% sequence similarity to reference strains and deposited in GenBank under accession number PV875550.1. Optimization experiments demonstrated that bacterial growth and pigment production were maximal at 37 °C, pH 8.0, and 3% (w/v) NaCl, yielding approximately 67.4 mg/L crude pigment after 72 h of cultivation. Ultraviolet–visible spectroscopy revealed a major absorption peak at 210.8 nm, while Fourier transform infrared (FT-IR) spectroscopy detected hydroxyl, aliphatic C–H, aromatic C=C, and glycosidic-associated functional groups, suggesting the presence of a carotenoid-associated pigment. However, definitive structural identification requires chromatographic and spectrometric analyses, such as HPLC–MS/MS and nuclear magnetic resonance (NMR) spectroscopy. The extracted pigment exhibited strong concentration-dependent antioxidant activity, achieving 52.66%, 76.27%, and 85.29% 2,2-diphenyl-1-picrylhydrazyl (DPPH) radical scavenging activity at concentrations of 1, 3, and 5 mg/mL, respectively. The pigment also displayed antimicrobial activity against Escherichia coli (15.6 ± 0.4 mm inhibition zone), Salmonella spp., Bacillus subtilis , and Staphylococcus aureus . Furthermore, treatment with the bacterial suspension enhanced seed germination in Cicer arietinum (90%) and Vigna radiata (95%) without observable phytotoxic effects. Collectively, these findings identify mangrove-derived E. himgiriensis as a promising source of multifunctional natural bioactive pigments with potential applications in the food, pharmaceutical, agricultural, and environmental biotechnology sectors. Nevertheless, the preliminary spectroscopic characterization, absence of definitive pigment identification, and lack of mechanistic and in vivo validation represent important limitations that warrant further investigation.