Isolation and characterization of Shouchella clausii MJ1G0B, a novel halotolerant bacterium from Madura salt ponds: a putative Valenciaxanthin-like xanthophyll with antioxidant and antibacterial activities.

Shouchella clausii MJ1G0B (GenBank: MW308498.1), a halotolerant bacterium isolated from Madura salt ponds, East Java, Indonesia, was characterized through integrated phenotypic, molecular, pigment, spectroscopic, and bioactivity analyses. The Gram-positive isolate grew at 0-15% NaCl (optimum 0% NaCl), with distinct temporal windows for carotenoid accumulation (84 h) and antibacterial secondary metabolite production (120 h). 16S rRNA sequencing (1,418 bp) confirmed 99.42% identity with S. clausii and 99.27% with S. rhizosphaerae. The dominant yellow-orange xanthophyll (Spot 3; Rf 0.98), purified by preparative two-dimensional thin-layer chromatography (TLC), showed UV-Vis absorption in the UV region (λmax 210, 245, and 325 nm) deviating from the expected visible-range absorption (400-550 nm) of xanthophyll carotenoids, possibly due to a cis-configured or structurally disrupted polyene system. Fourier-transform infrared (FTIR) spectroscopy revealed O-H, aliphatic C-H, and C = C groups; liquid chromatography-mass spectrometry (LC-MS) detected [M + H]⁺ at m/z 413 (C₂₈H₄₄O₂, MW 412 g/mol), supporting identification as a Valenciaxanthin-like xanthophyll. Antioxidant activity improved after purification (IC₅₀ 999 ppm) relative to crude extracts (1,101-1,557 ppm). The secondary metabolite extract exhibited moderate antibacterial activity against Staphylococcus aureus (IC₅₀ 0.689 mg/mL) and Escherichia coli (IC₅₀ 0.789 mg/mL). To our knowledge, this is the first report of a Valenciaxanthin-like pigment from a bacterial source in an Indonesian hypersaline ecosystem.