Harnessing the seaweed holobiont: antibacterial and antioxidant lead compounds from epiphytic microbiota

Seaweed-associated epiphytic bacteria represent a promising source of bioactive metabolites with antimicrobial and antioxidant properties. In this study, 26 culturable epiphytic bacterial isolates were obtained from five marine macroalgae, namely Ulva lactuca, Amphiroa anceps, Halimeda opuntia, Kappaphycus alvarezii, and Sargassum wightii. Preliminary screening using the agar overlay method revealed that six isolates (EB-30, EB-38, EB-42, EB-47, EB-48, and EB-50) exhibited antibacterial activity against at least two pathogenic test strains, accounting for 23% of the total isolates. Crude extracellular extracts demonstrated inhibitory effects in the disc diffusion assay, with inhibition zones ranging from 8 to 21 mm at 1 mg/mL. Among these, EB-38 showed the strongest antibacterial activity against Bacillus subtilis MTCC 441, Staphylococcus aureus MTCC 96, and Klebsiella pneumoniae MTCC 432, with minimum inhibitory concentrations ranging from 62.5 to 250 µg/mL. Antioxidant activity assessed by the DPPH radical scavenging assay showed concentration-dependent effects, with EB-38 extract exhibiting the highest scavenging efficiency (71.13 ± 1.03%) and the lowest IC₅₀ value (0.41881 ± 0.02544 mg/mL). Total phenolic content ranged from 7.7 ± 0.78 to 16.98 ± 1.73 mg GAE/g extract. Molecular identification based on partial 16 S rRNA gene sequencing revealed the isolates as Halobacillus sp. (EB-30), Exiguobacterium aestuarii (EB-38), Bacillus licheniformis (EB-42), Rossellomorea aquimaris (EB-47), Bacillus sp. (EB-48), and Pseudomonas sp. (EB-50). GC–MS analysis revealed bioactive compounds, including Pyrrolo[1,2-a] pyrazine-1,4-dione derivatives, fatty acid methyl esters, tert-butylated phenols, and 13-Docosenamide. However, further mechanistic investigations are required to elucidate the molecular basis of the observed antibacterial and antioxidant effects.