OSMAC guided bioprospecting of Atlantic sponges reveals novel bacterial species and evidence for the antimicrobial thiazole alkaloid agrochelin II

The deep sea remains a major reservoir of underexplored microbial diversity and biosynthetic novelty. Here, we describe three bacterial species isolated from Atlantic sponges, including Stappia quadratibracata sp. nov., Bacillus crepusculi sp. nov., and Psychrobacter noctis sp. nov. Genome sequencing, phylogenomics, and phenotypic characterization confirmed their novelty and biosynthetic potential. A targeted ‘One Strain Many Active Compounds’ (OSMAC) screen revealed previously silent antibacterial activity from Stappia quadratibracata sp. nov., only when grown with the carbon source succinate. Metabolomics and molecular networking analysis indicated that this activity was attributable to an unstable thiazole alkaloid with spectral data closely related to, but distinct from, the epimeric siderophores agrochelin and massiliachelin. Spectroscopic studies, in tandem with comparative analysis of the biosynthetic gene cluster for this metabolite, are consistent with agrochelin II, a previously unreported thiazole alkaloid diastereoisomer. Agrochelin II exhibits iron-enhanced antibacterial activity against Staphylococcus aureus, underscoring the ecological role of iron acquisition in microbial competition. Our findings highlight the value of OSMAC-guided bioprospecting in uncovering antimicrobial metabolites from sponge-associated bacteria.