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 <i>Stappia quadratibracata</i> sp. nov., <i>Bacillus crepusculi</i> sp. nov., and <i>Psychrobacter noctis</i> 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 <i>Stappia quadratibracata</i> 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 <i>Staphylococcus aureus,</i> 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.IMPORTANCEBioactive microbial natural products remain the preeminent source of new lead compounds for drug development. Due to the increasingly high levels of strain and compound rediscovery from terrestrial environments, the deep-ocean is increasingly considered an attractive starting point for bioprospecting programs, which seek to isolate and characterize novel chemical scaffolds. Here, we use a combination of genomics, metabolomics, and chemical analysis, to establish the biosynthetic potential of three bacterial species isolated from deep-ocean Atlantic sponges and report the discovery and characterization of a new antimicrobial thiazole alkaloid, agrochelin II. Our findings demonstrate the usefulness of integrated cultivation-screening-metabolomics-genomics pipelines for microbial metabolite discovery and identify the genus <i>Stappia</i> as a hitherto neglected source of bioactive natural products.