New Pyrroline Isolated from Antarctic Krill-Derived Actinomycetes Nocardiopsis sp. LX-1 Combining with Molecular Networking.

Antarctic krill ( Euphausia superba ) of the Euphausiidae family comprise one of the largest biomasses in the world and play a key role in the Antarctic marine ecosystem. However, the study of E. superba -derived microbes and their secondary metabolites has been limited. Chemical investigation of the secondary metabolites of the actinomycetes Nocardiopsis sp. LX-1 (in the family of Nocardiopsaceae ), isolated from E. superba , combined with molecular networking, led to the identification of 16 compounds a - p (purple nodes in the molecular network) and the isolation of one new pyrroline, nocarpyrroline A ( 1 ), along with 11 known compounds 2 - 12 . The structure of the new compound 1 was elucidated by extensive spectroscopic investigation. Compound 2 exhibited broad-spectrum antibacterial activities against A. hydrophila , D. chrysanthemi , C. terrigena , X. citri pv. malvacearum and antifungal activity against C. albicans in a conventional broth dilution assay. The positive control was ciprofloxacin with the MIC values of <0.024 µM, 0.39 µM, 0.39 µM, 0.39 µM, and 0.20 µM, respectively. Compound 1 and compounds 7 , 10 , and 11 displayed antifungal activities against F. fujikuroi and D. citri , respectively, in modified agar diffusion test. Prochloraz was used as positive control and showed the inhibition zone radius of 17 mm and 15 mm against F. fujikuroi and D. citri , respectively. All the annotated compounds a - p by molecular networking were first discovered from the genus Nocardiopsis . Nocarpyrroline A ( 1 ) features an unprecedented 4,5-dihydro-pyrrole-2-carbonitrile substructure, and it is the first pyrroline isolated from the genus Nocardiopsis . This study further demonstrated the guiding significance of molecular networking in the research of microbial secondary metabolites.