**4. Conclusions**

In this work, *Limnoraphis* sp. CCNP1324 was revealed to be a new producer of aeruginosamides. Some of the peptides were cytotoxic against a breast cancer cell line. The cytotoxic activity of these compounds is probably determined by the unknown amino acid residues in *N*-terminal position. Unfortunately, the data collected with MS/MS were insufficient to resolve their structures. LC-MS/MS analyses of samples are key elements of bioassay-guided fractionation and structure characterization of bioactive metabolites. Due to high sensitivity and selectivity, trace amounts of the compounds in complex matrices can be detected. However, like any technique, it has also some limitations. The unequivocal elucidation of peptide structure with unknown modifications is impossible or bears a high risk of error. Therefore, in our future work, the chromatographic conditions have to be further optimized, to isolate the bioactive peptides in sufficient amounts for structural analysis by NMR.

**Supplementary Materials:** The following are available online at http://www.mdpi.com/1660-3397/18/9/446/s1, Figures S1–S13. Enhanced product ion mass spectrum of aeruginosamides: AER595, AEG603, AEG657, AER667, AER681a, AER681b, AER683, AER685, AEG705, AEG707, AEG721, AER735, AER749, Figure S14. 1H NMR Spectrum of aeruginosamide AEG671 in DMSO-d6, Figure S15. COSY Spectrum of aeruginosamide AEG671 in DMSO-d6, Figure S16. TOCSY Spectrum of aeruginosamide AEG671 in DMSO-d6, Figure S17. ROESY Spectrum of aeruginosamide AEG671 in DMSO-d6, Figure S18. HSQC Spectrum of aeruginosamide AEG671 in DMSO-d6, Figure S19. HMBC Spectrum of aeruginosamide AEG671 in DMSO-d6.

**Author Contributions:** Conceptualization, M.C., A.K. and H.M.-M.; methodology, M.C., H.M.-M.; formal analysis, M.C., K.S.; investigation M.C. (HPLC and LC-MS/MS analysis, MTT assays), K.S. (MTT assays), E.W. (NMR analysis), H.M.-M. (LC-MS/MS analysis); writing—original draft preparation, M.C.; writing—review and editing, M.C., K.S., E.W., A.K., H.M.-M.; visualization, M.C.; funding acquisition, A.K., H.M.-M. All authors have read and agreed to the published version of the manuscript.

**Funding:** This study was financially supported by the National Science Centre in Poland (project number NCN 2016/21/B/NZ9/02304 to HMM), and by the statutory programme of the Institute of Oceanology, PAS (grant No.II.3).

**Conflicts of Interest:** The authors declare no conflict of interest.
