No abstract

Bacteria of the phylum Planctomycetes occur ubiquitously in marine environments and play important roles in the marine nitrogen-and carbon cycle, for example as scavengers after phototrophic blooms. Here, we describe the isolation and characterization of the planctomycetal strain Enr13 T isolated from a Posidonia sp. biofilm obtained from seawater sediment close to Panarea Island, Italy. Phylogenetic tree reconstruction based on 16S rRNA gene sequences and multi-locus sequence analysis supports the delineation of strain Enr13 T from characterized species part of the phylum of Planctomycetes. HPLC-MS analysis of culture broth obtained from strain Enr13 T revealed the presence of lipophilic metabolites, of which the major compound was isolated by preparative reversed-phase HPLC. The structure of this compound, named stieleriacine D (1), was elucidated utilizing HRESIMS, 1D-and 2D-NMR data as a new N-acylated dehydrotyrosine derivative. Its biosynthesis was proposed based on an in silico gene cluster analysis. Through analysis of the MS/MS spectrum of 1 and its minor derivative, stieleriacine E (2), it was possible to assign the structure of 2 without isolation. 1 showed antibacterial activity, however, the wide distribution of structurally related compounds indicates a potential role as a signaling molecule.

In the course of our survey to study the metabolic potential of two species of a new helotialean genus Polyphilus, namely P. frankenii and P. sieberi, their crude extracts were obtained using different cultivation techniques, which led to the isolation and characterization of two new naphtho-α-pyranone derivatives recognized as a monomer (1) and its 6,6′-homodimer (2) together with two known diketopiperazine congeners, outovirin B (3) and (3S,6S)-3,6-dibenzylpiperazine-2,5-dione (4). The structures of isolated compounds were determined based on extensive 1D and 2D NMR and HRESIMS. The absolute configuration of new naphtho-α-pyranones was determined using a comparison of their experimental ECD spectra with those of related structural analogues. 6,6′-binaphtho-α-pyranone talaroderxine C (2) exhibited potent cytotoxic activity against different mammalian cell lines with IC50 values in the low micromolar to nanomolar range. In addition, talaroderxine C unveiled stronger antimicrobial activity against Bacillus subtilis rather than Staphylococcus aureus with MIC values of 0.52 µg mL−1 (0.83 µM) compared to 66.6 µg mL−1 (105.70 µM), respectively.