Carbon-Source Effects on Growth and Secondary Metabolism in the Marine Bacteroidota Tenacibaculum mesophilum and Fulvivirga kasyanovii

Marine Bacteroidota are recognized bacterial producers of bioactive metabolites, yet their biosynthetic potential remains cryptic under standard laboratory conditions. Here, we developed chemically defined media for <i>Fulvivirga kasyanovii</i> 48LL (Cytophagia) and <i>Tenacibaculum mesophilum</i> fLL (Flavobacteriia) to evaluate the effect of environmentally relevant carbon sources on growth and secondary metabolism. <i>F. kasyanovii</i> utilized 31 of 34 tested carbon sources whereas <i>T. mesophilum</i> grew on only five substrates, underscoring a distinct nutritional preferences. Substrate significantly influenced the antibacterial activity of <i>F. kasyanovii</i> extracts. Growth on <i>β</i>-1,3-glucan, glycerol, poly(<i>β</i>-hydroxybutyrate) (PHB), fish gelatin, or pectin resulted in extracts generating the largest inhibition zones (10-13 mm) against <i>Bacillus subtilis</i> or <i>Rossellomorea marisflavi</i>. Genome analysis revealed <i>F. kasyanovii</i> to be enriched in biosynthetic gene clusters (BGCs), notably harboring a ~570 kb genomic island comprising five large NRPS/PKS-type clusters. Quantitative PCR confirmed carbon-source-dependent regulation of these operons: glucose induced BGC1, BGC3, and BGC4, while <i>κ</i>-carrageenan and PHB upregulated BGC2. Conversely, yeast-peptone medium (analogous to standard marine broth) repressed transcription across all active clusters. These findings demonstrate that naturally occurring carbon sources can selectively activate cryptic BGCs and modulate antibacterial activity in <i>F. kasyanovii</i>, suggesting that similar strategy can be used for natural-product discovery in marine Bacteroidota.