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

## TL;DR

This study shows how different carbon sources affect the growth and production of bioactive compounds in two marine bacteria, revealing new ways to activate hidden biosynthetic pathways.

## Contribution

The study identifies specific carbon sources that activate cryptic biosynthetic gene clusters in marine Bacteroidota, offering a strategy for natural-product discovery.

## Key findings

- F. kasyanovii utilized 31 of 34 tested carbon sources, while T. mesophilum grew on only five.
- Certain carbon sources like β-1,3-glucan and PHB significantly increased antibacterial activity in F. kasyanovii extracts.
- Quantitative PCR showed carbon-source-dependent regulation of biosynthetic gene clusters in F. kasyanovii.

## Abstract

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 Fulvivirga kasyanovii 48LL (Cytophagia) and Tenacibaculum mesophilum fLL (Flavobacteriia) to evaluate the effect of environmentally relevant carbon sources on growth and secondary metabolism. F. kasyanovii utilized 31 of 34 tested carbon sources whereas T. mesophilum grew on only five substrates, underscoring a distinct nutritional preferences. Substrate significantly influenced the antibacterial activity of F. kasyanovii extracts. Growth on β-1,3-glucan, glycerol, poly(β-hydroxybutyrate) (PHB), fish gelatin, or pectin resulted in extracts generating the largest inhibition zones (10–13 mm) against Bacillus subtilis or Rossellomorea marisflavi. Genome analysis revealed F. kasyanovii 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 κ-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 F. kasyanovii, suggesting that similar strategy can be used for natural-product discovery in marine Bacteroidota.