Algal–Bacterial Interaction-Driven Secondary Metabolites From Phaeobacter inhibens and Their Anti-Allergic Effects on Th2 Cell Immune Response

The Roseobacter clade, a versatile Rhodobacteraceae lineage, comprises up to 20% of marine bacteria and drives key biogeochemical cycles. Phaeobacter inhibens, a representative model species, is associated with the alga Emiliania huxleyi, exhibiting a dual lifestyle that alternates between promoting symbiotic growth and displaying pathogenicity during algal senescence. In this study, we investigated the metabolic responses of P. inhibens cultured with sinapic acid, an algal-derived lignin catabolite known to modulate algal–bacterial interactions. Detailed LC–MS/UV-guided analysis of the sinapic acid-treated culture identified 10 metabolites, including two new compounds, roseochelins C (1) and D (2). All isolated compounds were tested for anti-allergic effects in Th2 cell–mediated immune responses, and sinatryptin A (4) showed the strongest activity by reducing Th2 cytokine production and blocking Th2 differentiation through inhibition of the IL-4/STAT6–GATA3/IRF4 pathway. These findings expand the chemical diversity of algal–bacterial interactions, and anti-allergic assays of the isolated metabolites highlight the potential of Roseobacter-derived compounds as novel bioactive resources.