Indole Acetic Acid: A Key Metabolite That Protects Marine Sulfitobacter mediterraneus Against Oxidative Stress

For marine bacteria, the phycosphere is attractive as a major source of labile nutrients, but it also presents challenges due to the accumulation of stressors, such as reactive oxygen species (ROS) from algal metabolisms. Therefore, successful colonization of bacteria in the phycosphere requires an efficient mechanism to fight against oxidative stress, which is still a missing piece in studying bacteria-algae interactions. Here, we demonstrate that a common metabolite, indole acetic acid (IAA), enables the Roseobacter clade <i>Sulfitobacter mediterraneus</i> SC1-11, an IAA-producer, to resist hydrogen peroxide (H₂O₂) stress and that IAA biosynthesis can be activated by low concentrations of H₂O₂. Proteomics and metabolomics analyses revealed that bacteria consume high amino acid levels when exposed to H₂O₂ stress, while exogenous supplementation with IAA effectively protects bacteria from ROS damage and alleviates amino acid starvation by upregulating several proteins responsible for replication, recombination, and repair, as well as two proteins involved in amino acid transport and metabolism. Furthermore, the supplementation of some amino acids, such as arginine, also showed a significant protective effect on bacteria under H₂O₂ stress. This study highlights an unprecedented role of IAA in regulating amino acid metabolisms for resisting oxidative stress, which may be a specific strategy for adapting to the phycosphere.