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 Sulfitobacter mediterraneus 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.

Although the kojic acid gene cluster in Aspergillus oryzae was identified in 2010, the functions of neighbouring genes remain poorly understood. Here, we characterise Aokap7, a gene adjacent to this cluster that encodes a novel nucleus-localised zinc finger protein without transcriptional activation activity. Disruption of Aokap7 markedly accelerated spore germination, hyphal growth, and conidial formation, but impaired kojic acid production. Overexpression of kojR or laeA restored kojic acid production in the Aokap7 disruption strain, whereas their disruption abolished it, indicating that Aokap7 regulates kojic acid production by kojR and laeA. Furthermore, disrupting Aokap7 in the AozfA overexpression or disruption strains reversed their kojic aicd production, suggesting that Aokap7 acts downstream of AozfA. The Aokap7 mutant also exhibited reduced expression of reactive oxygen species (ROS)-scavenging genes and heightened oxidative stress sensitivity. Biochemical assays revealed that Aokap7 preferentially binds to the motif 5’-CGGCTCGG-3’, and directly interacts with the AoGPX1 promoter. Disruption of AoGPX1 increased the sensitivity of A. oryzae to oxidative stress. Our findings elucidate the pivotal role of Aokap7 in coordinating growth, oxidative stress response and kojic acid production, advancing understanding of the regulatory network of kojic acid synthesis in A. oryzae.