Flavobacterium adiutor sp. nov., a newly identified freshwater bacterium potentially supporting Microcystis growth via oxidative stress reduction

<i>Microcystis</i> blooms deteriorate water quality and pose significant threats to aquatic ecosystems. Understanding how micro-organisms contribute to their formation is essential for developing effective control strategies. Although <i>Flavobacterium</i> species frequently coexist with <i>Microcystis</i> in natural environments, their role in <i>Microcystis</i> bloom formation remains largely unexplored. In the present study, a novel Gram-negative, designated strain RS13.1^T, was isolated from freshwater during a <i>Microcystis</i> bloom. Phylogenetic analysis based on 16S rRNA gene sequences placed strain RS13.1ᵀ within the genus <i>Flavobacterium</i>, closely related to <i>Flavobacterium limi</i> THG-AG6.4ᵀ (98.89%) and <i>Flavobacterium hibisci</i> THG-HG1.4ᵀ (97.98%). The average nucleotide identity and digital DNA-DNA hybridization between strain RS13.1^T and <i>Flavobacterium</i> species were below 89.39% and 37.60%, respectively. The growth of <i>Microcystis</i> increased by 26.6 % after 7 days when cultured with strain RS13.1ᵀ at 10% (v/v), demonstrating its growth-promoting effect. Furthermore, <i>Microcystis</i> exhibited tolerance to 20 mg l^-1 H₂O₂, and its photosynthesis remained unaffected even with exposure to 5 mg l^-1 H₂O₂ in the presence of RS13.1^T. Genome annotation of strain RS13.1^T revealed the presence of genes associated with carbohydrate-active enzyme, vitamin, heme and catalase biosynthesis, verifying mutualistic interaction between strain RS13.1^T and <i>Microcystis</i>. Overall, these findings suggest that strain RS13.1^T may contribute to the formation and persistence of <i>Microcystis</i> bloom by alleviating oxidative stress and supplying beneficial nutrients. Based on polyphasic features, strain RS13.1ᵀ was identified as a distinct species within the genus, for which the name <i>Flavobacterium adiutor</i> sp. nov. is proposed. The type strain is <i>Flavobacterium adiutor</i> RS13.1^T (=KCTC 102193^T=LMG 33484^T).