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

Microcystis 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 Flavobacterium species frequently coexist with Microcystis in natural environments, their role in Microcystis bloom formation remains largely unexplored. In the present study, a novel Gram-negative, designated strain RS13.1 T , was isolated from freshwater during a Microcystis bloom. Phylogenetic analysis based on 16S rRNA gene sequences placed strain RS13.1ᵀ within the genus Flavobacterium , closely related to Flavobacterium limi THG-AG6.4ᵀ (98.89%) and Flavobacterium hibisci THG-HG1.4ᵀ (97.98%). The average nucleotide identity and digital DNA–DNA hybridization between strain RS13.1 T and Flavobacterium species were below 89.39% and 37.60%, respectively. The growth of Microcystis increased by 26.6 % after 7 days when cultured with strain RS13.1ᵀ at 10% (v/v), demonstrating its growth-promoting effect. Furthermore, Microcystis exhibited tolerance to 20 mg l −1 H 2 O 2 , and its photosynthesis remained unaffected even with exposure to 5 mg l −1 H 2 O 2 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 Microcystis . Overall, these findings suggest that strain RS13.1 T may contribute to the formation and persistence of Microcystis 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 Flavobacterium adiutor sp. nov. is proposed. The type strain is Flavobacterium adiutor RS13.1 T (=KCTC 102193 T =LMG 33484 T ).