Flagellimonas algicida sp. Nov.: A Novel Broad-Spectrum Algicidal Bacterium Targeting Harmful Algal Bloom Species and Genomic Insights into Its Secondary Metabolites

A novel Gram-negative bacterium, designated strain SN16^T, was isolated from a harmful algal bloom (HAB). Strain SN16^T exhibited potent, broad-spectrum algicidal activity against the colony-forming alga <i>Phaeocystis globosa</i> and eight other HAB-causing species, highlighting its potential as a promising candidate for the biological control of HABs. A phylogenetic analysis of 16S rRNA gene sequences placed strain SN16^T within the genus <i>Flagellimonas</i>. The average nucleotide identity (ANI) and digital DNA-DNA hybridization (dDDH) values between strain SN16^T and its relatives were 75.4-91.4% and 19.3-44.0%, respectively. These values fall below the established thresholds for species delineation, confirming that SN16^T represents a novel species. A chemotaxonomic analysis revealed its dominant cellular fatty acids to be iso-C_15:0 and iso-C_15:1 G. The major polar lipid was phosphatidylethanolamine, and the primary respiratory quinone was menaquinone-6. Genome mining identified 11 biosynthetic gene clusters (BGCs), including those encoding for terpenes, ribosomal peptide synthetases, and non-ribosomal peptide synthetases. By integrating BGC analysis with the observed algicidal activities, we predicted that pentalenolactone and xiamycin analogues are the likely causative compounds. Based on this polyphasic evidence, strain SN16^T is proposed as a novel species of the genus <i>Flagellimonas</i>, named <i>Flagellimonas algicida</i> sp. nov. This is the first report of <i>Flagellimonas</i> species exhibiting broad-spectrum algicidal activity, including activity against the colonial form of <i>P. globosa</i>-a key ecological challenge in HAB mitigation.