A potential algicidal bacterium against Spirogyra gracilis blooms: identification, algicidal activity, algicidal mode, and metabolomic profiling

The use of algicidal Bacillus species has been considered as an effective and environmental-friendly treatment strategy to control harmful algal blooms. However, little information is available on Bacillus species against harmful Spirogyra. In this study, an isolate (A4) was found to have a strong algicidal activity against S. gracilis, and was identified molecularly and phenotypically as B. subtilis. Its significant algicidal effects were obtained at 3.0 × 10⁶ to 3.0 × 10⁹ CFU mL⁻¹, 15 °C to 35 °C, and photoperiods of 14 h:10 h, 24 h:0 h and 0 h:24 h (light/dark). In addition, the cell-free filtrate of isolate A4 could cause cell wall rupture and increase MDA, POD, CAT and SOD levels in S. gracilis, indicating an algicidal mode of indirect attack. The comparative LC–MS/MS-based metabolomics analysis further revealed that the differential metabolites and relevant metabolic pathways, especially the increased algicidal metabolites and their biosynthesis pathways such as pyrocatechol and benzoate degradation, could probably contribute to the efficient algicidal activity of isolate A4. The findings of this study provide valuable insights into the biological control of harmful Spirogyra using B. subtilis.