Fermentation-process verification of algicidal bacterium Bacillus cereus CZBC1 and algicidal effect of its fermentation products on Microcystis aeruginosa

To facilitate the industrial application of the algicidal bacterium Bacillus cereus CZBC1, this study employed 2 L and 50 L fermenter systems to scale up and validate the optimal fermentation process previously established in shake flask experiments, and assessed process reproducibility by comparing the dynamic changes in cell density, morphological evolution and process parameters under the two scales, and systematically evaluated the algicidal efficacy of different fermentation products and their dosages against Microcystis aeruginosa. The optimal fermentation parameters for CZBC1 in the 50 L fermenter were determined as follows: wheat bran and molasses (combined) at 6.84 g·L⁻¹, yeast extract 3.36 g·L⁻¹, NaCl 1 g·L⁻¹, KH₂PO₄ 0.2 g·L⁻¹, MgSO₄ 0.2 g·L⁻¹, FeCl₃ 0.1 g·L⁻¹, 3.08% (φ) MnSO₄ solution 1 mL·L⁻¹, and CaCO₃ 0.1 g·L⁻¹. Fermentation was carried out with a filling volume of 70% (φ), an inoculum size of 1% (φ), a temperature of 30 °C, an agitation speed of 500 r·min⁻¹, an aeration rate of 0.6 (V·V⁻¹·min), a fermenter pressure of 0.03–0.05 MPa, and a controlled pH of 6.5. After 24 h of fermentation, the sporulation rate exceeded 90%. In the 2 L fermenter, the cell concentration reached a peak of 4.12×10⁹ CFU·mL⁻¹ at 24 h, with a sporulation rate exceeding 90% at 36 h; in the 50 L fermenter, the peak cell concentration of 3.72×10⁹ CFU·mL⁻¹ was achieved at 16 h, and the sporulation rate exceeded 90% by 24 h, indicating that scale-up significantly accelerated both cell proliferation and sporulation. Algicidal assays revealed significant differences in the efficacy of the various fermentation products: the crude fermentation broth and non-activated spore powder exhibited no algicidal activity, whereas the supernatant, activated spore powder, and vegetative cells all showed potent algicidal effects, achieving 10-day algicidal rates of 97.45%, 86.73%, and 97.10%, respectively, suggesting that the algicidal activity is mainly dependent on extracellular metabolites and the metabolically active state of the cells. Furthermore, dose-response analysis demonstrated that, in an experimental system with an initial M. aeruginosa concentration of 8.31×10⁵ cell·mL⁻¹, the effective threshold concentration of activated spore powder was no less than 10⁵ CFU·mL⁻¹; when the concentration increased to 10⁶ CFU·mL⁻¹, the algicidal rate reached 99.01%, highlighting its excellent potential for cyanobacterial bloom control. In summary, these findings confirm the promising application prospects of the CZBC1 strain and its fermentation process in the biological control of cyanobacterial blooms.