Bacillus cereus strain L7 lyses Cylindrospermopsis raciborskii through intercellular contact

Cylindrospermopsis raciborskii is a new type of toxic filamentous cyanobacteria commonly found in tropical regions but has recently spread to temperate areas. It is considered an invasive species that poses a serious threat to drinking water sources and human health. Bacillus cereus strain L7, previously isolated and identified by our group, was found to have efficient algicidal activity against C. raciborskii FACHB-1503 in this study. The growth period and initial concentration of L7 had a significant impact on algicidal activity, with L7 culture in the decline phase exhibiting the highest algicidal activity (I = 99.62 %). The ideal environmental conditions for the coculture system were 30 °C, pH = 8, and a light-dark cycle (12 h light: 12 h dark). In experiments to confirm the algicidal mode of L7, the pellet resuspension with R2A medium had the highest algicidal efficiency (I = 98.54 %, t = 6 d), even higher than that of the L7 culture. In contrast, sterile filtrate and pellet resuspension with sterile water had no discernible impact on the algal growth, indicating that L7 performed algicidal activity through direct contact with algal cells. Following 6 days of coculture of L7 culture with C. raciborskii solution, the activity of the PSII system and the photosynthetic efficiency of algal cells were both zero. Following SEM and TEM results, it was discovered that the process of L7 lysis of algal cells begins with the disruption of their multicellular filamentous structure, rupture of the cell wall, efflux of the cell contents, and complete disappearance of the algal cell structure. This is the first research to demonstrate that L7 lyses C. raciborskii directly, which serves as an excellent strategic direction for launching innovative freshwater bloom control strategies.