ORIGINAL ABSTRACT

Glyphosate-based herbicides are among the most widely used agricultural chemicals globally, and their widespread application presents risks to environmental health and aquatic ecosystems. Continuous glyphosate inputs disrupt phytoplankton communities, potentially triggering harmful algal blooms. This study examines the proliferation of microalgal species exposed to low glyphosate concentrations (0.05 mg/L) and various phosphorus sources, with a particular focus on C-P and C-O-P bond phosphonates, which have been insufficiently studied in previous research. We hypothesized that cyanobacteria might exhibit a competitive growth advantage over other algal species when exposed to C-P bond glyphosate, especially under phosphorus-limited conditions. In monoculture experiments, Microcystis aeruginosa and Peridinium umbonatum var. inaequale significantly increased their biomass when cultured with C-P bond phosphonates, whereas Scenedesmus bijuga failed to thrive under similar conditions. Peridinium umbonatum var. inaequale also displayed increased soluble protein content in response to glyphosate stress, indicating an adaptive stress response. In co-culture experiments, M. aeruginosa demonstrated greater tolerance to glyphosate than P. umbonatum var. inaequale, though biomass increases were not significantly correlated with soluble protein or APA. Sediment-water interface experiments revealed that glyphosate exposure significantly promoted cyanobacterial biomass, which was approximately five times greater than that of the inorganic phosphorus group. Notably, when cyanobacterial biomass exceeded 20% of the total, Cyanophyta replaced Chlorophyta as the dominant group, suggesting a potential competitive advantage under low-dose glyphosate exposure. These findings highlight that glyphosate may promote cyanobacterial dominance by altering phytoplankton community composition, potentially contributing to the increased frequency of harmful algal blooms in nutrient-limited aquatic environments.