Beyond acute toxicity: How nanoplastics and norfloxacin jointly trigger cyanobacterial blooms by modulating lag-phase metabolism

Existing research on pollutant-induced algal blooms has primarily focused on high-concentration exposures during the logarithmic growth phase, largely overlooking the lag phase, a critical window for bloom initiation, under combined exposure at environmentally relevant levels. This study investigates the physiological and metabolic responses of Microcystis aeruginosa during the lag and logarithmic phases following exposure to environmentally relevant concentrations of norfloxacin (NOR), alone and in combination with carboxyl-modified polystyrene nanoplastics (PS-NPs). Our results demonstrate that PS-NPs significantly enhance the growth-promoting effect of low-dose NOR during the lag phase in a growth-stage-dependent manner, with these early-stage cells exhibiting more pronounced responses and subtle nitrogen metabolism disturbances, while logarithmic-phase cells showed only weak physiological changes. PS-NPs further amplified these stage-specific differences. Exposure to the combined L-NOR-NP treatment during the lag phase was associated with moderate antioxidant responses and did not induce significant membrane lipid peroxidation, while photosynthetic efficiency was sustained, facilitating toxin accumulation, and induced a distinct metabolic pattern characterized by stable carbon flux and selective nitrogen activation, in stark contrast to the general suppression of both carbon and nitrogen metabolism observed under high-concentration exposures. In contrast, L-NOR-NP exposure during the logarithmic phase induced oxidative stress and inhibited carbon metabolism, resulting in insufficient energy supply and limited growth promotion. These findings suggest that traditional high-concentration exposure paradigms may underestimate the ecological risks of emerging contaminants, and that lag-phase metabolic activation, characterized by stable carbon flux and selective nitrogen activation, could serve as a potential early-warning biomarker for cyanobacterial blooms.