Dose-response pattern of marine macroalgae Gracilariopsis heteroclada to three fluoroquinolones and the cause-effect relationship of dose, growth, toxicity and sorption

Fluoroquinolone (FQ) antibiotics, as emerging micropollutants, have been shown to inhibit the growth of aquatic plants. Conversely, these affected plants simultaneously exhibit FQ uptake capacity. However, the response patterns and removal efficiency of macroalgae exposed to FQs, and the cause-effect relationships between these processes, remain unclear. This study investigated the growth and toxicity responses of Gracilariopsis heteroclada to norfloxacin, enrofloxacin, and lomefloxacin over 96 h exposure at doses of 0–128 mg/L. Residual FQ concentrations in seawater were determined after 72 h. Structural equation modeling was applied to explore the relationships between initial dose, algal responses, and FQ sorption. Growth response patterns of G. heteroclada under each FQ exposure followed hormetic effects, with bell-shaped or Hill dose-response curves. FQs exhibited limited toxicity to G. heteroclada, with most toxicity curves indicating a lack of 50 % inhibition before 96 h. After 72 h, FQ removal rates ranged from 54.18 % ± 6.10 % to 97.93 % ± 0.17 %. Norfloxacin showed the highest removal rate and enrofloxacin the lowest. Growth response was the primary factor directly influenced FQ sorption (regression coefficient: 7.044), while toxicity had an inverse effect (−1.683). The specific growth rate acted as both cause and effect of the removal rate and terminal concentration, and the percent inhibition of the specific growth rate served as an important mediating variable. These findings suggest that environmental trace levels of FQs can stimulate the growth of G. heteroclada, which demonstrated strong tolerance to high FQ concentrations. However, reduced removal efficiencies at environmentally relevant doses and acute exposure constrain bioremediation applications, while chronic exposure risks necessitate further ecotoxicity assessments.