Physiological characteristics of microalgae in response to glyphosate and tetracycline regulated by exogenous melatonin

Microalgae exhibit significant promise in wastewater bioremediation owing to their exceptional environmental adaptability. This investigation systematically evaluated the effects of glyphosate (herbicide) and tetracycline (antibiotic) on four microalgal species Chlorella sorokiniana , Scenedesmus quadricauda , Chlorella sp., and Chaetoceros sp., while elucidating melatonin's regulatory role in stress resilience. The results showed that 800 μg/L glyphosate promoted growth in all species, with the cell counts of S. quadricauda increasing by 42.25 %. Low glyphosate concentrations stimulated C. sorokiniana growth, while high concentrations inhibited it. Tetracycline (10 μg/L) enhanced growth, with C. sorokiniana biomass increasing by 25.77 % at 50 μg/L. Organic pollutants increased malondialdehyde (MDA) content and decreased catalase (CAT) activity. Glyphosate inhibited CAT activity in Chaetoceros sp. by 28.00 %. Melatonin (10 μg/L) significantly improved stress resistance , increasing S. quadricauda biomass by 49.58 % at 10 μg/L glyphosate. Melatonin attenuated MDA levels (18.55 % to 56.11 % reduction) and markedly elevated CAT activity (108.37 % increase) in Chlorella sp. Melatonin promoted lipid accumulation in S. quadricauda , Chlorella sp., and Chaetoceros sp., with the lipid content of Chlorella sp. reaching 2.58 times than the control under combined melatonin-glyphosate treatment. This study provides critical insights for optimizing microalgae-based treatment systems targeting glyphosate- and tetracycline-contaminated wastewater, particularly through melatonin-mediated stress mitigation strategies. • Marine algae may exhibit greater glyphosate tolerance than freshwater species. • Low-dose glyphosate and melatonin synergistically enhanced microalgal growth. • Melatonin-mediated stress regulation in microalgae may be time-dependent.