Mitigating antibiotic pollution using cyanobacteria: Removal efficiency, pathways and metabolism

The occurrence of pharmaceuticals and personal care products (PPCPs) in wastewater poses huge environmental threats, even at trace concentrations, and novel approaches are urged due to the inefficiencies of conventional wastewater treatment plants, especially when processing contaminants at high concentrations. Meanwhile, another widespread problem in the aquatic domain is the occurrence of harmful algal blooms (HABs) which cause serious damage to the ecosystem, but have rarely been investigated for possible valorization. This study investigated the possibilities, mechanisms, and effects of toxin release of using a harmful cyanobacterial species, Microcystis aeruginosa (M. aeruginosa), in order to remove the widely used drug, tetracycline, at high concentration. The results were compared with the performance obtained by the use of the hitherto generally-selected chlorophyte alga Chlorella pyrenoidosa (C. pyrenoidosa) for tetracycline concentrations of 10-100 mg L^-1. M. aeruginosa exhibited a much more effective and rapid tetracycline removal (over 98.0% removal in 2 days) than did C. pyrenoidosa (36.7%-93.9% in 2 days). A comprehensive kinetic investigation into probable removal pathways indicated that, theoretically, bio-remediation dominated the process by M. aeruginosa (71.6%), while only accounting for 20.5% by C. pyrenoidosa. Both microalgae promoted the hydrolysis of tetracycline under conditions of increased pH and inhibited abiotic photolytic reactions by the shading effect to the water column, when compared with control experiments. Although identical degradation by-products were identified from treatments by both microalgal species, distinct by-products were also confirmed, unique to each treatment. Moreover, the growth of M. aeruginosa biomass exhibited strong tolerance to tetracycline exposure and released significantly lower levels of microcystin-LR, compared with the control systems. This study supports the possibility of reusing HABs species for the effective remediation of antibiotics at high concentrations. We have further suggested possible mechanisms for remediation and demonstrated control of toxin release.