Defensive responses of microalgal-bacterial granules to tetracycline in municipal wastewater treatment

Nowadays, tetracycline has been frequently detected in municipal wastewater, posing a pressing threat for wastewater treatment. This study investigated the defensive responses of microalgal-bacterial granules to tetracycline. It was found that the physical structure of microalgal-bacterial granules tended to shift from individual granules to loosely inter-connected agglomerates. In response to tetracycline, microalgae instead of bacteria in granules were found to produce more low molecular weight polysaccharides in extracellular polymeric substances (EPS), which increased from 0.26 mg C/g VSS in the control to 17.81 and 25.15 mg C/g VSS after being exposed to 1 and 10 mg/L of tetracycline, respectively. It was further revealed that tetracycline could bind to tryptophan in EPS proteins, and this action in turn could help to alleviate the direct toxicity of tetracycline to microorganisms in granules. Moreover, it appeared that the abundance of Pseudomonas-carrying tetracycline resistant genes increased substantially, together with gradual disappearance of Cyanobacteria.