Size-specific mediation of the physiological responses and degradation ability of microalgae to sulfamerazine by microplastics

Antibiotics and microplastics (MPs) are two classes of emerging contaminants that are commonly found in various water environments. However, how different sized MPs affect the toxicity and biodegradation of antibiotics remains poorly understood. We investigated the effects of polystyrene (PS) MPs with different particle sizes (100 nm and 30 μm) on the physiological responses and degradation behavior of Phaeodactylum tricornutum to sulfamerazine (SMR). Results showed that microalgae growth was inhibited by SMR, and MPs especially those of smaller size exacerbated the inhibitory effects of SMR on microalgae, including decreasing the content of chlorophyll a, carotenoids, malondiadehyde and superoxide dismutase activity. MPs exhibited low adsorption towards SMR, and MPs especially 30 μm MPs strengthened SMR photodegradation through leaching more organic chemicals. In comparison, 100 nm MPs obstructed the light, resulting in insignificant effects on photodegradation. Apart from photodegradation, SMR could be bioaccumulated and biodegraded by microalgae, and biodegradation was the main removal mechanism. The overall influence of MPs on SMR degradation by microalgae was a balance of the promotion on photodegradation and negative effects on microalgae growth, with the degradation efficiency and rate of SMR significantly lower in treatment of 100 nm MPs (0.0128 ± 0.0012 day^-1, 30.13 ± 0.36 %) than treatments without MPs (0.0155 ± 0.0011 day^-1, 32.90 ± 3.11 %) or with 30 μm MPs (0.0165 ± 0.0013 day^-1, 34.46 ± 2.52 %). Overall, this study reveals the size-specific effects of MPs on the toxicity and degradation behavior of SMR, providing novel insights into the combined effects of SMR and MPs.