Mechanism and Carbon-Induced Enhancement of Metronidazole Photodegradation by Tetradesmus obliquus

Metronidazole (MNZ) is a nitroimidazole antibiotic that is poorly removed by conventional biological wastewater treatment, motivating interest in microalgae-based tertiary treatment strategies. This study characterizes the concentration-dependent toxicity, degradation kinetics, and removal mechanism of MNZ by Tetradesmus obliquus monoculture, and examines whether external carbon supplementation can enhance its degradation capacity. Across a concentration range of 1–9 mg/L, MNZ inhibited T. obliquus growth and photosynthetic pigment content in a dose-dependent manner, while degradation efficiency declined correspondingly, from 25.5% at 1 mg/L to 12.8% at 9 mg/L. Dark-condition controls combined with reactive oxygen species quenching showed that MNZ removal was dominated by extracellular organic matter (EOM)-mediated indirect photodegradation, with triplet-state EOMs (3EOM*) identified as the dominant reactive species. Supplementation with exogenous glucose (0–200 mg/L) induced a heterotrophic metabolic shift that enhanced MNZ photodegradation in a dose-dependent manner; EEM-PARAFAC fluorescence analysis linked this enhancement to selective enrichment of protein-like, tryptophan/tyrosine-associated EOM components rather than a uniform increase in total EOM content. These findings indicate that the carbon-responsive enhancement of EOM photoreactivity previously reported for a methanotroph–T. obliquus co-culture system reflects a conserved algal metabolic response rather than a phenomenon contingent on bacterial partnership, and establish a mechanistic and kinetic baseline for algal-mediated antibiotic photodegradation independent of bacterial co-culture.