Algal organic matter accelerates the photodegradation of tetracycline: Mechanisms, degradation pathways and product toxicity

Algae are ubiquitous in eutrophic surface waters and they play important roles in the sunlight-driven transformation of environmental contaminants. The present study aims to investigate the role of algal organic matter (AOM) separated from Microcystis aeruginosa at different growth stages in tetracycline photodegradation along with the related photosensitive mechanisms. The results showed that AOM, both extracellular organic matter (EOM) and intracellular organic matter (IOM) at different growth phases all promote tetracycline photodegradation, but in different ways and performances. EOM exerts greater photolysis effectiveness than IOM with logarithmic phase (LP) organic matter more effective than that from the algae’s declining growth phase (DP). The promoting efficiencies of tetracycline photodegradation by four AOM types follow the order of LP-EOM > DP-EOM > LP-IOM > DP-IOM, accounting for 1.0420 h−1, 0.8281 h−1, 0.6527 h−1 and 0.4974 h−1, respectively. Such discrepant effectiveness is related to the different chemical constituents and structures of EOM and IOM. Scavenger experiments further demonstrated that triplet-state AOM (3AOM*) contributes the most to tetracycline degradation (30.01%∼48.51%), followed by OH (1.68%∼10.66%) and 1O2 (1.34%∼10.08%). The steady-state concentrations of 3AOM*, OH and 1O2 were measured with probe compounds in the range of 13.950 ∼ 155.234 L/mol, 2.494 × 10−17 ∼ 27.636 × 10−17 M and 1.410 × 10−14 ∼ 26.493 × 10−14 M, respectively. Interestingly, the photolytic intermediates show increasing toxicity to Photobacterium phosphoreum at first which then decreases with the extension of irradiation. These results provide comprehensive insight into the transport and environmental behavior of antibiotics in a eutrophic aqueous environment.