Impacts of trace ofloxacin on autotrophic denitrification process driven by pyrite/sulfur: Performance, microbial community evolution and metagenomic analysis

In this work, ofloxacin (OFL), a kind of frequently detected antibiotic in groundwater, was selected to explore its impact (at ng/L-µg/L-level) on denitrification performance in an autotrophic denitrification system driven by pyrite/sulfur (FeS₂/S⁰). Results showed that OFL restrained nitrate removal efficiency, and the inhibition degree was positively related to the concentration of OFL. After being exposed to increased OFL (200 ng/L-100 µg/L) for 69 days, higher inhibition of electron transport activity (ETSA), enzyme activities of nitrate reductase (NAR), and nitrite reductase (NIR) were acquired. Meanwhile, the extracellular protein (PN) content of sludge samples was remarkably stimulated by OFL to resist the augmented toxicity. OFL contributed to increased microbial diversity and sulfur/sulfide oxidation functional genes in ng/L-level bioreactors, whereas led to a decline in µg/L level experiments. With OFL at concentrations of 200 ng/L and 100 µg/L, the whole expression of 10 key denitrification functional genes was depressed, and the higher the OFL concentration, the lower the expression level. However, no significant proliferation of antibiotic resistance genes (ARGs) either in 200 ng/L-OFL or 100 µg/L-OFL groups was observed. Two-factor correlation analysis results indicated that Thiobacillus, Anaerolineae, Anaerolineales, and Nitrospirae might be the main hosts of existing ARGs in this system.