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24The study evaluates the chronic impact of the antibiotic tetracycline on the biodegradation of 25 organic substrate under anaerobic conditions. The experiments involved an anaerobic 26 M A N U S C R I P T A C C E P T E D ACCEPTED MANUSCRIPT2 sequencing batch reactor fed with a synthetic substrate mixture including glucose, starch and 1 volatile fatty acids, and operated in a sequence of different phases with gradually increasing 2 tetracycline doses of 1.65 -8.5 mg/L, for more than five months. Tetracycline exerted a 3 terminal/lethal effect at 8.5 mg/L on the microbial community under anaerobic conditions, 4 which caused the inhibition of substrate/COD utilization and biogas generation and leading to 5 a total collapse of the reactor. The microbial activity could not be recovered and re-started 6 within a period of more than 10 days, even after stopping tetracycline dosing. At lower doses, 7 substrate utilization was not affected but a reduction of 10-20% was observed in the 8 biogas/methane generation, suggesting that substrate utilization of tetracycline to the 9 biomass was limiting their bioavailability. During the experiments, tetracycline was partially 10 removed either through biodegradation or conversion into its by-products. The adverse long-11 term impact was quite variable for fermenting heterotrophic and methanogenic fractions of 12 the microbial community based on changes inflicted on the composition of remaining/residual 13 organic substrate. 14 15 16

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