Persistence of the antibiotic sulphametoxazole in river water microcosms

Many recent studies have reported the ubiquitous occurrence of antibiotics in aquatic ecosystems. Antibiotics can both kill natural microbial populations and act as a pressure for the selection of resistant bacteria. Following administration, antibiotics are only partially metabolized and, therefore, a large amount is excreted unaltered or as active metabolites via urine and faeces; consequently, human antibiotics reach wastewater treatment plants. The latter are not specifically designed for antibiotic removal, and consequently these molecules are released directly into the receiving environments. The fate and effects on health ecosystem of these compounds is not still well known. Sulfamethoxazole (SMX) is one of the most widely synthesized sulfonamides, prescribed to treat urinary infections; it is also used in veterinary practice, aquaculture and livestock breeding. Its mechanism of action is based on the inhibition of folic acid synthesis in bacteria. SMX has been detected in groundwater, in effluents of WWTPs and in surface water, including drinkable water. SMX has been reported to be not readily biodegradable, resistant to hydrolysis and the photo-degradation as a possible degradation process in surface waters. Data on biodegradation in natural water ecosystem are quite scarce. In this context, the aim of the present work was to investigate the persistence of the antibiotic sulphametoxazole in river water, focusing on both the biodegradation and photodegradation processes. For this purpose, two different microcosm experiments were set up using river water treated with 500 µg/L of SMX in the presence or absence (river water sterilized) of the natural microbial community. In order to evaluate SMX biodegradation, the first experiment was performed in the dark. In the second one, river water microcosms were incubated under UV-light, in order to evaluate the possible photodegradation of SMX. At fixed times, water sample were collected from microcosms for measuring SMX residual concentrations over time by-using HLPC-UV. The disappearance time of 50% of the initial SMX concentration (DT50) was evaluated in both experiments. Moreover, the effects of the antibiotic on the natural microbial community were assessed in terms of cell vitality, abundance and phylogenetic diversity comparing the treated river water microcosms with those no treated one.