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Sulfonamide antibiotics undergo transformation in the aquatic environment through biodegradation, photolysis, or hydrolysis. In this study, the residual antibacterial activity of 11 transformation products (TPs) of sulfamethoxazole (SMX) was investigated with regard to their in vitro growth and luminescence inhibition on Vibrio fischeri (30 min and 24 h exposure). Two transformation products, 4-hydroxy-SMX and N(4)-hydroxy-acetyl-SMX, were synthesized in-house and confirmed by nuclear magnetic resonance and high-resolution mass spectrometry. Results of individual compound experiments showed that TPs modified at the para amino group still exhibit clear antibacterial effects, whereas TPs resulting from breakdown of the SMX structure lost this mechanism of action. 4-NO2- and 4-OH-SMX were found to inhibit growth to a clearly greater extent than the parent compound, SMX. In contrast, the N(4)-acetyl- and N(4)-hydroxy-acetyl-derivatives retain less than 10 and 5% of the effect of SMX on growth and luminescence inhibition, respectively. The effect of a mixture of para-modified TPs was observed to be additive. Considering the homologous series of sulfa drugs widely prescribed and their common mechanism of action, the potential environmental impact must consider the total amount of sulfonamide antibiotics and their derivative TPs, which might end up in a water body. Extrapolating the results obtained here for the para TPs of SMX to other sulfa drugs and determining the persistence and occurrence of these compounds in the aquatic environment is required for improved risk assessment.

Nanoparticles (NP) are ubiquitous in environmental and technical aquatic systems. Understanding the role and the fate of NP in these systems is an interdisciplinary challenge requiring innovative experimental, theoretical and analytical approaches and critical reflection of classical concepts. This contribution critically reviews the outstanding properties of NP and the resulting consequences for their behaviour in environmental and technical aquatic systems considering natural NP which are mostly geogenic or biogeogenic, and engineered NP. Owing to the severe lack of data on the occurrence of NP in environmental aquatic systems, it is a key task of researchers to further develop analytical methods for the sensitive detection of NP directly in aqueous samples. There is urgent need for standardisation of analytical methods for detection and characterisation of NP, and for toxicity tests to assess possible adverse effects of NP. In this context, NP reference materials have to be defined as a common fundament for research in this field.