Effects of sulfonamide antibiotics on aquatic microbial community composition and functions.

Knowledge on interactions among microbial communities colonizing various streambed substrata (e.g. cobbles, sediment, leaf-litter etc.) is essential when investigating the functioning of stream ecosystems. However, these interactions are often forgotten when assessing the responses of aquatic microbial communities to chemical contamination. Using a stream microcosm approach, the respective impact of two sulfonamide antibiotics (sulfamethoxazole and sulfamethazine) on the ability of microbial heterotrophs to decompose alder leaves was investigated in the presence or absence of periphyton. Our hypothesis suggested that sulfonamides would negatively impair microbial litter decomposition and that periphyton could possibly alleviate this effect by stimulating microbial decomposer activity through a priming effect. Results showed that the presence of periphyton enriched water with oxygen and labile dissolved organic carbon forms. However, these labile organic carbon sources did not stimulate leaf-litter decomposition but mostly decoupled microbial decomposer activity from particulate organic matter to dissolved organic matter through negative priming. Also, the two sulfonamide molecules did not affect the leaf-litter decomposition process but significantly decreased bacterial biomass accrual on leaves. The reduction of bacteria was concomitant with an increase in biomass-specific β-glucosidase activity and this was attributed to a stress response from bacteria to sulfonamides. Further research looking at microbial interactions would provide for better assessment of chemical contamination effects in communities and processes in stream ecosystems.