Céline Roose-Amsaleg

Nitrous oxide production kinetics during nitrate reduction in river sediments

A significant amount of nitrogen entering river basins is denitrified in riparian zones. The aim of this study was to evaluate the influence of nitrate and carbon concentrations on the kinetic parameters of nitrate reduction as well as nitrous oxide emissions in river sediments in a tributary of the Marne (the Seine basin, France). In order to determine these rates, we used flow-through reactors (FTRs) and slurry incubations; flow-through reactors allow determination of rates on intact sediment slices under controlled conditions compared to sediment homogenization in the often used slurry technique. Maximum nitrate reduction rates (R(m)) ranged between 3.0 and 7.1microg Ng(-1)h(-1), and affinity constant (K(m)) ranged from 7.4 to 30.7mg N-NO(3)(-)L(-1). These values were higher in slurry incubations with an R(m) of 37.9microg Ng(-1)h(-1) and a K(m) of 104mg N-NO(3)(-)L(-1). Nitrous oxide production rates did not follow Michaelis-Menten kinetics, and we deduced a rate constant with an average of 0.7 and 5.4ng Ng(-1)h(-1) for FTR and slurry experiments respectively. The addition of carbon (as acetate) showed that carbon was not limiting nitrate reduction rates in these sediments. Similar rates were obtained for FTR and slurries with carbon addition, confirming the hypothesis that homogenization increases rates due to release of and increasing access to carbon in slurries. Nitrous oxide production rates in FTR with carbon additions were low and represented less than 0.01% of the nitrate reduction rates and were even negligible in slurries. Maximum nitrate reduction rates revealed seasonality with high potential rates in fall and winter and low rates in late spring and summer. Under optimal conditions (anoxia, non-limiting nitrate and carbon), nitrous oxide emission rates were low, but significant (0.01% of the nitrate reduction rates).

The effect of environmental and therapeutic concentrations of antibiotics on nitrate reduction rates in river sediment

The use of antibiotics in both human and veterinary medicine has led to increased presence of these compounds and antibiotic resistance in the environment. In this study, the effect of low, environmentally relevant (mg L(-1)) concentrations of vancomycin (VA), flumequine (FLU), and sulfamethoxazole (SMX) on nitrate reduction rates was studied in river sediments. Nitrate reduction rates were determined by supplying intact sediments for several weeks with both nitrate and antibiotics (ng L(-1), μg L(-1), and mg L(-1) concentrations), including a non-amended control. Furthermore the concentrations of the three investigated antibiotics were measured in the initial (natural) sediments and the sediments supplied with the antibiotics. The antibiotic concentrations in the sediments decreased (on average 62% for FLU and 93% for SMX) during the experiments, indicating loss of antibiotics due to sorption or (bio) degradation. Nitrate reduction rates were not affected by environmental concentrations of VA, FLU and SMX. FLU and SMX only partially inhibited nitrate reduction rates at high, therapeutic concentrations by 41 and 39% respectively. The three tested antibiotics significantly enhanced the production of nitrite, an intermediate in dissimilatory nitrate reduction. Nitrite production increased 1.9 and 1.4 fold for environmental VA concentrations (107 and 187 μg L(-1) respectively), application of 58 mg L(-1) SMX resulted in a 7.5 fold increase and augmented 16 and 8.5 fold in the presence of respectively 13 μg L(-1) and 52 mg L(-1) FLU. Even though inhibition of nitrate reduction rates was observed at therapeutic antibiotic concentrations, nitrate reduction proceeded under all experimental conditions, indicating the presence of resistance toward these antibiotics among the nitrate reducing bacteria. The accumulation of nitrite suggests that the nitrite reduction step was more affected than the overall nitrate reduction process.

Exposure to vancomycin causes a shift in the microbial community structure without affecting nitrate reduction rates in river sediments

Antibiotics and antibiotic resistance genes have shown to be omnipresent in the environment. In this study, we investigated the effect of vancomycin (VA) on denitrifying bacteria in river sediments of a Waste Water Treatment Plant, receiving both domestic and hospital waste. We exposed these sediments continuously in flow-through reactors to different VA concentrations under denitrifying conditions (nitrate addition and anoxia) in order to determine potential nitrate reduction rates and changes in sedimentary microbial community structures. The presence of VA had no effect on sedimentary nitrate reduction rates at environmental concentrations, whereas a change in bacterial (16S rDNA) and denitrifying (nosZ) community structures was observed (determined by polymerase chain reaction–denaturing gradient gel electrophoresis). The bacterial and denitrifying community structure within the sediment changed upon VA exposure indicating a selection of a non-susceptible VA population.