Nitrate reduction coupled with microbial oxidation of sulfide in river sediment

Nitrate (NO (3) (-) ) is often considered to be removed mainly through microbial respiratory denitrification coupled with carbon oxidation. Alternatively, NO (3) (-) may be reduced by chemolithoautotrophic bacteria using sulfide as an electron donor. The aim of this study was to quantify the NO (3) (-) reduction process with sulfide oxidation under different NO (3) (-) input concentrations in river sediment. Under NO (3) (-) input concentrations of 0.2 to 30 mM, flow-through reactors filled with river sediment from the Pearl River, China, were used to measure the processes of potential NO (3) (-) reduction and sulfate (SO (4) (2-) ) production. Molecular biology analyses were conducted to study the microbial mechanisms involved. Simultaneous NO3 (-) removal and SO4 (2-) production were observed with the different NO (3) (-) concentrations in the sediment samples collected at different depths. Potentially, NO (3) (-) removal reached 72 to 91 % and SO (4) (2-) production rates ranged from 0.196 to 0.903 mM h(-1). The potential NO (3) (-) removal rates were linearly correlated to the NO (3) (-) input concentrations. While the SO (4) (2-) production process became stable, the NO (3) (-) reduction process was still a first-order reaction within the range of NO (3) (-) input concentrations. With low NO (3) (-) input concentrations, the NO (3) (-) removal was mainly through the pathway of dissimilatory NO (3) (-) reduction to NH (4) (+) , while with higher NO (3) (-) concentrations the NO (3) (-) removal was through the denitrification pathway. While most of NO (3) (-) in the sediment was reduced by denitrifying heterotrophs, sulfide-driven NO (3) (-) reduction accounted for up to 26 % of the total NO (3) (-) removal under lower NO (3) (-) concentrations. The vertical distributions of NO (3) (-) reduction and SO (4) (2-) production processes were different because of the variable bacterial communities with depth.