Nitrate bioreduction dynamics in hyporheic zone sediments under cyclic changes of chemical compositions

Abstract

Hyporheic zone (HZ) is the hotspot of contaminant biotransformation in ecosystems that affects water quality in both groundwater and river water. HZ is characterized by its dynamic changes in water flow direction that lead to the cyclic changes in aqueous chemical compositions such as dissolved oxygen (DO), nitrate, dissolved organic carbon, and their concentrations. Here we report a study on the effects of cyclic changes in the chemical conditions on the rates of nitrate bioreduction in sediments from a hyporheic zone, and the corresponding dynamics of the synthesis and decay of nitrate reductases. The results indicated that the rate of nitrate bioreduction in the second and third cycles was faster than that in the first one because the nitrate reductases produced in the first cycle still partially maintained their reactivity. The presence of DO significantly decreased the rates of nitrate reduction and nitrate reductase synthesis, but such an inhibition effect diminished in the later cycle. The coupled changes in nitrate concentrations and functional enzymes and genes abundances were generally consistent with an enzyme-based biogeochemical kinetic model. Overall the results indicated that enzyme growth dynamics and functions were complexly affected by the cyclic changes of environmental conditions, and implied that HZ sediments have a memory-type system behavior in reducing nitrate that would be affected by the frequency and magnitude of groundwater and river water exchanges, and chemical compositions in the groundwater and river water.