Integrated Understanding of Organic Carbon Concentration, Chemistry, and Thermodynamics on Hyporheic Metabolism.

Inputs of dissolved organic carbon (DOC) and nutrients from groundwater (GW) and surface water (SW) to the hyporheic zone strongly influence biogeochemical processes. Multiple field-based studies along the Columbia River have shown important influences of DOC character (chemistry and thermodynamics) on hyporheic zone biogeochemical function. To evaluate specific hypotheses derived from these field studies, we developed a non-invasive method to measure sediment oxygen consumption using fiber-optic sensors. This allowed us to analyze differences in metabolic rates within hyporheic zone sediments in response to changes in DOC character and concentration using batch incubations with hyporheic sediments from the Hanford site. We studied four compounds characteristic of GW (i.e. serine, ascorbate) and SW (i.e. lysine, propionate) at three different concentrations (1X, 10X, 30X). The two DOC sources supply contrasting thermodynamic profiles, with GW compounds being more thermodynamically favorable than SW compounds. Oxygen consumption rates were linear from 0 to 6 hr of incubation, and only 10 g of sediment were required for each treatment. Analysis revealed no significant relationship between respiration rates and the thermodynamic character of added DOC, although concentration effects were significant. This indicates that microbial communities were able to rapidly alter their metabolic machinery in order to oxidize the different DOC substrates. Multi-omic analyses are being pursued to reveal how microbial communities regulate/shift the expression of metabolic pathways to maintain consistent overall respiration rates despite changes in DOC characteristics. This work will provide a fundamental understanding of metabolic controls, in addition to the function of hydrobiogeochemical processes occurring in the groundwater/surface water mixing zones, and will enable concrete implementation of process-based predictions of river corridor health in watershed-scale models.