A New Approach for Determining In Situ Microbial Response to Geochemical Perturbations

Understanding the changes in microbial activity that occur as a result of geochemical perturbations (ex. recharge) is critical for predicting chemical fate and transport in natural systems and providing insight into nutrient and carbon cycling. Adequate assessment of the linkages between geochemistry and microbial community in situ has been challenging however, because current techniques do not allow for direct measurement. To address this issue a new technique, Native Organism Geochemical Experimentation Enclosures (NOGEEs), was developed to further evaluate the role of microorganisms through direct measurement of the effect of geochemical perturbations on a native microbial community. NOGEEs were designed to 1) trap a native microbial community in situ, 2) isolate the community, and 3) introduce and remove test solutions to measure resulting reactions rates. Two experiments were conducted using NOGEEs at the Norman Landfill research site in Norman, OK. The first experiment consisted of repeated introductions of a sulfate test solution over time. Findings showed increased rates of sulfate reduction corresponding to an increase in the abundance of sulfate reducing microorganisms. The second experiment tested the importance of changes in sulfate concentration and temperature on sulfate reduction rates. These NOGEE experiments were designed to evaluate differences in sulfate reduction rates for three sulfate concentrations (10, 25, and 100 mg/L SO₄^2-) during both warm and cold seasonal temperatures. Geochemical results indicated that higher concentrations of sulfate resulted in faster sulfate reduction rates. Variability in rates determined during the two different seasons indicated that warmer temperatures also resulted in faster sulfate reduction rates. Molecular analyses of the sulfate reducing population supported the geochemical data. Results from these experiments demonstrate the feasibility of this technique for in situ quantification of microbial reaction rates coincident with changes in the microbial population in response to a geochemical perturbation.