Karina Urmann

Assessment of microbial methane oxidation above a petroleum‐contaminated aquifer using a combination of in situ techniques

[1] Emissions of the greenhouse gas CH 4 , which is often produced in contaminated aquifers, are reduced or eliminated by microbial CH 4 oxidation in the overlying vadose zone. The aim of this field study was to estimate kinetic parameters and isotope fractionation factors for CH 4 oxidation in situ in the vadose zone above a methanogenic aquifer in Studen, Switzerland, and to characterize the involved methanotrophic communities. To quantify kinetic parameters, several field tests, so-called gas push-pull tests (GPPTs), with CH 4 injection concentrations ranging from 17 to 80 mL L -1 were performed. An apparent V max of 0.70 ±0.15 mmol CH 4 (L soil air) -1 h -1 and an apparent K m of 0.28 ± 0.09 mmol CH 4 (L soil air) -1 was estimated for CH 4 oxidation at 2.7 m depth, close to the groundwater table. At 1.1 m depth, K m (0.13 ± 0.02 mmol CH 4 (L soil air) -1 ) was in a similar range, but V max (0.076 ± 0.006 mmol CH 4 (L soil air) -1 h -1 ) was an order of magnitude lower. At 2.7 m, apparent first-order rate constants determined from a CH4 gas profile (1.9 h -1 ) and from a single GPPT (2.0 ± 0.03 h -1 ) were in good agreement. Above the groundwater table, a V max much higher than the in situ CH 4 oxidation rate prior to GPPTs indicated a high buffer capacity for CH 4 . At both depths, known methanotrophic species affiliated with Methylosarcina and Methylocystis were detected by cloning and sequencing. Apparent stable carbon isotope fractionation factors a for CH 4 oxidation determined during GPPTs ranged from 1.006 to 1.032. Variability was likely due to differences in methanotrophic activity and CH 4 availability leading to different degrees of mass transfer limitation. This complicates the use of stable isotopes as an independent quantification method.

Response of methanotrophic activity and community structure to temperature changes in a diffusive CH4/O2 counter gradient in an unsaturated porous medium

Microbial methane oxidation is a key process in the global methane cycle. In the context of global warming, it is important to understand the responses of the methane-oxidizing microbial community to temperature changes in terms of community structure and activity. We studied microbial methane oxidation in a laboratory-column system in which a diffusive CH(4)/O(2) counter gradient was maintained in an unsaturated porous medium at temperatures between 4 and 20 degrees C. Methane oxidation was highly efficient at all temperatures, as on average 99 +/- 0.5% of methane supplied to the system was oxidized. The methanotrophic community that established in the model system after initial inoculation appeared to be able to adapt quickly to different temperatures, as methane emissions remained low even after the system was subjected to abrupt temperature changes. FISH showed that Type I as well as Type II methanotrophs were probably responsible for the observed activity in the column system, with a dominance of Type I methanotrophs. Cloning and sequencing suggested that Type I methanotrophs were represented by the genus Methylobacter while Type II were represented by Methylocystis. The results suggest that in an unsaturated system with diffusive substrate supply, direct effects of temperature on apparent methanotrophic activity and community may be of minor importance. However, this remains to be verified in the field.