Recovery of in-situ methanotrophic activity following acetylene inhibition

Methane (CH4) is the second most important greenhouse gas after carbon dioxide (CO2). To understand CH4 cycling, quantitative information about microbial CH4 oxidation in soils is essential. Field methods such as the gas push-pull test (GPPT) to quantify CH4 oxidation are often used in combination with specific inhibitors, such as acetylene (C2H2). Acetylene irreversibly binds to the enzyme methane monooxygenase, but little is known about recovery of CH4 oxidation activity after C2H2 inhibition in situ, which is important when performing several experiments at the same location. To assess recovery of CH4 oxidation activity following C2H2 inhibition, we performed a series of GPPTs over 8 weeks at two different locations in the vadose zone above a petroleum hydrocarbon-contaminated aquifer in Studen, Switzerland. After 4 weeks a maximum recovery of 30% and 50% of the respective initial activity was reached, with a subsequent slight drop in activity at both locations. Likely, CH4 oxidation activity and CH4 concentrations were too low to allow for rapid recovery following C2H2 inhibition at the studied locations. Therefore, alternative competitive inhibitors have to be evaluated for application in conjunction with GPPTs, especially for sites with low activity.