WHERE DO THE METHANOGENS LIVE IF NOT DOWN-UNDER? MICROBIAL METHANE CYCLING IN AN UNUSUAL ‘UPSIDE DOWN’ CHEMICAL- GRADIENT IN LIGHTNING LAKE, OKLAHOMA

Lake sediments can be a substantial source of the major greenhouse gas methane. A redoxcline usually stratifies the sediment horizons covering a microbial metabolic repertoire ranging from an oxic horizon at the top of the sediment that represents aerobic microbial processes, to oxygen-depleted horizons with increasing sediment depth that display diverse anaerobic processes. Methane-producing archaea, also called methanogens, are abundant in the anoxic sediment layers producing methane from bacterial metabolic products like acetate, H₂ + CO₂, and methyl compounds. Lightning Lake, located in Stillwater, Oklahoma, harbors large amounts of methane that can if disturbed be released into the atmosphere. Surprisingly, the lake exhibits an ‘upside-down’ chemical gradient profile presenting the hot spots for methane production at the upper sediment layers, followed by the sulfate-reduction-, iron (III) reduction-, and microaerophilic zones with increasing depths. In this study, we aim to unravel the methanogenic potential and diversity hosted by this ‘upside-down’ lake profile and determine if and why that profile might support extensive methane formation from Lightning Lake.