Page 141 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

showed highest methanogenesis rates in July and lowest rates in September, which indicates a positive temperature dependency (Mu et al. 2016a). However, although methanogens show a positive response to warming, the methanogenesis rates seem mainly affected by labile organic matter and substrate availability in lakes (Matheus Carnevali et al. 2015). A study on a thermokarst bog in interior Alaska showed a general substrate limitation as controlling factor of microbial GHG production rates (Neumann et al. 2016). Similarly, laboratory-based experiments on the sediment of a small Alaskan arctic lake showed that acetate availability limited methanogenesis rates (Hershey, Northington and Whalen 2014). A similar observation was made by de Jong et al. with incubations of thermokarst lake sediments from Utqiagvik, Alaska, in which substrate amendment stimulated acetoclastic and methylotrophic methanogens, indicating substrate limitation in the sediment (de Jong et al. 2018).
Aerobic methanotrophs function as important methane filter in thermokarst lakes
Methanotrophs function as a methane sink by oxidizing CH4 to CO2 (Fig. 2). CH4 is oxidized by both aerobic bacteria and anaerobic prokaryotes that can use a suite of alternative electron acceptors. Aerobic methanotrophy is thought to function as the main CH4 filter in permafrost environments (Dean et al. 2018). Aerobic methanotrophic bacteria belong to the Alphaproteobacteria (type II), Gammaproteobacteria (type I), and the Verrucomicrobia phylum (Trotsenko and Murrell 2008; Op den Camp et al. 2009; Semrau, DiSpirito and Yoon 2010). A study on a stratified humic, boreal lake in southern Finland showed that methanotrophs can function as an efficient CH4 filter, consuming up to 80% of the CH4 produced in the lake sediment (Kankaala et al. 2006). A study on thermokarst lakes that formed on collapsing lithalsas aligned in a north-south transect showed that methanotrophy in a northern lake was 10% of the CH4 emission rate, whereas at the southernmost lake it accounted for 60% of the CH4 emission rate. Even though total emissions of GHG were still higher at the southernmost lake, these data indicate that warmer temperatures have a disproportionate positive effect on methanotrophic activity (Matveev, Laurion and Vincent 2018). Besides temperature, aerobic methanotrophic activity is dependent on dissolved oxygen and CH4 concentrations. A study of 30 Alaskan lakes, including thermokarst and non-thermokarst lakes along a latitudinal transect, showed that the overall winter methanotrophic activity was controlled mainly by oxygen
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