Page 177 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
Thermokarst lakes, which are widespread in the Arctic and subarctic landscape, are important greenhouse gas (GHG) sources in a warming world (Osterkamp et al. 2009; Deshpande et al. 2015; Schuur et al. 2015; Matveev et al. 2016). Methane (CH4) emissions are of special interest due to their strong global warming potential of 34 for 100 years (compared to the climate impact of CO2) (Myhre et al. 2013). It is important to consider the time horizon because of the relatively short lifetime of CH4 in the atmosphere. Currently, thermokarst lakes release an estimated 4.1 ± 2.2 Tg CH4 y-1, which equals 2.2% of global wetland CH4 emissions (Saunois et al. 2016; Wik et al. 2016). In a warming world, however, both the increase in organic matter bioavailability and elevated temperatures can induce microbial respiration, resulting in rapid oxygen depletion, production of intermediates, and subsequent stimulation of methanogenesis in these lakes (van Huissteden et al. 2011; Deshpande et al. 2017; Dean et al. 2018).
Several studies indicate that CH4 production in high-latitude wetlands is mainly limited by substrate availability and, to a lesser extent, by low temperatures (Valentine, Holland and Schimel 1994; Hershey, Northington and Whalen 2014; Matheus Carnevali et al. 2015; de Jong et al. 2018; Chang et al. 2019). With warming-induced thaw progression, the release of labile organic matter is expected to increase (Ewing et al. 2015; Mueller et al. 2015). Under anoxic conditions, this has the potential to increase CH4 emissions from thermokarst lakes. However, experimental data on the link between warming, increased substrate availability and CH4 production in these lake sediments is not well explored.
In our previous study we performed combined experimental warming and substrate amendments on thermokarst lake sediments from Utqiaġvik, Alaska, to mimic the expected warming-induced increase of in situ substrates (de Jong et al. 2018). Amendments were performed in separate triplicate incubations to investigate the substrate-specific responses of the microbial community. The increase in temperature from 4°C to 10°C reduced methanogenic lag phases and increased methanogenesis rates with up to 30% for acetate (acetoclastic methanogenesis) and 38% for trimethylamine (TMA) (methylotrophic methanogenesis). Hydrogenotrophic methanogenesis did not seem to play a major role in this ecosystem (3-5% conversion efficiency to CH4). Total CH4 production was not affected by temperature, which indicated that substrate availability was the main controlling factor.
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