Page 170 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 7. Methane cycling in Arctic thermokarst lake sediments
slightly increased compared to their abundance in the original sediment, which is in line with the postulation that “Candidatus Methanoflorens stordalenmirensis” of the order Methanocellales might be a key player in the CH4 positive feedback loop in thawing permafrost (McCalley et al. 2014; Mondav et al. 2014). The CH4 production rates in the control incubations (1.0 and 1.7 μmol CH4 gdw-1 d−1 at 4°C and 10°C respectively) were similar as previously observed in permafrost soil, where CH4 production ranged between 0.1 and 0.9 μmol CH4 (g wet soil)-1 d−1 after 1 week of incubation at 10°C (Barbier et al. 2012), assuming the same wet to dry weight conversion factor of 3 as was measured in our study. Methanol amendment (2 mM) might have had an inhibitory effect at both temperatures, possibly related to toxicity, since the observed methanogenic activity was lower than the control. Similar observations were made in a methanogenic mesocosm study with Arctic sediment amended with methanol (10 mM) (Blake et al. 2015). Curiously, standard incubations for meso- and thermophilic methylotrophic methanogenic cultures include similar and even higher concentrations of methanol without any of the potential toxic effects observed in our study (Doerfert et al. 2009; Mochimaru et al. 2009; Allan et al. 2014). Although there was no methanogenic activity observed with H2/MeOH as substrates, the clade Terrestrial Miscellaneous Group, belonging to the class Thermoplasmata, increased in relative abundance in these cultures. The class Thermoplasmata contains the recently found methanogenic lineage Methanomassiliicoccales (Dridi et al. 2012; Iino et al. 2013) that use H2 and methanol or methylamines as substrates for methanogenesis, which were also found in thaw ponds and permafrost affected wetlands (Crevecoeur et al. 2015; Yang et al. 2017).
The CH4 produced by the methanogens is commonly oxidized to CO2 by methanotrophs in the sediment. In this study, oxidation rates increased by 57% at 10°C compared to 4°C, which is in line with previous observations from experiments in serum bottles (Knoblauch et al. 2008; Duc, Crill and Bastviken 2010; He et al. 2012; Stackhouse et al. 2017). Knoblauch et al. (2008) observed highest oxidation rates at 22°C in Siberian permafrost soils and an increase at 10°C compared to 0°C and 6°C, although rates were almost 100-fold different between their two sampling sites. He et al. (2012) studied Alaskan Arctic lake sediment, where the oxidation rates ranged between 0.3 and 1.3 μmol CH4 gdw-1 d−1 at 4°C (5 days of incubation) and 27.3-29.5 μmol CH4 gdw-1 d−1 at 10°C (4 days of incubation). We observed rapid enrichment of the
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