Page 153 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Introduction
In Arctic and sub-Arctic ecosystems, thermokarst lakes cover about 20-40% of the permafrost regions (Osterkamp et al. 2009; Koch, Gurney and Wipfli 2014; Abbott et al. 2015; Schuur et al. 2015; Narancic et al. 2017). Thermokarst lakes evolve through horizontal and vertical permafrost degradation, which creates characteristic eroding shorelines from where sediment is slumping into the lake. These lakes are generally shallow (<3.0 m) and range in size from a few square meters to hundreds of square kilometers (Grosse, Jones and Arp 2013; Wik et al. 2016). Due to warming, there is a retreat or melting of permafrost that may result in an increase in lake numbers and expansion of their area (Karlsson, Jaramillo and Destouni 2015). Interestingly, these thermokarst lakes contribute significantly to methane (CH4) emissions (Walter Anthony et al. 2006, 2008; Olefeldt et al. 2016; Wik et al. 2016), releasing an estimated 4.1 ± 2.2 Tg CH4 y-1 (Wik et al. 2016). With the prediction of 20 more ice-free days on Arctic water bodies before 2079 (Dibike et al. 2011; Prowse et al. 2011), CH4 emissions from thermokarst lakes are expected to increase by 30% (Wik et al. 2016).
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The majority of globally emitted CH4 (60-70%) originates from biogenic sources (Kirschke et al. 2013; Myhre et al. 2013). Methane is produced by methanogenic archaea as the terminal step of microbial decomposition of organic matter in anoxic environments. Methanogenic substrates include hydrogen/carbon dioxide (H2/CO2) (hydrogenotrophic methanogenesis), methylated compounds including methanol (MeOH) (methylotrophic methanogenesis), acetate (acetoclastic methanogenesis), and methoxylated aromatic compounds (methoxydotrophic methanogenesis) (Garrity, Bell and Lilburn 2004; Cheng et al. 2007; Thauer and Shima 2008; Dridi et al. 2012; Mayumi et al. 2016). In permafrost soils, H2/CO2 and acetate are the main substrates (Kotsyurbenko et al. 2001; Basiliko et al. 2003; Nozhevnikova et al. 2003; Galand et al. 2005; Kotsyurbenko 2005b; Metje and Frenzel 2007; McCalley et al. 2014; Yang et al. 2017), but there are contradicting reports on the respective predominant methanogenesis pathway in thawing permafrost. Several studies found an association of acetoclastic methanogenesis with thawing permafrost (Høj, Olsen and Torsvik 2008; Barbier et al. 2012; McCalley et al. 2014; Mondav et al. 2014; Blake et al. 2015; Coolen and Orsi 2015; Gill et al. 2017; Voigt et al. 2017a), while other data suggests an increase in hydrogenotrophic
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