Page 138 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 6. Roles of thermokarst lakes in a warming world
thawed permafrost soils release more nutrients than soils that have already been exposed to thawing, which boosts initial microbial activity leading to GHG production (Mackelprang et al. 2011; Reyes and Lougheed 2015). Radiocarbon age profiles, which were obtained by analyzing the 14C composition of CH4 ebullition seeps from Yedoma thermokarst lakes showed oldest CH4-carbon origins at lake margins and younger profiles towards the center, indicating rapid degradation of freshly thawed old permafrost C associated with lake expansion (Brosius et al. 2012; Wik et al. 2016). A study of thermokarst lake taliks found the highest CH4 production potentials in organic-rich lake sediments and freshly thawed permafrost soils (Heslop et al. 2015). In addition, the exposure of organic matter to sunlight within lakes transforms dissolved organic matter into microbiologically more easily accessible substrates (Laurion and Mladenov 2013). These findings illustrate how the input of freshly thawed organic matter can lead to peak emissions of GHGs from thermokarst lakes (Wik et al. 2016).
Overall, the microbial degradation of organic matter is highly dependent on oxygen availability. Under oxic conditions microbial mineralization mainly results in CO2 production, whereas anoxic conditions also lead to CH4 production. Despite their shallow depths (<4 m) thermokarst lakes and ponds are generally stratified, which results in separation of oxic and anoxic habitats (Rossi, Laurion and Lovejoy 2013; Deshpande et al. 2015). The constant input of organic matter during thaw stimulates microbial decomposition and leads to the formation of anoxic, methanogenic conditions toward the bottom (Deshpande et al. 2017). Thermokarst formation therefore affects local hydrology and limits oxygen availability, which in turn enhances the CH4 production potential (Grosse et al. 2011; Hayes et al. 2014; Lawrence et al. 2015).
Methanogenesis pathways are highly location specific
Methane is produced during the final step in the anaerobic degradation of organic matter (Conrad 2002) (Fig. 2). The main pathways for CH4 production in permafrost environments are hydrogenotrophic methanogenesis, which uses hydrogen (H2) and CO2 as substrates, and acetoclastic methanogenesis, which uses acetate as substrate (Kotsyurbenko 2005b; Wagner et al. 2005; Hershey, Northington and Whalen 2014; McCalley et al. 2014; Mondav et al. 2014; Liebner et al. 2015). There is, however, conflicting evidence as to which methanogenic pathway is dominant in thermokarst lake sediments (Brosius et al. 2012; Heslop et al. 2015; Heslop,
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