Page 200 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 9. Long-term warming effects on permafrost soil microbial communities
temperature, nutrient availability, and vegetation influence soil organic carbon (SOC) decomposition and the resulting ratio of CO2 to CH4 emissions (Ganzert et al. 2007; McCalley et al. 2014; Mackelprang et al. 2016). Long-term carbon release from permafrost is associated with SOC quality (Schädel et al. 2014). After the initial degradation of labile organic matter fractions from plant organic matter, microbes have access to less labile organic matter fractions. These organic compounds are mainly degraded by microbial guilds with cellulase and hemicellulase activity (Schädel et al. 2014; Woodcroft et al. 2018). Therefore, the microbial community is expected to shift towards a population that degrades less labile organic matter on the long-term.
Increasing global efforts have recognized the need to understand the microbial ecology of thawing permafrost to better predict its role and fate in a warmer world, especially its impact on the global carbon budget (Schuur et al. 2008; Jansson and Taş 2014). Repeated freeze-thaw cycles modify the active layer communities to conserve energy and obtain nutrients from a diversity of substrates through aerobic and anaerobic processes as well as adaptation to survival under dynamic freeze-thaw conditions (Hultman et al. 2015). In contrast, microbial communities in permafrost layers can be very well conserved and of ancient origin (Rivkina et al. 2004; Mackelprang et al. 2011; Hultman et al. 2015; Holm et al. 2020). In extreme cryogenic environments, the microbial community is likely to maintain a high level of stress tolerance due to long-term cold exposure (Jansson and Taş 2014; Mackelprang et al. 2017). Permafrost thawing leads to shifts in microbial diversity, abundance, and activity within days to months (Mackelprang et al. 2011; Allan et al. 2014; de Jong et al. 2018). After thawing the microbial community can rapidly respond and induce biogeochemical cycling of various elements. The taxonomic and functional shifts, especially enrichment of genes involved in the carbon and nitrogen cycle as well as respiratory processes were identified from short-term thaw experiments and field studies on permafrost thawing (Mackelprang et al. 2011; Tas et al. 2014; Tveit et al. 2015; Singleton et al. 2018). Tveit et al. (2015) observed that under increasing temperatures, the microbiota in a permafrost-affected peatland modulates the metabolic and trophic interactions to maintain high fermentation rates and CH4 production. Additionally, Hultman et al. (2015) found low but detectable ferric iron [Fe(III)] reduction capacities in permafrost, whereas Fe(III) reduction was not detected in a thermokarst bog. In addition,
198