Page 218 - 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
conditions (Wells and Wilkins 1996) and they are able to ferment plant polysaccharides in soils (Boutard et al. 2014). Exposure to anoxic conditions led to an increase in relative abundance of this lineage within permafrost-affected soils in the Lena Delta (Wagner, Kobabe and Liebner 2009) and High Arctic permafrost soil from Spitsbergen, Norway (Hansen et al. 2007). A study on Greenland permafrost-affected soils described a positive correlation of Clostridia with hydrolytic and oxidative enzyme activities (Gittel et al. 2014). These findings together with our observations suggest a potential role of Clostridia in the degradation of organic matter under thaw exposure and long-term warming. In addition, dominance and increase of Parcubacteria (Candidate Division OD1) and Microgenomatia (Candidate Division OP11) within the Patescibacteria superphylum (Rinke et al. 2013) have been observed in permafrost thaw ponds (Vigneron et al. 2013; Wurzbacher et al. 2017) and permafrost-affected soils (Frey et al. 2016) similar to our observation. These taxa seem to be important players in a warming Arctic.
The long-term incubations also resulted in an overall decrease in abundance of methanogenic archaea, as briefly discussed above in the context of GHG production rates. This observation is in contrast with several short-term incubation studies on permafrost soils showing an increase in methanogen abundance (Mackelprang et al. 2011; McCalley et al. 2014). The decrease of methanogens observed here is likely explained by the much longer incubation period that resulted in an overall (relative) decrease in many genes and pathways involved in energy metabolisms, and not just by genes and gene clusters involved methanogenesis. Despite the overall decrease of methanogen abundance, an overall increase of members of the hydrogenotrophic Rice Cluster II clade closely affiliated with “Candidatus Methanoflorens stordalenmirensis” was observed. This methanogenic archaeon is a key species in thawing permafrost (Liebner et al. 2015) that can be used to predict CO2 to CH4 carbon emission ratios (McCalley et al. 2014; Mondav et al. 2014). At the same time, Methanosarcinaceae were only detected in AL before the incubation, and Methanosaetaceae decreased in all samples. Both families have been observed in permafrost peatlands and soils and their existence is linked to acetate availability (Yavitt and Seidmann-Zager 2006; Ganzert et al. 2007). Although this is in contrast with the study of Wei et al. (2018) that showed a shift to Methanosarcinales after thaw exposure, it further supports the formation of a less acetate-driven system, which is linked to the degradation of labile organic matter and the formation of a more hydrogen-driven system
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