Page 142 - 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
availability, whereas summer methanotrophic activity was limited by CH4 availability (Martinez-Cruz et al. 2015). In a comparative study on different lake types, Yedoma thermokarst lakes showed highest methanotrophy rates, which is linked to their relatively higher carbon inputs (Sepulveda-Jauregui et al. 2015). This is in line with the link between high methanogenesis rates and high methanotrophy rates, as found in a study on a small Alaskan lake (Hershey, Northington and Whalen 2014).
Methanotrophs are also detected in low-oxygen bottom waters and lake sediments (Crevecoeur et al. 2017). The study of Crevecoeur et al. using pmoA gene transcripts found presence of methanotrophs regardless of local oxygen conditions (Crevecoeur et al. 2017). Osudar et al. report on a correlation between CH4 oxidation rates and CH4 concentrations in Siberian lakes and rivers that were all emitting CH4 (Osudar et al. 2016). These findings indicate that even thermokarst lakes that have a large potential for methanotrophy remain net CH4 sources.
A 16S rRNA gene-based study of Crevecoeur et al. on shallow ponds of the Nunavik subarctic region in Québec showed that methanotrophs were the most abundant bacterial group in all water bodies (median 4.9%, maximum 27%) (Crevecoeur et al. 2015). This is in line with a 16S rRNA gene-based study on four subarctic thermokarst ponds near the coast of Hudson Bay, Canada, which found that 20% of clone library sequences belonged to methanotrophic bacteria (Methylobacter, Crenothrix, and Methylocystis) (Rossi, Laurion and Lovejoy 2013). A pmoA transcript-based quantification study on thermokarst ponds in subarctic Québec showed dominance of type I methanotrophs, belonging mainly to Methylobacter species (Crevecoeur et al. 2017). Methylobacter is a dominant member of the aerobic methanotrophic community in many permafrost-affected landscapes, both in soil and lake environments (Liebner et al. 2009; Graef et al. 2011; He et al. 2012; Tveit et al. 2013).
Verrucomicrobial methanotrophs have also been detected in ponds of sporadic and discontinuous permafrost in Québec, Canada (1-6% of bacterial community 16S rRNA gene reads) (Crevecoeur et al. 2015; Comte et al. 2016). Their diversity in thermokarst lakes might be underestimated due to the fact that Verrucomicrobial methanotrophs are not picked up by standard primers (He et al. 2012). During permafrost thaw progression, submerged or floating peat mosses can cover the water surface giving rise to a more effective methane filter due to methane-oxidizing bacteria (MOB) colonizing the peat moss (Blodau et al. 2008; van Winden
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