Page 144 - 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
currently no experimental evidence on the activity of nitrate-, manganese-, or iron-dependent AOM in thermokarst lakes.
Nitrite-dependent CH4 oxidation is carried out by “Candidatus Methylomirabilis” belonging to the bacterial NC10 phylum. These bacteria produce oxygen for CH4 activation via an intra- aerobic pathway (Ettwig et al. 2010). So far, these bacteria have not been detected in permafrost soils or thermokarst environments. However, primer-based studies are prone to miss bacteria of the phylum NC10, which stresses the importance for future metagenomics studies to identify the full microbial methanotrophic potential of these ecosystems (Luesken et al. 2011; Crevecoeur et al. 2017).
Marine influence introduces anaerobic methanotrophic archaea (ANME) capable of sulfate-dependent anaerobic oxidation of methane
Due to rising sea levels, coastal thermokarst lakes and lacustrine depressions can be converted into thermokarst lagoons (Ruz, Héquette and Hill 1992; Romanovskii et al. 2000). Projections of estimated sea level rise range between 0.18 and 0.59 m globally, and the Arctic Ocean is expected to face the greatest magnitude of increase (Yin, Griffies and Stouffer 2010; Barnhart et al. 2014). This change will affect both coastal erosion rates and salinification of coastal permafrost and thermokarst lakes (Barnhart et al. 2014). Thermokarst lagoons form the transitional state between a freshwater and saltwater environment (Schirrmeister et al. 2018).
Such a transitioning process is nowadays visible on the Bykovsky Peninsula and the Yana River Delta in Siberia (Romanovskii et al. 2000). Within the Bykovsky Peninsula and the Khorogor Valley of Siberia, thermokarst processes have affected over half of the area, and around 4% of the area is covered by lagoons (Grosse et al. 2005). A marine connection introduces sulfate into CH4-rich environments. Under anoxic conditions, spatial co-occurrence of CH4 and sulfate can induce sulfate-dependent anaerobic oxidation of methane (sulfate AOM) (Fig. 2) (Boetius et al. 2000).
Schirrmeister et al. calculated that the Bykovsky Peninsula in Siberia contains 1.68 ± 0.04 Mt of organic carbon in the upper 6 meters (Schirrmeister et al. 2018). Observed CH4 emissions from the Ivashkina Lagoon range between 5 and 24 g m-2 per day and occurred through bubble seeps (Shakhova et al. 2015). This finding indicates that methanotrophy could limit diffusive CH4 emissions from the lagoon. Sequences for anaerobic sulfate-dependent AOM (ANME-2a,b
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