Page 252 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Chapter 11. Integration and outlook
under both oxic and anoxic conditions. Besides the advantage of real-time tracking of GHG
fluxes, this method can accurately assess ebullitive fluxes.
AOM might play a stronger role as methane filter in thermokarst lakes
In coastal areas, rising sea levels caused by warming lead to the transition of freshwater thermokarst lakes into brackish to saltwater thermokarst lagoons, which alters the conditions for carbon and CH4 cycling communities (Capone and Kiene 1988; Ruz, Héquette and Hill 1992; Romanovskii et al. 2000; Logares et al. 2009). Projections of disproportionate sea level rise in the Arctic Ocean highlight the relevance of this transition state in the future (Yin, Griffies and Stouffer 2010).
The influx of sulfate (SO42-) at millimolar concentrations into a CH4-rich ecosystem introduces conditions feasible for sulfate-dependent methane oxidation (S-AOM) (Holmer and Storkholm 2001). Indeed, sequences of anaerobic sulfate-dependent AOM archaea were detected in our studies on thermokarst lagoons (S. Liebner, personal communication), as well as in thermokarst lakes (Chapter 7, 8 and unpublished data), and in several other studies on permafrost drained lakes, soils, and deep submarine permafrost sediments (Kao-Kniffin et al. 2015; Shcherbakova et al. 2016; Winkel et al. 2018).
In collaboration with the German Research Centre for Geosciences in Potsdam, we further investigated the potential geochemical and microbial changes upon the conversion of freshwater thermokarst lakes to brackish and marine thermokarst lagoons (unpublished data). Field observations show that thermokarst lagoons have a higher potential for CH4 oxidation compared to thermokarst lakes (Spangenberg et al. 2020). However, compared to freshwater thermokarst lakes, these lagoons experience lower levels of ebullitive emissions (Schindler 2019). This suggests that marine conditions have the potential to reduce CH4 emissions from these lakes. Nonetheless, thermokarst lagoons are still considered sources of CH4 (Shakhova and Semiletov 2007; Schirrmeister et al. 2018). There currently is no strong evidence for S- AOM as an important CH4 filter in permafrost-affected ecosystems, but S-AOM in thermokarst lagoons could provide a stronger CH4 sink in the future.
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