Page 139 - Microbial methane cycling in a warming world From biosphere to atmosphere Michiel H in t Zandt

Walter Anthony and Zhang 2017). The availability of methanogenic substrates may differ between thermokarst lake regions, or even single lakes, due to differences in local physicochemical parameters such as pH and organic matter types and inputs (Matheus Carnevali et al. 2015). A study on thaw lakes in the Arctic Foothills province of Alaska found that the acetoclastic pathway dominated over the hydrogenotrophic pathway with a 2:1 ratio that further increased with sediment depth (Lofton, Whalen and Hershey 2015). A study on polygonal ponds, trough ponds, and lakes on Bylot Island, a continuous permafrost zone of the Eastern Canadian Arctic, also designated acetoclastic methanogenesis as the dominant process (Bouchard et al. 2015). In contrast with the aforementioned studies, Blodau et al. showed that, for a thermokarst pond and adjacent thermokarst depressions in Igarka, Northern Siberia, CH4 was produced mainly through hydrogenotrophic methanogenesis (Blodau et al. 2008). When thermokarst lakes develop into fully thawed thermokarst bogs, methanogenesis may shift from acetoclastic to hydrogenotrophic, as found in a microcosm study in peat bog samples from west Siberia by Kotsyurbenko et al. due to the low pH (3.8) of the bog (Kotsyurbenko 2005b).
A molecular survey, based on methanogenic marker genes, of thaw lakes that formed upon the collapse of palsas (frozen peat cores) and lithalsas (mineral core mounds) found both hydrogenotrophic Methanomicrobiales and versatile Methanosarcinales in near-bottom waters (Crevecoeur, Vincent and Lovejoy 2016). The dominance of Methanomicrobiales in functional gene (methyl-coenzyme M reductase, mcrA) transcript data on these thaw lakes indicates that hydrogenotrophic methanogenesis is the major pathway (Crevecoeur, Vincent and Lovejoy 2016). However, a study of transcriptional activity in methanogenic archaea showed co- occurrence of Methanomicrobiales and Methanosarcinales in thawed permafrost, but only an increase in transcriptional activity in thawed permafrost for acetoclastic Methanosarcinales (Wei et al. 2018). A study based on 16S rRNA gene amplification by Matheus Carnevali et al. showed that the archaeal population of thermokarst lakes on the north slope of Alaska is highly diverse and not yet characterized properly with possibly novel taxa with poorly characterized metabolism involved in methanogenesis (Matheus Carnevali et al. 2018). These findings stress the need for activity-based studies to gain insight into methanogenesis dynamics in the field.
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