First evidence for cold-adapted anaerobic oxidation of methane in deep sediments of thermokarst lakes

M Winkel,K Martinez-Cruz,F Horn,J K Heslop,S Liebner,R Rijkers,K M Walter Anthony,A Sepulveda-Jauregui

doi:10.1088/2515-7620/ab1042

Abstract

Microbial decomposition of thawed permafrost carbon in thermokarst lakes leads to the release of ancient carbon as the greenhouse gas methane (CH4), yet potential mitigating processes are not understood. Here, we report δ13C–CH4 signatures in the pore water of a thermokarst lake sediment core that points towards in situ occurrence of anaerobic oxidation of methane (AOM). Analysis of the microbial communities showed a natural enrichment in CH4-oxidizing archaeal communities that occur in sediment horizons at temperatures near 0 °C. These archaea also showed high rates of AOM in laboratory incubations. Calculation of the stable isotopes suggests that 41 to 83% of in situ dissolved CH4 is consumed anaerobically. Quantification of functional genes (mcrA) for anaerobic methanotrophic communities revealed up to 6.7 ± 0.7 × 105 copy numbers g−1 wet weight and showed similar abundances to bacterial 16S rRNA gene sequences in the sediment layers with the highest AOM rates. We conclude that these AOM communities are fueled by CH4 produced from permafrost organic matter degradation in the underlying sediments that represent the radially expanding permafrost thaw front beneath the lake. If these communities are widespread in thermokarst environments, they could have a major mitigating effect on the global CH4 emissions.

Highlights

Permafrost contains about 1307 Pg carbon (C), with a substantial amount (450 Pg C; >25% of thaw-susceptible C) found in the Yedoma regions (Walter Anthony et al 2014, Strauss et al 2017)
Pore water and bubble methane analysis Radiocarbon dating of CH4-rich bubbles from the borehole that reflects the transitional permafrost revealed a C age of 21.1 ± 0.08 kyr, in comparison to CH4 of bubbles from natural ebullition events that showed a mixture of old and young carbon
We found that anaerobic oxidation of methane (AOM) occurs in thermokarst lake sediments and mitigates diffusive CH4 emission from these lakes

Summary

Introduction

Permafrost contains about 1307 Pg carbon (C), with a substantial amount (450 Pg C; >25% of thaw-susceptible C) found in the Yedoma regions (Walter Anthony et al 2014, Strauss et al 2017). Most of the C is stored in deep layers of undisturbed permafrost soils and organic-rich thermokarst (thaw) lake sediments (Strauss et al 2013, Walter Anthony et al 2014). Thermokarst lakes are estimated to cover 1.3 × 106 km and store 102 Pg carbon (Olefeldt et al 2016). In the Northern Hemisphere, thermokarst lakes are hotspots of methane (CH4) emission (Walter et al 2006) through multiple gas transport modes (Sepulveda-Jauregui et al 2015), and are estimated to emit 4.1 ± 2.2 Tg CH4 per year (Wik et al 2016). Emissions from thermokarst lakes are expected to increase fivefold by 2100 (Schneider von Deimling et al 2015, Walter Anthony et al 2018). For better projections of CH4 fluxes from high-arctic thermokarst lakes, we have to understand the molecular processes affecting CH4 during its migration from deeper sediment layers to the atmosphere

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