Anaerobic oxidation of methane (AOM) at the thaw front of subsea permafrost

Introduction: Thawing arctic subsea permafrost is a source of organic carbon in deep sediment layers. The permafrost that is at its thermal equilibrium releases biologically produced methane and a deep sulfate-methane transition zone (SMTZ) is formed due to sulfate-rich overlaying marine sediment layers. The process of methane oxidation in this anaerobic environment has been suggested1 but AOM associated microbial communities remain to be identified. Objectives: We aimed at providing evidence for anaerobic methanotrophic (ANME) archaeal communities at the deep SMTZ of the north-east Siberian Laptev Sea shelf. Material and methods: Two sediment cores were retrieved (77 m and 47.4 m deep) from the coastal shelf north of Cape Mamontov Klyk ‘C2’ (11.5 km offshore) and west to the Buor Khaya Peninsula ‘BK2’ (800 m offshore), respectively. Methane and sulfate concentrations as well as 13C isotope values of CH4 were measured and correlated with molecular analysis of microbial communities along the thaw front. Results: At the thaw front of BK2, at 23.7 meters below sea floor (mbsf) biologically produced methane (13 C= -70‰ VPDB) gets oxidized (13C= -29.8 ‰ VPDB)1. At the same depth, we found an increase in functional genes of methanogenic archaea (mcrA) and sulfate reducing bacteria (dsrB) analysed by quantitative PCR. Massive parallel tag-sequencing of the 16S rRNA gene showed an increase of ANME-2a/2b and ANME-2d sequences towards the thaw front in both cores. At the thaw front of the BK2 core, typical ANME-2 partners of the Desulfobacterales2were found to dominate the sulfate reducing bacterial community, whereas Desulfobaccasequences dominate in all samples of the C2 core. Theoretical methane oxidation rates (0.4-6 nmol cm-3d-1)1based on estimated methane fluxes showed higher values than typically found in subsurface sediments and are more similar to rates of margin SMTZs3. Conclusion: Our data indicate that active anaerobic methane oxidizer communities at the thaw front of subsea permafrost prevent methane from being released into the water column and subsequently to the atmosphere. Further analyses on lipid biomarkers and 14C-CH4isotopic rate measurements will determine how active these communities are in situ. 1. Overduin, P. P. et al.Methane oxidation following submarine permafrost degradation: Measurements from a central Laptev Sea shelf borehole. J. Geophys. Res. Biogeosciences2014JG002862 (2015). 2. Schreiber, L., Holler, T., Knittel, K., Meyerdierks, A. & Amann, R. Identification of the dominant sulfate-reducing bacterial partner of anaerobic methanotrophs of the ANME-2 clade. Environ. Microbiol.12, 2327-2340 (2010). 3. Knittel, K. & Boetius, A. Anaerobic Oxidation of Methane: Progress with an Unknown Process. Annu. Rev. Microbiol. 63, 311-334 (2009)