ALBERT

Description: Introduction: Submarine arctic permafrost was formed when sea level rise flooded terrestrial permafrost and warmed the frozen sediments during the Holocene. This thawing permafrost may play a major role in global warming as it stores huge amounts of organic carbon. Hitherto, the extent and importance of microbial activity on carbon transformations as well as the reactions of microorganisms to the environmental changes accompanying the inundation of permafrost by sea water are poorly understood. Objectives: We investigated the impact of sea level rise on methane cycle associated microbial communities in degrading permafrost of the western and central Laptev Sea shelf, Siberian. Material and methods: Two sediment cores were retrieved (77 m and 52 m deep) from the coastal shelf north of Cape Mamontov Klyk ‘C2’ (11.5 km offshore) and west of Buor Khaya Peninsula ‘BK2’ (800 m offshore), respectively. Chemical parameters such as total organic carbon (TOC), methane concentrations and 13 C isotope values were measured and correlated with molecular analysis of microbial communities along the cores. Results: Frozen sediment was encountered at 35.5 (C2) and 28 (BK2) meters below sea level (mbsl), respectively. Methane concentrations varied between 0.21 and 284.31 nmol g-1with highest values in the frozen permafrost and lowest values in the overlaying unfrozen sediments. Low methane concentrations in the unfrozen sediments of BK2 (16.25-28.20 mbsl) correlated with the highest carbon isotope values of methane (-29.8 ‰ VPDB) indicating microbial oxidation of methane under in situ conditions in the thawing permafrost. Bacterial cell numbers (16S rRNA) and functional genes (mcrA) of methanogenic archaea and sulphate reducing bacteria (dsrB) analysed by quantitative PCR often peaked at high methane or TOC concentrations in the frozen permafrost and showed specific 13CH4isotopic values indicating distinct methanogenic populations. Conclusion : Our data give first insights into how the inundation of permafrost by sea water influences the abundance of active members of the microbial methane cycle both along thawed and still frozen sediments. Further analysis of amplicon sequencing and quantitative analysis by fluorescence in situ,hybridization will give a better overview of these highly dynamic microbial populations.

Description: 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).