Biogeochemical evidence of aerobic and anaerobic methane oxidation in active mud volcanoes of the Canadian Beaufort Sea

Abstract

Active mud volcanoes (MVs) of the Canadian Beaufort Sea were investigated based on a multidisciplinary approach to better understand methane-drived “biogeosystems”. The main goals of this thesis were i) to investigate microbial methane oxidation processes, i.e. anaerobic oxidation of methane (AOM) and aerobic oxidation of methane (MOx) and ii) to identify some key environmental factors determining the niche diversification of methanotrophic communities in these MV systems. Firstly, in the three MVs (MV282, MV420, and MV740) located on the continental slope at different water depths (282, 420, and 740 m), evidence of active methane migration was shown by a mousse-like texture and depleted fluids (particularly sulfate) in the sediment cores investigated. In this regard, diagnostic 13C-depleted archaeal lipids and 16S rDNA sequencing data provided further evidence that AOM was mediated by anaerobic methanotrophic archaea (ANME) clades ANME-2c and ANME-3. Secondly, the diverse 13C-depleted lipid biomarkers (irregular isoprenoids, isoprenoidal dialkyl glycerol diethers and C30 hopanoids) were characteristic at a pingo-like feature (PLF) site located on the continental shelf at 107 m water depth. However, MV740 showed scarce and minor signatures of these compounds. These results suggest that different methane fluxes were responsible for the AOM and MOx processes at the PLF site resulting in different biogeochemical signals compared to the MV site. Thirdly, three distictive chemosynthethic fields, i.e. bare organisms (BO), free-living bacteria mats (BM), and siboglinid tubeworms (ST)) were discovered at MV420 during the remotely operated vehicle (ROV) operation. The estimated methane fluxes were higher at BO (0.06 mmol cm-2 y-1) and BM (0.04 mmol cm-2 y-1) sites than at the ST site (0.01 mmol cm-2 y-1). The lipid biomarkers and 16S rRNA gene sequence data showed that methane oxidations were differently mediated via MOx-related bacteria (Methlyococcales) and/or AOM-related archaea (ANME-2 and ANME-3) at three fields of MV420. Accordingly, it appears that the main mechanisms determining the horizontal zonation of methanotrophic communities were related to different methane fluxes. Fourthly, siboglinid tubeworms inhabiting MV420 were designated as Oligobrachia haakonmosbiensis BS1. Chemosynthetic bacteria communities inhabiting siboglinid tubeworms showed bulk organic carbon and fatty acids (FAs) characteristics indicating presence of sulfur-oxidizing symbionts in worms. On the other hand, the distribution of other bacterial communities (e.g. autotrophs and/or heterotrophs) inhabiting the tube sections was likely associated with the utilization of ambient carbons supplied from sediments and bottom seawater. Fifthly, geochemical parameters and lipid biomarkers obtained from two siboglinid tubeworm fields (STFs; STF1 and STF2) at MV420 showed discriminative AOM and MOx signals. These differences appear to be associated with the supply of byproducts (sulfide and dissolved inorganic carbon) derived from the AOM. In summary, the biogeochemical characteristics of active Canadian MVs investigated indicate that AOM and MOx processes are being actively involved in removing methane ascending via MV structures. Both processes are mainly controlled by gas and fluid transport processes. Methanotophic activities also support the life of chemosynthetic organisms on the surface.