This study analyzed cored sediments retrieved from sites distributed across a transect of the Lei-Gong-Hou mud volcanoes in eastern Taiwan to uncover the spatial distributions of biogeochemical processes and community assemblages involved in methane cycling. The profiles of methane concentration and carbon isotopic composition revealed various orders of the predominance of specific methane-related metabolisms along depth. At a site proximal to the bubbling pool, the methanogenic zone was sandwiched by the anaerobic methanotrophic zones. For two sites distributed toward the topographic depression, the methanogenic zone overlaid the anaerobic methanotrophic zone. The predominance of anaerobic methanotrophy at specific depth intervals is supported by the enhanced copy numbers of the ANME-2a 16S rRNA gene and coincides with high dissolved Fe/Mn concentrations and copy numbers of the Desulfuromonas/Pelobacter 16S rRNA gene. Assemblages of 16S rRNA and mcrA genes revealed that methanogenesis was mediated by Methanococcoides and Methanosarcina. pmoA genes and a few 16S rRNA genes related to aerobic methanotrophs were detected in limited numbers of subsurface samples. While dissolved Fe/Mn signifies the presence of anaerobic metabolisms near the surface, the correlations between geochemical characteristics and gene abundances, and the absence of aerobic methanotrophs in top sediments suggest that anaerobic methanotrophy is potentially dependent on iron/manganese reduction and dominates over aerobic methanotrophy for the removal of methane produced in situ or from a deep source. Near-surface methanogenesis contributes to the methane emissions from mud platform. The alternating arrangements of methanogenic and methanotrophic zones at different sites suggest that the interactions between mud deposition, evaporation, oxidation and fluid transport modulate the assemblages of microbial communities and methane cycling in different compartments of terrestrial mud volcanoes.

Abstract. Terrestrial mud volcanoes (MVs) represent the surface expression of conduits tapping fluid and gas reservoirs in the deep subsurface. Such plumbing channels provide a direct, effective means to extract deep microbial communities fueled by geologically produced gases and fluids. The drivers accounting for the diversity and composition of these MV microbial communities, which are distributed over a wide geographic range, remain elusive. This study characterized the variation in microbial communities in 15 terrestrial MVs across a distance of ∼ 10 000 km on the Eurasian continent to test the validity of distance control and physiochemical factors in explaining biogeographic patterns. Our analyses yielded diverse community compositions with a total of 28 928 amplicon sequence variances (ASVs) taxonomically assigned to 73 phyla. While no true cosmopolitan member was found, ∼ 85 % of ASVs were confined within a single MV. Community variance between MVs appeared to be higher and more stochastically controlled than within MVs, generating a slope of the distance–decay relationship exceeding those for marine seeps and MVs as well as seawater columns. For comparison, physiochemical parameters explained 12 % of community variance, with the chloride concentration being the most influential factor. Overall, the apparent lack of fluid exchange renders terrestrial MVs a patchy habitat, with microbiomes diverging stochastically with distance and consisting of dispersal-limited colonists that are highly adapted to the local environmental context.

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Material and methods Sampling sites and proceduresMuddy fluids from bubbling pools and a total of 16 sediment cores from the adjacent mud platform were retrieved from 15 MVs across the Eurasian continent during 2011 to 2017 (Fig. 1; plotted using the ggmap package (Kahle & Wickham, 2013) in R; Table S1) for geochemical and molecular analyses. In brief, bubbling fluids and cores were collected using sterilized cups and liners, respectively. The lengths of the cores ranged between 20 and 160 cm. Samples were transported to the nearby laboratory or accommodation within 5 hours after retrieval. The cores were immediately sectioned at an interval of 1.5 to 5 cm (Table S1) with the average depth of individual sectioned intervals as the representative depth. For gas geochemistry, we preserved 6 mL of sediments in a 36-mL serum bottle with 10 mL of 1 M NaOH, and sealed with a butyl rubber and an aluminum ring. Following the gas sampling, 3 mL of sediments were collected in a 15-mL centrifuge tube for the determination of water content. Samples for pore water content were subject to freeze drying. The weight difference was used to calculate the water weight content or porosity assuming the density of dry sediment was 2.5g cm -3 and the pore space was completely saturated with pore water. For aqueous geochemistry, the remaining sediments were placed in a 50-mL centrifuge tube and centrifuged at 8,2000 x g for 15 minutes to collect pore water. The obtained pore water was decanted from the centrifuge tube, 0.22-µm-filtered using syringe filters and split it into two fractions with one for ion chromatographic analyses of anion abundances and the other for dissolved inorganic carbon (DIC). For molecular analyses, sediments were placed in a 50-mL centrifuge tube. Upon arriving at the laboratory, anion and DNA samples were stored in a 4°C refrigerator and a −80°C freezer, respectively, until further analysis.