Variations in microbial community compositions and processes imposed under contrast geochemical contexts in Sicilian mud volcanoes, Italy

Abstract

Terrestrial mud volcanoes represent surface features of channels for subsurface methane transport and, therefore, constitute an important source of methane emission from natural environments. How microbial processes regulate methane emissions in terrestrial mud volcanoes has yet to be fully addressed. This study demonstrated the geochemical characteristics and microbial communities of four mud volcano and seep sites in two geological settings of Sicily, Italy. At sites within the accretionary wedge that exhibited higher methane and sulfate concentrations, the communities were dominated by members capable of catalyzing methane and sulfate metabolisms and organic degradation. In particular, both anaerobic and aerobic methanotrophs were abundant and their abundance distribution coincided with the geochemical transition. In contrast, the sites near Mount Etna were characterized by high fluid salinity, CO₂, and low methane and sulfate concentrations, with communities consisting of halophilic organic degraders and sulfur metabolizers, along with a minor presence of aerobic methanotrophs. Substantial variations in community composition and geochemistry across spatial and vertical redox gradients suggest that physicochemical contexts imposed by the geology, fluid path, and source characteristics play a vital role in shaping community composition and cycling of methane, sulfur and organic carbon in Sicily mud volcanoes.

Keywords: AOM; Sicily; halophile; methane; microbial community; mud volcano.

Figure 1

Topographic map (A) and field photos (B) for sampling sites of Sicily, Italy. Geological structures and general characteristics of sampled mud pools are provided in (A). The exact locations of cores retrieved for analyses are marked with a cylinder symbol in (B).

Figure 2

Geochemical characteristics of fluids collected from mud pools and sediments from (A) Aragona (AR), (B) Comitini (COM), (C) Paterno (PA01), and (D) Vallone (PA02). Cross symbols marked at 0 cm depth represent the concentrations or isotopic compositions of fluids from mud pools. Open circles represent the concentrations below the detection limit. Panels filled with gray color and marked with N.A. indicate that the data is not available because the volume or concentration was too low for analyses. Data for the concentration of chloride, sulfate, methane and δ¹³C-methane were adopted from Tu et al. (2022).

Figure 3

Community compositions based on 16S rRNA gene amplicons for samples from Aragona (A) and Paterno (B). Methane concentrations (solid circles) and isotopic compositions (open circles; only for site Aragona) in the left panel are provided to constrain the possible methane metabolisms. Two primer sets (see Table 1 for detailed information) were used to amplify individual bacterial and archaeal communities of each sample. Archaeal community compositions for the mud pool and bottom sediments at Paterno could not be obtained because the DNA concentrations were too low.

Figure 4

Copy numbers of 16S rRNA gene for different taxonomic groups. (A) Results for site Agragona (AR). (B) Results for site Vallone (PA02). All the reported values represent the averages of triplicate analyses. The standard deviations were less than 20% of the average values and too small to be shown. The gray area for the ANME1 group from site PA02 represents the gene abundances were below the detection level.

Figure 5

Community variance based on PCoA analysis using different primer sets. (A) Archaeal community variance. (B) Bacterial community variance. The open triangles represent the communities for site AR, whereas the solid circles represent the communities for site PA02. Ordination of significant geochemical parameters is overlaid for comparison. The detailed information for individual primer set is described in Table 1.