Abstract
The hydrothermal sediments of Guaymas Basin, an active spreading center in the Gulf of California (Mexico), are rich in porewater methane, short-chain alkanes, sulfate and sulfide, and provide a model system to explore habitat preferences of microorganisms, including sulfate-dependent, methane- and short chain alkane-oxidizing microbial communities. In this study, hot sediments (above 60°C) covered with sulfur-oxidizing microbial mats surrounding a hydrothermal mound (termed “Mat Mound”) were characterized by porewater geochemistry of methane, C2–C6 short-chain alkanes, sulfate, sulfide, sulfate reduction rate measurements, in situ temperature gradients, bacterial and archaeal 16S rRNA gene clone libraries and V6 tag pyrosequencing. The most abundantly detected groups in the Mat mound sediments include anaerobic methane-oxidizing archaea of the ANME-1 lineage and its sister clade ANME-1Guaymas, the uncultured bacterial groups SEEP-SRB2 within the Deltaproteobacteria and the separately branching HotSeep-1 Group; these uncultured bacteria are candidates for sulfate-reducing alkane oxidation and for sulfate-reducing syntrophy with ANME archaea. The archaeal dataset indicates distinct habitat preferences for ANME-1, ANME-1-Guaymas, and ANME-2 archaea in Guaymas Basin hydrothermal sediments. The bacterial groups SEEP-SRB2 and HotSeep-1 co-occur with ANME-1 and ANME-1Guaymas in hydrothermally active sediments underneath microbial mats in Guaymas Basin. We propose the working hypothesis that this mixed bacterial and archaeal community catalyzes the oxidation of both methane and short-chain alkanes, and constitutes a microbial community signature that is characteristic for hydrothermal and/or cold seep sediments containing both substrates.
Highlights
The Guaymas Basin in the Gulf of California is a young marginal rift basin characterized by active seafloor spreading and rapid deposition of organic-rich sediments derived from both, highly productive overlying waters as well as terrigenous input
The investigated sediment samples were located at the base of a small hydrothermal mound termed “Mat Mound,” overgrown with Riftia and microbial mats of white sulfur bacteria which indicated diffuse venting of sulfidic fluids through the porous matrix of the mound (Figure 1)
The complex profile of δ13C-CH4 in core 4484-3 most likely reflects influences of microbial methane production and oxidation and of advective mixing, but it remains within the relatively narrow range of –42.4 to –39.5 (Figure 2). This range is close to the values reported previously for hydrothermal methane in Guaymas Basin, a mixture originating from thermal degradation of buried photosynthetic biomass in the sediments, and from hydrothermal fluids (Welhan, 1988; Pearson et al, 2005)
Summary
The Guaymas Basin in the Gulf of California is a young marginal rift basin characterized by active seafloor spreading and rapid deposition of organic-rich sediments derived from both, highly productive overlying waters as well as terrigenous input. Low-molecular weight organic acids (Martens, 1990) and ammonia (Von Damm et al, 1985) are produced in large amounts As these substrates are entrained into hydrothermal circulation and reach cooler overlying sediments, they are assimilated by dense microbial communities in the surficial sediments (Pearson et al, 2005). The benthic microbial communities of Guaymas Basin mediate methanogenesis (Jones et al, 1983), methane oxidation (Kallmeyer and Boetius, 2004), and sulfate reduction (Jørgensen et al, 1990, 1992; Elsgaard et al, 1994; Weber and Jørgensen, 2002; Kallmeyer et al, 2003), among other processes This active microbial interface, termed “microbial gauntlet” (Teske et al, 2014), could modulate hydrothermal carbon mobilization and release into the environment (Lizarralde et al, 2011)
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