The origin of methanethiol in midocean ridge hydrothermal fluids

Abstract

Simple alkyl thiols such as methanethiol (CH 3 SH) are widely speculated to form in seafloor hot spring fluids. Putative CH 3 SH synthesis by abiotic (nonbiological) reduction of inorganic carbon (CO 2 or CO) has been invoked as an initiation reaction for the emergence of protometabolism and microbial life in primordial hydrothermal settings. Thiols are also presumptive ligands for hydrothermal trace metals and potential fuels for associated microbial communities. In an effort to constrain sources and sinks of CH 3 SH in seafloor hydrothermal systems, we determined for the first time its abundance in diverse hydrothermal fluids emanating from ultramafic, mafic, and sediment-covered midocean ridge settings. Our data demonstrate that the distribution of CH 3 SH is inconsistent with metastable equilibrium with inorganic carbon, indicating that production by abiotic carbon reduction is more limited than previously proposed. CH 3 SH concentrations are uniformly low (∼10 −8 M) in hightemperature fluids (>200°C) from all unsedimented systems and, in many cases, suggestive of metastable equilibrium with CH 4 instead. Associated low-temperature fluids (<200°C) formed by admixing of seawater, however, are invariably enriched in CH 3 SH (up to ∼10 −6 M) along with NH + 4 and low-molecular-weight hydrocarbons relative to high-temperature source fluids, resembling our observations from a sediment-hosted system. This strongly implicates thermogenic interactions between upwelling fluids and microbial biomass or associated dissolved organic matter during subsurface mixing in crustal aquifers. Widespread thermal degradation of subsurface organic matter may be an important source of organic production in unsedimented hydrothermal systems and may influence microbial metabolic strategies in cooler near-seafloor and plume habitats. methyl mercaptan | biogeochemistry | origin of life | prebiotic chemistry S ince their discovery in 1977, seafloor hot spring fluids have been widely proposed as a potential source of organic molecules necessary for early life to emerge and thrive on a HadeanArchaean Earth (1-5), and for metabolic energy and fixed carbon in modern hydrothermal systems (6, 7). Abiotic (nonbiological) reduction of inorganic carbon (CO 2 or CO) to methanethiol (methyl mercaptan, CH 3 SH) is considered a crucial first step in the putative transition from prebiotic to primitive metabolic chemistry, leading to the emergence of hyperthermophilic microbial life (8-13). Specifically, methanethiol is the presumptive abiotic precursor of acetyl thioester (8, 12, 13)-the functional moiety of the Acetyl-CoA coenzyme central to many ancient metabolic pathways-and a sustainable abiotic source of acetyl thioesters is a key feature of models proposing the emergence of primordial metabolism in hydrothermal settings (5). Alkyl thiols are additionally implicated in the synthesis of the key metabolite pyruvate (10), which is speculated to have led to a primordial protometabolic network in a hydrothermal setting (14). In modern hot spring envi...

The geochemistry of methane isotopologues

“…These are consistent with their expected low formation temperatures. Furthermore, thermogenic methane sampled from a hydrothermal vent in the Guaymas Basin, Gulf of California (Welhan and Lupton, 1987), yielded a ∆ 13 CH 3 D temperature of 326 +170 −95°C , within error of the measured vent temperature (299°C; Reeves et al, 2014). Therefore, our data provide independent support of the hypothesis that 13 CH 3 D abundance reflects the temperature at which methane is generated in these sedimentary basins (Stolper et al, 2014a).…”

“…In some cases, however, it was necessary to combine aliquots from duplicate samples collected in separate samplers deployed in the same hydrothermal fluid during a submersible dive (Table 3.1). Due to the exceedingly low concentration of dissolved CH 4 in ambient bottom seawater (<10 −8 M, Reeves et al, 2014;McDermott et al, 2015) relative to concentrations in endmember vent fluids (samples regressed to zero Mg content) (Table 3.2), inadvertent entrainment of seawater during fluid collection has no effect on the isotopic composition of vent-fluid derived methane measured during this study.…”

“…The sample was taken in 2008 using a isobaric gas-tight sampler (Table 2.1) and poisoned with mercuric chloride (Seewald et al, 2002). Fluid properties and geochemical data associated with this sample have been previously published (Reeves et al, 2014). We assumed a value of +4 ± 2‰ for the δD water of the vent fluid based on previous observations of Guaymas Basin hydrothermal fluids (Shanks et al, 1995).…”

“…[5] Measured vent temperature and [H 2 ] are from Reeves et al (2014), and δD water was assumed based on Shanks et al (1995).…”

Pathways for abiotic organic synthesis at submarine hydrothermal fields

“…Similarly, abiotic production of other carboxylic acids (e.g., acetic, propanoic, and butanoic) is likely kinetically inhibited, because these species are below detection (< 1 μmol/kg). The organosulfur compound methanethiol (CH 3 SH) is present in Von Damm fluids at abundances that do not reflect metastable equilibrium with ΣCO 2 and has been attributed to thermal alteration of microbial biomass or other sources of preexisting organic matter (27). The presence of other potential metastable species, such as amino acids, was not investigated; however, if they formed on mixing, it is likely that their concentrations are below the precision of ΣCO 2 analysis (∼100 μmol/kg).…”