Toshiki Nagakura, Yuki Morono, Motoo Ito, Jens Kallmeyer

Guaymas Basin, located off the Gulf of California, is a hydrothermally active marginal basin. Due to steep geothermal gradients and localized heating by sill intrusions, microbial substrates like short-chain fatty acids and hydrocarbons are abiotically produced from sedimentary organic matter at comparatively shallow depths. However, the potential of hydrocarbons as microbial substrates for both catabolic and anabolic metabolism remains elusive. We thus analyzed the effect of hydrocarbons on microbial sulfate reduction rates (SRR) and uptake of hydrocarbons by microorganisms using NanoSIMS. Sediment samples were recovered during IODP Exp. 385. Two sites, U1545C and U1546D, have a distance of roughly 1 km, and their sedimentary sequence and current geothermal gradients are almost identical, but Site U1546D experienced the intrusion of a sill. Since emplacement, the sill has thermally equilibrated with the surrounding sediment. For SRR measurements, sediment samples were amended with four aliphatic and four aromatic hydrocarbons or methane. Incubations were carried out at in-situ temperature and pressure for 10 days. For NanoSIMS analysis, sediment samples were incubated with stable-isotope labeled hydrocarbons (hexadecane-d34 + benzene-13C6 or 13C-methane) and 15NH4Cl at in-situ temperature and pressure for 42 days. Our results show that SRR increases upon the addition of either methane and hydrocarbons in samples from near the seafloor at Site U1545C. Methane addition also stimulated SRR around the sulfate-methane transition zone (SMTZ) at Site U1545C. In contrast, SRR did only increase at Site U1546D below the SMTZ, when the sample was incubated with methane, but did not show any reaction on hydrocarbon addition. Despite the relatively short incubation time of only 42 days we succeeded in detecting hydrocarbon and nitrogen uptake in some samples from both sites. Assimilation also mostly occurred in samples near the seafloor. Consequently, these data indicate the potential of microorganisms in Guaymas Basin to metabolize hydrocarbons.