Sulfate reduction is the quantitatively most important process to degrade organic matter in anoxic marine sediment and has been studied intensively in a variety of settings. Guaymas Basin, a young marginal ocean basin, offers the unique opportunity to study sulfate reduction in an environment characterized by organic-rich sediment, high sedimentation rates, and high geothermal gradients (100–958°C km−1). We measured sulfate reduction rates (SRR) in samples taken during the International Ocean Discovery Program (IODP) Expedition 385 using incubation experiments with radiolabeled 35SO42− carried out at in situ pressure and temperature. The highest SRR (387 nmol cm−3 d−1) was recorded in near-surface sediments from Site U1548C, which had the steepest geothermal gradient (958°C km−1). At this site, SRR were generally over an order of magnitude higher than at similar depths at other sites (e.g., 387–157 nmol cm−3 d−1 at 1.9 mbsf from Site U1548C vs. 46–1.0 nmol cm−3 d−1 at 2.1 mbsf from Site U1552B). Site U1546D is characterized by a sill intrusion, but it had already reached thermal equilibrium and SRR were in the same range as nearby Site U1545C, which is minimally affected by sills. The wide temperature range observed at each drill site suggests major shifts in microbial community composition with very different temperature optima but awaits confirmation by molecular biological analyses. At the transition between the mesophilic and thermophilic range around 40°C–60°C, sulfate-reducing activity appears to be decreased, particularly in more oligotrophic settings, but shows a slight recovery at higher temperatures.

<p>Guaymas Basin, located in the Gulf of California, Mexico, is a young marginal ocean basin with high sedimentation rates of >1 mm/year, active seafloor spreading, and steep geothermal gradients in its sediment. It hosts a unique subseafloor biosphere as these conditions lead to the thermal cracking of sedimentary organic matter and the production of bioavailable organic carbon compounds and hydrocarbons already at shallow depths. The abundance and diversity of potential microbial substrates raise the question of which substrates are being used for catabolic and anabolic microbial metabolism. We thus analyzed the microbial uptake of hydrocarbons using nanoscale secondary ion mass spectrometry (nano-SIMS) analysis after incubation with stable-isotope labeled substrates. Incubations were carried out with samples from two IODP Exp. 385 drill sites, Site U1545 with undisturbed sedimentary strata and a temperature gradient of 225°C/km, and Site U1546 with a sill intrusion led to temporary heating of the sediment. The temperature gradient of 221°C/km indicates thermal equilibration with the surrounding sediment since sill emplacement. Incubations were carried out with <sup>13</sup>C-benzene + <sup>2</sup>H-hexadecane or <sup>13</sup>C-methane at in-situ temperature (4-62°C) and pressure (25 MPa) for 42 days. Additionally, sulfate reduction rates (SRR) were measured by incubating the samples with four aliphatic hydrocarbons + four aromatic hydrocarbons or methane and radioisotope-labeled <sup>35</sup>SO<sub>4</sub><sup>2-</sup> at in-situ temperature (4-63°C) and pressure (25 MPa) for 10 days. The nano-SIMS analyses reveal that a few samples showed detectable microbial assimilation of hydrocarbons. Nitrogen (from <sup>15</sup>NH<sub>4</sub>Cl in the medium) was assimilated in some samples incubated with methane. The assimilation mostly occurred in samples from near the seafloor (2 and 44 mbsf). We hypothesize that the relatively short incubation time of 42 days was insufficient to detect extremely small incorporation rates in deep sediments. The results of the SRR measurements indicate that a mixture of hydrocarbons and methane increases the SRR in samples from near the seafloor (2 mbsf) and around the sulfate-methane transition zone (44 and 55 mbsf) but not in samples from greater depths. Our results show that anaerobic microorganisms in Guaymas Basin can use hydrocarbons for anabolic and catabolic metabolism in this extreme environment.</p>

<p>Sulfate reduction is the quantitatively most important process to degrade organic matter in anoxic marine sediment and has been studied intensively in a variety of settings. Guaymas Basin, a young marginal ocean basin, offers the unique opportunity to study sulfate reduction in an environment characterized by organic-rich sediment, high sedimentation rates, and high geothermal gradients (100-958°C km<sup>-1</sup>). We measured sulfate reduction rates (SRR) in samples of the International Ocean Discovery Program (IODP) Expedition 385 using incubation experiments with radiolabeled <sup>35</sup>SO<sub>4</sub><sup>2-</sup> carried out at in-situ pressure and temperature. Site U1548C, outside of a circular hydrothermal mound above a hot sill intrusion (Ringvent), has the highest geothermal gradient (958°C km<sup>-1</sup>) of all eight sampling sites. In near-surface sediment from this site, we measured the highest SRR (387 nmol cm<sup>-3</sup> d<sup>-1</sup>) of all samples from this expedition. At Site U1548C SRR were generally over an order of magnitude higher than at similar depths at other sites. Site U1546D also had a sill intrusion, but it had already reached thermal equilibrium and SRR were in the same range as nearby Site U1545C, which is minimally affected by sills. The wide temperature range found in the stratigraphic section at each drill site leads to major shifts in microbial community composition with very different temperature optima. At the transition between the mesophilic and thermophilic range around 40 to 60°C, sulfate-reducing activity appears to be decreased, particularly in more oligotrophic settings but shows a slight recovery at higher temperatures.</p>