Microbial hydrocarbon uptake and the effect of hydrocarbons on microbial sulfate reduction

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 ¹³C-benzene + ²H-hexadecane or ¹³C-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 ³⁵SO₄^2- 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 ¹⁵NH₄Cl 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.