Anaerobic oxidation of petroleum hydrocarbons in enrichment cultures from sediments of the Gorevoy Utes natural oil seep under methanogenic and sulfate-reducing conditions

This article presents the first experimental data on the ability of microbial communities from sediments of the Gorevoy Utes natural oil seep to degrade petroleum hydrocarbons under anaerobic conditions. Like in marine ecosystems associated with oil discharge, available electron acceptors, in particular sulfate ions, affect the composition of the microbial community and the degree of hydrocarbon conversion. The cultivation of the surface sediments under sulfate-reducing conditions led to the formation of a more diverse bacterial community and greater loss of n-alkanes (28%) in comparison to methanogenic conditions (6%). Microbial communities of both surface and deep sediments are more oriented to degrade polycyclic aromatic hydrocarbons (PAHs), to which the degree of the PAH conversion testifies (up to 46%) irrespective of the present electron acceptors. Uncultured microorganisms with the closest homologues from thermal habitats, sediments of mud volcanoes and environments contaminated with hydrocarbons mainly represented microbial communities of enrichment cultures. The members of the phyla Firmicutes, Chloroflexi, and Caldiserica (OP5), as well as the class Deltaproteobacteria and Methanomicrobia, were mostly found in enrichment cultures and belong to the “core” of microorganisms The influence of gas-saturated fluids may be responsible for the presence in the bacterial 16S rRNA gene libraries of the sequences of “rare taxa”: Planctomycetes, Ca. Atribacteria (OP9), Ca. Armatimonadetes (OP10), Ca. Latescibacteria (WS3), Ca. division (AC1), Ca. division (OP11), and Ca. Parcubacteria (OD1), which can be involved in hydrocarbon oxidation.

A method to differentiate hydrocarbon source (oil vs methane) in authigenic carbonate rock from seeps

<p>Abstract</p><p>Numerous marine hydrocarbon seeps have been discovered in the past three decades, the majority of which are dominated by methane-rich fluids. However, an increasing number of modern oil seeps and a few ancient oil-seep deposits have been recognized in recent years. Oil seepage exerts significant control on the composition of the seep-dwelling fauna and may have impacted the marine carbon cycle through geological time to a greater extent than previously recognized. Yet, distinguishing oil-seep from methane-seep deposits is difficult in cases where δ<sup>13</sup>C<sub>carb</sub> values are higher than approximately -30‰ due to mixing of different carbon sources. Here, we present a comparative study of authigenic carbonates from oil-dominated (site GC232) and methane-dominated (site GC852) seep environments of the northern Gulf of Mexico, aiming to determine the geochemical characteristics of the two types of seep carbonates. We analyzed (1) Major and trace element compositions of carbonates, (2) total organic carbon (TOC), total nitrogen (TN) and carbon isotope (δ<sup>13</sup>C<sub>TOC</sub>) of residue after decalcification, (3) sulfur isotope signatures of chromium reducible sulfur (CRS, δ<sup>34</sup>S<sub>CRS</sub>) and residue after CRS extraction (δ<sup>34</sup>S<sub>TOS </sub>), as well as (4) sulfur contents (TOS) of residue after CRS extraction. Carbonates from the studied oil seep are dominated by aragonite and exhibit lower δ<sup>34</sup>S<sub>CRS</sub> values, suggesting carbonate precipitation close to the sediment surface. In addition, oil-seep carbonates are characterized by higher TOC and TOS contents and higher TOC/TN ratios, as well as less negative δ<sup>13</sup>C<sub>TOC</sub> values compared to methane-seep carbonates, probably reflecting a contribution of residual crude oil enclosed in oil-seep carbonates. Very low δ<sup>13</sup>C<sub>TOC</sub> values (as low as −68.7‰, VPDB) and low TOC/TN ratios of methane-seep carbonates indicate that the enclosed organic matter is derived mainly from the biomass of methanotrophic biota. This study presents new geochemical data that will allow the discrimination of oil-seep from methane-seep deposits. Although some of the geochemical patterns are likely to be affected by late diagenesis, if applied with caution, such patterns can be used to discern the two end-member types of seepage – oil seeps and methane seeps – in the geological record.</p>