Hydrocarbon Degradation in Caspian Sea Sediment Cores Subjected to Simulated Petroleum Seepage in a Newly Designed Sediment-Oil-Flow-Through System

Abstract

The microbial community response to petroleum seepage was investigated in a whole round sediment core (16 cm length) collected nearby natural hydrocarbon seepage structures in the Caspian Sea, using a newly developed Sediment-Oil-Flow-Through (SOFT) system. Distinct redox zones established and migrated vertically in the core during the 190 days-long simulated petroleum seepage. Methanogenic petroleum degradation was indicated by an increase in methane concentration from 8 μM in an untreated core compared to 2300 μM in the lower sulfate-free zone of the SOFT core at the end of the experiment, accompanied by a respective decrease in the δ¹³C signal of methane from -33.7 to -49.5‰. The involvement of methanogens in petroleum degradation was further confirmed by methane production in enrichment cultures from SOFT sediment after the addition of hexadecane, methylnapthalene, toluene, and ethylbenzene. Petroleum degradation coupled to sulfate reduction was indicated by the increase of integrated sulfate reduction rates from 2.8 SO₄^2-m^-2 day^-1 in untreated cores to 5.7 mmol SO₄^2-m^-2 day^-1 in the SOFT core at the end of the experiment, accompanied by a respective accumulation of sulfide from 30 to 447 μM. Volatile hydrocarbons (C2-C6 n-alkanes) passed through the methanogenic zone mostly unchanged and were depleted within the sulfate-reducing zone. The amount of heavier n-alkanes (C10-C38) decreased step-wise toward the top of the sediment core and a preferential degradation of shorter (<C14) and longer chain n-alkanes (>C30) was seen during the seepage. This study illustrates, to the best of our knowledge, for the first time the development of methanogenic petroleum degradation and the succession of benthic microbial processes during petroleum passage in a whole round sediment core.

Keywords: crude oil; methane; methanogenesis; n-alkanes; oxygen consumption; porosity; sulfate reduction; sulfide.

FIGURE 1

(A) Schematic diagram of the SOFT system for the simulation of petroleum seepage through whole round sediment cores. Artificial seawater was ventilated through the supernatant (P1, pump rate 25 μL min^-1) and aerated with an air pump (P2). Petroleum was supplied in by a pump (P3) at 3.5 μL min^-1 through two integrated channels within the bottom sealing. Vertically aligned rhizons (2.5 mm diameter) were permanently fixed for frequent extraction of porewater. Silicon-sealed holes (4 mm diameter) on the opposite side were used for microsensor measurements. From the oxic supernatant electron acceptors (e.g., O₂, sulfate) entered the sediment by diffusion (dashed white arrows). (B–D) Individual parts of the SOFT system (technical drawing). (B) Rubber stopper with two integrated steel channels (C) Upper cap with three small holes to guide tubing for aeration and seawater inflow/outflow (D) Iso-versinic tubing assembly of individual parts with the core liner.

FIGURE 2

(a) Map of Azerbaijan and the Caspian Sea. (b) Geographical map showing the push-core sampling area (red dot). Characteristic features like on- and offshore mud volcanoes (green dots), abandoned offshore wells and infrastructures (white spots and lines in the image), and a central oil slick area (dark gray area in the image) are indicated. FC1 and FC2 are nearby sites where geochemical analyzes were done by Jost (2014). Map was produced by using ArcGIS 10.2, and is based on a regional SAR image taken in 2004 by ENVISAT (© ESA (2010), European Space Agency, ESA).

FIGURE 3

(A) Temporal development of sediment microprofiles of dissolved oxygen after the start of the SOFT experiment. Values are mean of three separate vertical profiles but with different horizontal positions (±SD, n = 3). The dashed horizontal line represents the sediment-water interface. (B) Temporal development of the oxygen penetration depth (PD) and the diffusive oxygen uptake (DOU) generated from the mean oxygen profiles.

FIGURE 4

Temporal development of biogeochemical profiles in the Caspian Sea sediment during simulated petroleum seepage. “Untreated” shows conditions measured in a replicate core prior to the start of the SOFT experiment. “SOFT” shows condition developing over the course of 190 days in the SOFT core. Sulfate (black line with triangles), total sulfide (black line with squares), sulfate reduction rates (SRR, black checkered bars) and methane (black line with circles). Sulfide data were corrected for the shift in the electronic signal of the microsensors (between 0.5 to 1.5 mV). SRR bars (black) in the untreated core represent the average of two replicates, while the two SRR replicate values are shown as empty black squares and diamonds. In the final SOFT core (190 days) only one SRR replicate is shown. The sample at 9 cm depth was missed during radiotracer injection. Please consider the change of scale in some of the x-axes.

FIGURE 5

Vertical profiles of sediment parameters (A) total organic carbon (TOC), (B) C/N ratio, (C) porosity determined in the untreated Caspian Sea core (black line with circles) and the SOFT core (black line with squares) after 190 days of petroleum seepage.

FIGURE 6

Total cell numbers as detected by DAPI staining in the untreated and the final SOFT core.

FIGURE 7

δ¹³C of methane and methane concentration in two replicate Caspian Sea cores before (Untreated) and after the SOFT experiment (190 days, Final). The shaded area represents the methanogenic zone of the core and the non-shaded area represents the sulfate-reducing zone. For some depths δ¹³C values are missing, because methane concentrations were too low for analyzes.

FIGURE 8

Methane production in sulfate-free enrichment cultures in sediment samples from the sulfate-reducing (= sulfidogenic) and methanogenic zone of the SOFT core after the 190 days experiment.

FIGURE 9

Vertical distribution of volatile n-alkanes (from C1–C6: Methane, Ethane, Propane, n-Butane, i-Butane, Pentane, and Hexane) over depth in the Caspian Sea core at the end of the SOFT experiment (190 days). The shaded area represents the methanogenic zone of the core and the non-shaded area represents the sulfate-reducing zone in the SOFT core at 190 days.

FIGURE 10

(A) Absolute amounts of n-alkanes in the Caspian Sea core after the SOFT experiment (black, ng per g sediment) and in the original, unaltered petroleum (blue, ng per mg petroleum). Surface sediment (0–1 cm) is excluded, due to possible contaminations from the settled oil slick (see text). (B) Relative composition of individual n-alkanes with respect to the sum total of all n-alkanes analyzed. The red line shows the ratio of the weight of petroleum extract at each depth to the respective sediment weight and represents the movement of petroleum in the SOFT core.