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 SO42-m-2 day-1 in untreated cores to 5.7 mmol SO42-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.
Highlights
Geothermal action on kerogen in fine-grained sedimentary rocks leads to the formation of petroleum over geological timescales
Cores sampled upon arrival in Kiel will be called “untreated cores” in opposite to the SOFT core, which was used in the simulated petroleum seepage experiment
In order to test if the storage time had major effects on bacterial diversity in the sediment cores, a comparison of bacterial diversity was done between an untreated core that was sampled upon arrival in Kiel and an untreated core that was stored for 3 years at 0◦C in the dark
Summary
Geothermal action on kerogen in fine-grained sedimentary rocks leads to the formation of petroleum over geological timescales. Many studies have focused on the microbial degradation of spilled oil in the oceans’ water column (for example, Delvigne and Sweeney, 1988; Atlas, 1991; Prince et al, 2003; Jiménez et al, 2006; Prince et al, 2013), but relatively few studies investigated the microbial degradation of petroleum in hydrocarbon seeps (for example, Wenger and Isaksen, 2002; Wardlaw et al, 2008; Orcutt et al, 2010). Investigations of hydrocarbon seeps often capture only snapshots of biogeochemical features (for example, Bauer et al, 1988; Wenger and Isaksen, 2002; Orcutt et al, 2008; Wardlaw et al, 2008) and are unable to follow the succession of processes related to petroleum seepage through natural sediment. Only a few studies have focused on the microbial community and petroleum degradation in sediments from the Caspian Sea (Hassanshahian et al, 2012; Hassanshahian, 2014; Mahmoudi et al, 2015)
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