Processes of anaerobic oxidation of oil in bottom sediments of Lake Baikal

© Author(s) 2020. This work is distributed under the Creative Commons Attribution 4.0 License. The processes of anaerobic oil oxidation in bottom sediments of freshwater lakes have not been previously studied. At Lake Baikal, there are two known sites with natural oil seepages. One site is located at the estuary of the Bolshaya Zelenovskaya River (BZ) and has been known since the late 18th century, another site is located near Cape Gorevoy Utes (GU) (discovered in 2005). The oil collected during its release to the water surface at the GU site in 2005 was characterized by an extremely high content of n-alkanes and was identified as nonbiodegradable paraffin oil (Kontorovich et al., 2007). At present, chemical analysis of the oil composition has indicated its biodegradation (Gorshkov et al., 2020). This work aimed to study the processes of anaerobic oxidation of oil by the microbial community from the subsurface and deep sediments of the Gorevoy Utes natural oil seep. Samples of gas-hydrate and oil-bearing bottom sediments collected in 2018 from the Cape Gorevoy Utes (St. 5, GC-3) were investigated. To obtain enrichment cultures, we used sedimentary layers that corresponded to the core depths of 10–30 cm (GUI) and 100–120 cm (GUII). Cultivation was carried out at a temperature of 10°С in a mineral medium containing oil and an N2/CO2 gas mixture (90:10 v/v). Sulfate-reducing conditions were determined in microcosms with the addition of Na2SO4 (final concentration of 20 mM) to the medium, and Na2S (0.5 mM) was added as a reducing agent. Methanogenic microcosms were prepared as for sulfatereducing microcosms but without the addition of Na2SO4. Hydrocarbon gases in experimental vials were determined by a modified phase-equilibrium degassing method (Mizandrontsev et al., 2020). The degree of oil utilization by the microbial community was inferred from the loss of n-alkanes and PAHs determined by gas chromatography-mass spectrometry (Gorshkov et al., 2020) after one year of cultivation. To analyse clone libraries of the enrichment cultures, 30 clones with fragments of the 16S rRNA gene sequences were taken from each sample. The Sanger reaction with the use of BigDye Terminator Kit v. 3.1 (Applied Biosystems, United States) and analysis of the reaction products on an ABI 3130х1 gene analyzer were carried out at the SB RAS Genomics Core Facility, Novosibirsk. In the GUI sample, the highest loss of n-alkanes (28%) was determined during the cultivation of the microbial community under sulfate-reducing conditions. Under methanogenic conditions, a decrease in the amount of alkane oil fraction was insignificant and equal to 6%. In deep sedimentary layers, we observed the opposite pattern. In the cultures enriched with bicarbonate ion, the conversion of n-alkanes was 20%, and those enriched with sulfate ion – less than 1.5%. In the GUII, we recorded the highest conversion of PAHs (45–46%) under methanogenic and sulfate-reducing conditions, respectively. In the subsurface sedimentary layer, the loss of the total PAHs was lower than in the deep one. In the presence of sulfate in the enrichment