Bacterial weathering of fossil organic matter and organic carbon mobilization from subterrestrial Kupferschiefer black shale: long‐term laboratory studies

Abstract

A large part of the organic carbon present in the lithosphere is trapped in fossil organic matter deposited in sedimentary rocks. Only specialized microorganisms are able to degrade it contributing to the return of the carbon to the global cycle. The role of bacteria in this process is not yet completely understood. In the present laboratory studies, subterrestrial organic-rich ∼256-million-year-old Kupferschiefer black shale was exposed to the activity of an indigenous consortium of lithobiontic bacteria for 365 days under aerobic conditions. An interdisciplinary research approach was applied, consisting of a detailed comparison of the chemical composition of extractable bitumens as well as resistant to extraction kerogen of the unweathered black shale to that of the bioweathered and chemically weathered, identification of mobilized organic compounds and spectrometry-based determination of proteomic composition of the bacterial biofilm. The oxidative bioweathering of bitumens and kerogen was confirmed. The mobilization of organic carbon in the form of oxidized organic compounds, such as monohydroxy and dihydroxy alcohols, aldehydes, monocarboxylic and dicarboxylic acids and esters due to the microbial activity, was documented. The enzymes crucial for the aerobic metabolism of aliphatic and aromatic hydrocarbons such as monooxygenases and dehydrogenases were identified in the epilithic biofilm inhabiting the black shale.

The Oxidative Metabolism of Fossil Hydrocarbons and Sulfide Minerals by the Lithobiontic Microbial Community Inhabiting Deep Subterrestrial Kupferschiefer Black Shale

“…The pyrolytic analysis of black shale by the Rock-Eval method showed an unusually high ratio of oxygen to hydrogen in this rock compared with that of the unweathered black shale (Figure 8 ; Table S7 ) (Stasiuk et al, 2017 ), which may confirm the strong oxidation of kerogen. At the same time, we observed a considerably reduced (about 86%) content of C organic dominated by residual carbon (91%) (Figure 9A ).…”

“…In order to confirm the potential metabolic activity of LMC, we examined the geochemical properties of the 12-year-old weathered black shale and we compared these to the properties of the unweathered shale studied earlier (Stasiuk et al, 2017 ). Our research included the comparison of: (i) the properties of kerogen and organic carbon content, (ii) the qualitative and semi-quantitative comparative composition of extractable organic matter, and (iii) the sulfur content and its speciation.…”

“…The detailed data of Rock-Eval analysis are presented in Table S7 . The unweathered black shale is described in detail in Stasiuk et al ( 2017 ).…”

“…The samples were collected aseptically from outcrops (to a depth of 10 cm) and kept at −80 and −4°C until processing in the lab. The unweathered black shale originating from a freshly exposed outcrop (2 days) in Lubin mine has previously been described (Stasiuk et al, 2017 ).…”

“…In a subterrestrial mine the processes of the aerobic oxidation of fossil hydrocarbons and sulfide minerals occurring in the Kupferschiefer black shale may be intensive and may be proceed with the participation of either abiotic factors (oxygen, water) or microorganisms inhabiting the black shale. In previous laboratory studies, we described the long-term oxidation of Kupferschiefer kerogen by an indigenous bacterial consortium (Stasiuk et al, 2017 ). We reported that the bacterial oxidation of fossil hydrocarbons may be one of the processes responsible for kerogen weathering.…”