Comparative mineralogy, organic geochemistry and microbial diversity of the Autun black shale and Graissessac coal (France)

•Characterization carbonaceous material in the Autun shale and Graissessac coal.•Microbial diversity in the Autun shale and Graissessac coal.•Presence of nag-like genes in the biomass associated to the shale and coal.

There is growing evidence that microbial populations associated with rocks may be involved in weathering processes; in particular they may play a role in the degradation of fossil organic carbon widely present in sedimentary rocks. In this study, we investigate the microbial diversity associated with two organic-rich rocks exhibiting contrasted organic chemical structure: a typical type I aliphatic carbon-rich kerogen (Autun shale) and a coal rich in aromatic carbon (Graissessac coal). In both rocks, the mineralogy and the geochemistry of the organic matter were investigated. Moreover, the microbial diversity associated with these rocks was characterized using bacterial 16S rRNA gene analysis. In the Autun shale, one dominant species is observed (Ralstonia) whereas a larger microbial diversity is retrieved from the Graissessac coal. Altogether, we conclude that potential hydrocarbonoclastic microorganisms detected by the presence of nag-like genes potentially encoding dioxygenase may be associated with a possible biodegradation of fossil organic carbon in the two samples. We conclude that under favorable conditions, aerobic biodegradation could occur and oxidize a minor fraction of these two different kerogens using a similar enzymatic process.