Do mature hydrocarbons have an influence on acid rock drainage generation?

•Analyses indicate the presence of petroleum-derived hydrocarbons in ARD sediments.•ARD sediment hosts acidophilic bacteria able to degrade mature hydrocarbons.•Mature hydrocarbons were not used by the bacterial community during ARD generation.•ARD generation is likely primarily an atmospheric CO2 driven, autotrophic process.

The generation of acid rock drainage (ARD) is a biogeochemical process that causes severe ecological impacts, threatening human health worldwide. Microbes involved in acid drainage reactions are generally considered autotrophic but heterotrophic and mixotrophic microorganisms have often been identified at ARD sites. This raises questions about the role of organic matter naturally present at these sites, such as mature hydrocarbons, in promoting the microbial processes underpinning ARD generation. To investigate this, aerobic microcosm experiments were carried out using ARD samples collected at a well-characterised site in northern England (Mam Tor, Derbyshire). Organic analyses indicated the presence of substantial amounts of mature, petroleum-derived hydrocarbons and microbial analyses indicated that the sediment hosts acidophilic bacteria with the capability of degrading petroleum-derived compounds. However, the aerobic microcosm experiments indicated that these petroleum-derived hydrocarbons were not used by the bacterial community and, therefore, are not involved in the reactions that ultimately lead to ARD generation. These observations support a primary role for autotrophs in ARD generation.