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“The impact of four electron acceptors on hydrocarbon-induced methanogenesis was studied. Methanogenesis from residual hydrocarbons

may enhance the exploitation of oil reservoirs and may improve bioremediation. The conditions to drive the rate-limiting first hydrocarbon-oxidizing steps for the conversion of hydrocarbons into methanogenic substrates are crucial. Thus, the electron acceptors ferrihydrite, manganese dioxide, nitrate or sulfate were added to sediment microcosms acquired from two brackish water locations. Hexadecane, ethylbenzene or Selleckchem Gefitinib 1-13C-naphthalene were used as model hydrocarbons. Methane was released most rapidly from incubations amended with ferrihydrite and hexadecane. Ferrihydrite enhanced only hexadecane-dependent methanogensis. The rates of methanogenesis were negatively affected by Pictilisib in vivo sulfate and nitrate at concentrations of more than 5 and 1 mM, respectively. Metal-reducing Geobacteraceae and potential sulfate reducers as well as Methanosarcina were present in situ and in vitro. Ferrihydrite addition triggered the growth of Methanosarcina-related methanogens. Additionally, methane was removed concomitantly by anaerobic methanotrophy.

ANME-1 and -2 methyl coenzyme M reductase genes were detected, indicating anaerobic methanotrophy as an accompanying process [Correction added 16 December after online publication: ‘methyl coenzyme A’ changed to ‘methyl coenzyme M’ in this sentence]. The experiments presented here demonstrate the feasibility of enhancing methanogenic alkane degradation by ferrihydrite or sulfate addition in different geological settings. Roughly, one third of oil in reservoirs remains inaccessible (US Department of Energy, 2006). Since Zengler et al. (1999) reported the conversion of hexadecane to methane, it has been suggested that remaining energy can be recovered as methane gas (Anderson & Lovley, 2000; Head et al., 2003). Moreover, the conversion of hydrocarbons to carbon

dioxide (CO2) or methane represents a useful tool for CYTH4 bioremediation of oil-impacted ecosystems. The overall reaction kinetics of hydrocarbon biodegradation are controlled by the initial attack on hydrocarbons, where hydrocarbon biodegradation with oxygen as an electron acceptor is the energetically most favorable process. However, microbial methanogenesis usually requires anoxic conditions and methanogenesis, including the conversion of hexadecane to methane, is a slow process (Zengler et al., 1999; Feisthauer et al., 2010). The initial anaerobic activation of hexadecane may be irreversible and the removal of reaction products is unlikely to accelerate the initial steps or the overall degradation (Cravo-Laureau et al., 2005; Callaghan et al., 2006). However, β-oxidation and the release of electrons are essential steps in hydrocarbon biodegradation pathways (Fig. 1; Kniemeyer et al., 2003; Rabus, 2005; Callaghan et al., 2006).