5; Fig. S4 in Appendix S1). We studied the impact of ferrihydrite, manganese dioxide, nitrate and sulfate Entinostat clinical trial on hydrocarbon-dependent methanogenesis. Ferrihydrite accelerated hexadecane-dependent methanogenesis compared with sulfate or nitrate. Nitrate almost completely inhibited methanogenesis from

hexadecane and ethylbenzene (Figs 2 and 3a). This is not surprising because nitrate is a well-known inhibitor of methanogenesis (Klüber & Conrad, 1998). Furthermore, nitrate and high sulfate concentrations negatively influenced the conversion rates of hexadecane to methane (Figs 2 and 3a). However, in the presence of 2 mM sulfate, nitrate was not inhibitory (Fig. 3a), indicating that a sulfate-reducing hexadecane-degrading community prevailed. Bleomycin ic50 Adding sulfate in concentrations up to 5 mM to the sediment microcosms of Eckernförde Bay resulted in a significant increase of hexadecane-dependent methanogenesis (Fig. 3b). In contrast, concentrations higher than 5 mM strongly inhibited hexadecane-dependent methanogenesis. Possibly, sulfate addition stimulated the growth of new or other sulfate reducers, dominating substrate competition for intermediates with methanogens. In contrast, a previous study reported no inhibition of methanogenesis

by sulfate of up to 10 mM (Gieg et al., 2008). The inhibitory effect of 22 mM sulfate on ethylbenzene-dependent methanogenesis was less pronounced compared with hexadecane. For naphthalene, neither the inhibition nor the stimulation of methanogenesis was found with either electron acceptor (Fig. 4 and Table 1). This agrees with a recent study of contaminated sediments, where no stimulating effect of Fe(III) on PAH degradation was observed (Li et al., 2010). The impact of

electron acceptors on hydrocarbon-dependent methanogenesis demonstrates that (1) the concentration of the added electron acceptor is crucial for hexadecane-fed Phosphoprotein phosphatase methanogenesis and (2) the solubility of the electron acceptor appears to be important. Indeed, insoluble electron acceptors such as ferrihydrite or manganese dioxide had a stimulating effect on hexadecane-dependent methanogenesis (Fig. 2a). However, these electron acceptors are only locally bioavailable, which may result in microscale compartment formation. In contrast, theoretically possible products of hexadecane degradation, such as carbonate, acetate and H2, can freely diffuse and become available for methanogens in niches where other electron acceptors are depleted. In Zeebrugge microcosms, the observed increase of the total archaeal community and mcrA gene copies suggests that especially Methanosarcina species account for iron reduction as demonstrated by van Bodegom et al. (2004) (Fig. 5 and Supporting Information). Moreover, neither ferrihydrite or sulfate nor hexadecane or methane addition triggered the growth of Geobacteraceae. In conclusion, members of this family are probably less important for the respective processes (Fig. 5).