Two different ecosystems – contaminated harbor mud and pristine marine
sediment – were investigated to show that this approach is generally applicable. Methane evolved upon hexadecane, ethylbenzene or naphthalene addition in different sediment microcosms (Fig. 2 and Table 1). In most cases, conversion of hexadecane to methane was faster compared with aromatic hydrocarbons this website (Fig. 2 and Table 1). Exceptions were ethylbenzene microcosms with 2 mM sulfate, in which the conversion to methane was faster (58.1±0.6 nmol methane cm−3 day−1) than that in the respective hexadecane incubation (37.8±6.6 nmol methane cm−3 day−1). The observed rates were approximately one order of magnitude lower than those reported in a study of an inoculated oil field sediment core (Gieg et al., 2008). Apparently, inoculation using an enriched consortium was more efficient
than the stimulation of indigenous hydrocarbon degraders. In another study of a sediment-free methanogenic hexadecane-degrading enrichment culture, hexadecane-dependent methanogenesis was lower (13 nmol methane mL−1 day−1) than the rates Gefitinib solubility dmso observed in our experiments (Feisthauer et al., 2010). Presumably, a sediment-free enrichment culture never reaches cell densities of sediments (approximately 109 cells cm−3 sediment, Fig. S2 in Appendix S1), resulting in lower volume-related rates. Methanogenesis from naphthalene was in a picomolar range while other hydrocarbons induced methane release in nanomolar ranges (Fig. 2 and Table 1). The time lag between 13CO2 and 13CH4 evolution as well as the significant difference in δ13C-signature shifts (Fig. 4) indicate that methanogenesis played a minor role in naphthalene-degrading microcosms. Primarily, naphthalene seems to have been mineralized to CO2. Anaerobic oxidation of naphthalene and subsequent formation of CO2 was demonstrated under nitrate- (Bregnard, 1996) and sulfate-reducing Inositol monophosphatase 1 conditions (Langenhoff et al., 1989; Coates et al., 1996; Hayes et al., 1999; Musat et al., 2009).
Nevertheless, methanogenesis occurred in our naphthalene-degrading microcosms, a process that was suggested (Sharak Genthner et al., 1997; Chang et al., 2006), but hitherto never confirmed. Sharak Genthner et al. (1997) observed an inhibition of methanogenesis after naphthalene addition and concluded that naphthalene may be toxic to methanogens. In our microcosms, this seems unlikely because they were naturally exposed to various mineral oil compounds found in the sediments (Ministerie van de Vlaamse Gemeenschap, 2002). Regardless of naphthalene toxicity, methanogens possibly had better access to degradation products of hexadecane and ethylbenzene than to those of naphthalene. We therefore postulate that methanogens themselves were directly involved in the degradation chain of hexadecane and ethylbenzene, but not of naphthalene degradation.