For naphthalene incubations, the rates were calculated in a timeframe of 435 days without an intermediate measurement. Sediment DNA was extracted using a FastDNA Spin Kit for Soil DNA extraction kit (MP Biomedicals). Genes of interest were quantified using an Applied Biosystems StepOne thermocycler. 16S rRNA gene copy numbers of Archaea and Bacteria were determined as described previously (Takai & Horikoshi, 2000; Nadkarni et al.,

2002). The concentrations of mcrA and dsrA genes were investigated according to Nunoura et al. (2006) and Schippers & Nerretin (2006), respectively. Members of the Geobacteraceae were quantified using the method described by Holmes et al. (2002). Copy numbers FK506 order are expressed as copies cm−3 sediment. Members of the microbial community in the Zeebrugge sediment were identified by the incorporation of 16S rRNA gene sequence fragments of a clone library into an existing maximum-parsimony tree (version 102) provided by Pruesse et al. (2007). Fragments of 16S rRNA genes were obtained using the modified primer sets Ar109f (5′-ACKGCTCAGTAACACGT) and Ar912r (5′-CTCCCCCGCCAATTCCTTTA) for Archaea and 27f (5′-AGAGTTTGATCCTGGCTCAG) and 907r (5′-CCATCAATTCCTTTRAGTTT) for Bacteria (Liesack & Dunfield, 2004). Subsequently, cloning was performed using the pGEM-T vector system according to the manufacturer’s instructions (Promega). All sequencing was conducted at Seqlab Göttingen

Tanespimycin (Germany). Sequences were deposited at the GenBank online database Olopatadine under accession numbers HM598465–HM598629. Methanogenesis was observed in all Zeebrugge microcosms after 178 days. Without added hydrocarbons, the methanogenesis rates were 2.9, 0.8, 0.6, 0.3 or 0.8 nmol methane cm−3 day−1 for ferrihydrite, manganese dioxide, nitrate, 2 or 22 mM sulfate-amended

microcosms, respectively. The respective CO2 release rates in these controls ranged from 35.5 nmol CO2 cm−3 day−1 for ferrihydrite to 73.8 nmol CO2 cm−3 day−1 for nitrate. In microcosms containing Zeebrugge sediment with hexadecane, a significant increase of methanogenesis was observed compared with control experiments without hexadecane (Fig. 2a). Moreover, hexadecane-dependent methanogenesis rates were significantly different between microcosms with and without an added electron acceptor (Fig. 2a). Most prominently, ferrihydrite accelerated hexadecane-dependent methanogenesis to 87.3±2.3 nmol methane cm−3 day−1 compared with 37.8±6.6 nmol methane cm−3 day−1 in 2 mM sulfate incubations (natural harbor water). The increase of methanogenesis in manganese dioxide incubations to 45.9±1.9 nmol methane cm−3 day−1 was insignificant compared with 2 mM sulfate incubations (Fig. 2a). Adding 20 mM sulfate decreased methanogenesis to 2.1±1.1 nmol methane cm−3 day−1. Nitrate inhibited methanogenesis completely. However, the addition of hexadecane triggered CO2 release from the microcosms (Fig. 2a). The CO2 release rates ranged from 64.6±5.8 nmol CO2 cm−3 day−1 for 2 mM sulfate to 139.6±3.