Fig. S1. A. Sampling site. Picture from Figueiras and Rodas beaches in the Cíes Island where sampling was carried out. Red and blue shapes delimit the sampling sites at each beach. B. Oil patch in the seafloor of Figueiras beach in 2004 (left) and an example of a petroleum patch found at the bottom of Rodas cores in 2007 (right). C. Scheme of the core depth zones analysed in this work (left) and magnification of the oxic to anoxic transition zone typical of Figueiras sediment cores.

Fig. S2. Relative distribution of alkanes between most polluted samples. Data are compared with the Prestige oil fingerprinting described in Alzaga and colleagues (2004). The asterisk (*) indicates sum of nC17 and pristane.

Fig. S3. A. Rarefaction curve for 16S rRNA genes of microbial populations from the sediment sampled at Figueiras beach in 2004 (FI) and from Figueiras and Rodas beach in 2007 (FII and RII respectively). PET, tar aggregate collected at Figueiras in 2004; OX, TR and AN refer to the oxidized, transition and reduced zones as described in the text (see Table S1). B. Good's Index generated using MOTHUR. Clones were grouped into phylotypes at a level of ≥ 97% sequence similarity.

Fig. S4. Distribution of environmental origin of the closest relatives found in the 16S rRNA gene libraries with at least 97% of similarity.

Fig. S5. Neighbour-joining tree of myxobacteria based on 16S rRNA gene sequences available in the SSU r108 database (SILVA database). A representative sequence belonging to the same OTU from each sample was selected, together with its closest relatives from marine and terrestrial environments; the number of retrieved sequences in each OTU is indicated (xn). Sequences from Brinkhoff and colleagues (2012) were aligned with the SINA-online tool and added to the database. The circle structure of the tree was made using MEGA 5.05. Clusters delimited in the tree have a phylogenetic distance of at least 10% respect to their terrestrial counterparts. Only full length sequences were considered. GenBank sequence accession numbers are given in parentheses. Different colours indicate the sample origin, as specified in the legend.

Table S1. Sample description with some geophysical parameters.

Table S2A–B. Aliphatic hydrocarbon composition in the sediments from 2004 and 2007 sampling.

Table S3A–B. PAH composition in the sediment samples from 2004 and 2007 sampling.

Table S4A–B. Most probable number counts of hydrocarbon oxidizing bacteria from the first and the second sampling.

Table S5A. FISH counts of bacterial cells in the sediment samples.

Table S5B. Efficiency of probes against 16S rRNA gene libraries.

Table S6. Description of bacterial phylotypes from 16S rRNA gene libraries.

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