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Fig. S1. A. Seasonal analysis of epibacterial communities of Fucus vesiculosus by denaturing gradient gel electrophoresis (DGGE) to control intraspecific similarity of epibacterial communities at each sampling point and to compare the bacterial community composition of the surrounding seawater with surface-associated bacteria. The standard was composed of a mixture of 16S rRNA gene amplified bacterial strains. Band pattern of Standard top down: 1. Verrucomicrobia;2. Bacteroidetes; 3. Chloroplast DNA; 4. Planctomyces; 5. Epsilonproteobacteria; 6. Alphaproteobacteria, 7. Actinobacteria; 8. Deltaproteobacteria. Standard (1, 12, 23), Seawater: December (10), March (15), May (19), July (2), September (6); Fucus: December (11, 13, 14), March (16–18), May (20–22), July (3–5), September (7–9). B. Seasonal analysis of epibacterial communities of G. vermiculophylla by DGGE to control intraspecific similarity of epibacterial communities at each sampling point and to compare the bacterial community composition of the surrounding seawater with surface-associated bacteria. The standard was composed of a mixture of 16S rRNA gene amplified bacterial strains. Band pattern of Standard top down: 1. Verrucomicrobia; 2. Bacteroidetes; 3. Chloroplast DNA; 4. Planctomyces; 5. Epsilonproteobacteria; 6. Alphaproteobacteria, 7. Actinobacteria; 8. Deltaproteobacteria. Standard (1, 15, 23), Seawater: December (6), March (2), May (14), July (19), September (10); Gracilaria: December (7–9), March (3–5), May (16–18), July (20–22), September (11–13). C. Seasonal analysis of epibacterial communities of Ulva intestinalis by DGGE to control intraspecific similarity of epibacterial communities at each sampling point and to compare the bacterial community composition of the surrounding seawater with surface-associated bacteria. The standard was composed of a mixture of 16S rRNA gene amplified bacterial strains. Band pattern of Standard top down: 1. Verrucomicrobia; 2. Bacteroidetes; 3. Chloroplast DNA; 4. Planctomyces; 5. Epsilonproteobacteria; 6. Alphaproteobacteria, 7. Actinobacteria; 8. Deltaproteobacteria. Seawater: May (9), July (1), September (5); Ulva: May (10–12), July (2–4), September (6–8). D. Cluster analysis of DGGE banding patterns (see DGGE gels A, B and C) based on 16S rDNA amplified from epiphytic bacterial communities. DGGE banding patterns were analysed using the PRIMER software v.6.1.9 (Primer-E). Banding positions were assigned to ‘species’ and the program then compared ‘species’ composition within and among host algal species using analysis of similarity (ANOSIM). Based on the banding profiles, a presence–absence matrix was generated and Bray–Curtis values without transformation were calculated (Jackson, 1993). Sample similarities in per cent are shown by cluster analysis: (a) F. vesiculosus, (b) G. vermiculophylla, (c) U. intestinalis, (d) seawater control (SW). Algae were sampled in 2007 and 2008 in bimonthly intervals (D, December; M, March; MY, May; J, July, S, September). Generally, three individuals were analysed (1–3). Pairwise comparison of clusters was computed by analysis of similarity (ANOSIM) (Clarke, 1993). The ANOSIM global test was used as statistical test for the pairwise comparison of cluster groups. R-values close to 1 indicate the similarity within a group is higher than between different groups. The composition of bacterial communities on algal surfaces significantly differed from the microbial colonizer pool of the respective surrounding seawater (similarities: F. vesiculosus: < 60%, G. vermiculophylla: < 60% and U. intestinalis: < 60%). These findings were confirmed by statistical analysis based on the DGGE banding patterns. Algal biofilm composition differed from the bacterial community in the surrounding seawater. Fucus vesiculosus: ANOSIM global test R = 0.927; P = 0.001; G. vermiculophylla: ANOSIM global test R = 0.83; P = 0.002, U. intestinalis: ANOSIM global test R = 0.95; P = 0.005.

Fig. S2. A–F. Maximum parsimony (ARB) based 16S rRNA phylogenetic tree of bacterial rRNA genes found on the surface of Fucus vesiculosus (FV), Gracilaria vermiculophylla (GV) and Ulva intestinalis compared (UI) to the ambient bacterial planktic community in the surrounding seawater (UW) in July and December 2007. Sequences were grouped by ≥ 99% sequence identity. Bacterial 16S rRNA genes observed on the surface of F. vesiculosus are highlighted in brown. Sequences observed in biofilms of G. vermiculophylla are highlighted in red, those of U. intestinalis in green and sequences of the planktic bacterial community are coloured in blue. Maximum parsimony bootstrap values (1000 resamplings).

Fig. S3. A–H. Maximum parsimony (ARB) based 16S rRNA phylogenetic tree of bacterial rRNA genes found on the surface of (I) Fucus vesiculosus (FV), (II) Gracilaria vermiculophylla (GV) and (III) Ulva intestinalis (UI) at different sampling intervals. Fucus and Gracilaria: summer (July 2007 and July 2008) and winter (January 2007 and December 2007); Ulva: May, July, September 2007 and May 2008. Sequences were grouped by ≥ 99% sequence identity. Bacterial 16S rRNA genes observed on the surface of F. vesiculosus are highlighted in brown. Sequences observed in biofilms of G. vermiculophylla are highlighted in red and those detected in biofilms of U. intestinalis are marked in green. Maximum parsimony bootstrap values (1000 resamplings).

Fig. S4.Ulva intestinalis collected in July 2007 was heavily fouled by the blue mussel Mytilus edulis.

Table S1.SIMPER analysis of dissimilarities between summer and winter gene libraries of Fucus vesiculosus. OTUs grouped by 99% sequence similarity. Average dissimilarity between summer (FS) and winter (FW) was 88%. More than 60% (cumulative in per cent; Cum. %) of dissimilarity could be ascribed to the following OTUs.

Table S2.SIMPER analysis of dissimilarities between summer and winter gene libraries of Gracilaria vermiculophylla. OTUs grouped by 99% sequence similarity. Average dissimilarity between summer (GS) and winter (GW) was 89%. More than 30% (cumulative in per cent; Cum. %) of dissimilarity could be ascribed to the following OTUs.

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