Fig. S1. Phylogenetic tree of the Ulva thallus bacterial population. The tree resulted from a sequence alignment of 16S rDNA from bacterial clones and isolates obtained from the Ulva thallus at 97% sequence similarity. The reference strains were taken from GenBank. In brackets is the number of clones in each OTU. The tree topology is based on neighbour- joining and bootstrap analysis was performed with 1000 replications.


Fig. S2. AHL bioreporter activation assay.

A. Cell-free supernatant extracts from Shewanella spp. 79 pBBRIMCS and pMT01 were assayed with lux-based AHL bioreporters E. coli pSB536 (clear bars) and pSB1075 (cross-hatched bars).

B. Cell-free supernatant extracts from Sulfitobacter spp. 376 pBBRIMCS and pMT01 were assayed with lux-based AHL bioreporter E. coli pSB401. All strains harbouring pMT01 and therefore expressing the aiiA lactonase failed to activate the bioreporters showing that their cognate AHLs were being deactivated.


Fig. S3. AHL germination assays slides. Example images of Ulva zoospores germinated on (A) Sulfitobacter sp. pMT01 biofilms (AHL-deficient) and (B) Sulfitobacter sp. pBBRIMCS biofilms (AHL-producing). Bar = 50 μm.


Table S1. Phylogenetic identification and AHL signal molecule production in Ulva-associated marine bacteria. The ability of each strain to produce AHLs was assayed using lux-based AHL bioreporters, a positive result is indicated by + and a negative result by . AHL production and identity was confirmed by LC-MS/MS.

Table S2. Non-marine bacterial strains used in this study.

Table S3. Plasmids used in this study.

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