Algal surface-associated compounds mediate epibacterial colonisation of the marine macroalga Fucus vesiculosus

The brown macroalga Fucus vesiculosus carries a community of surface bacteria that is specific and distinct to this alga. To identify potential chemical drivers on the algal thallus surface that mediate the abundance and composition of alga-associated bacteria, the present study tested the effect of surface-available secondary algal metabolites on bacterial settlement and epiphytic biofilm formation. Chemical compounds present on algal thalli were separated and investigated in laboratory and field bioassays to test their effect on inhibition of bacterial growth and settlement. The crude algal surface extract evoked a strong effect against bacterial settlement. The polar fraction of this extract revealed a profouling effect while the apolar fraction revealed a statistically significant anti-adhesive effect for surface colonizing bacteria. The xanthophyll pigment fucoxanthin was identified as one of the main compounds causing the anti-adhesion effect on bacteria. In combination assays, the profouling effect of polar compounds associated with the algal surface was clearly outweighed and suppressed by fucoxanthin and another unidentified apolar surface compound. These results indicate that algal surface-available compounds quantitatively promote or reduce overall epiphytic bacterial colonization to different extents. Analysis of bacterial surface communities by 454 pyrosequencing of 16S rRNA gene amplicons demonstrated that the observed antifouling effect of fucoxanthin and another apolar algal surface compound was mainly due to a drastic decrease of Gammaproteobacteria belonging to the phylum of Vibrionales. The established bacterial surface community on the alga was instead dominated by Chromatiales and other unclassified Gammaproteobacteria.