Ecological role of a seaweed secondary metabolite for a colonizing bacterial community

Bacteria associated with seaweeds can both harm and benefit their hosts. Many seaweed species are known to produce compounds that inhibit growth of bacterial isolates, but the ecological role of seaweed metabolites for the associated bacterial community structure is not well understood. In this study the response of a colonizing bacterial community to the secondary metabolite (1,1,3,3-tetrabromo-2-heptanone) from the red alga Bonnemaisonia hamifera was investigated by using field panels coated with the metabolite at a range of concentrations covering those measured at the algal surface. The seaweed metabolite has previously been shown to have antibacterial effects. The metabolite significantly affected the natural fouling community by (i) altering the composition, (ii) altering the diversity by increasing the evenness and (iii) decreasing the density, as measured by terminal restriction fragment length polymorphism in conjunction with clone libraries of the 16S rRNA genes and by bacterial enumeration. No single major bacterial taxon (phylum, class) was particularly affected by the metabolite. Instead changes in community composition were observed at a more detailed phylogenetic level. This indicates a broad specificity of the seaweed metabolite against bacterial colonization, which is supported by the observation that the bacterial density was significantly affected at a lower concentration (0.02 μg cm⁻²) than the composition (1-2.5 μg cm⁻²) and the evenness (5 μg cm⁻²) of the bacterial communities. Altogether, the results emphasize the role of secondary metabolites for control of the density and structure of seaweed-associated bacterial communities.