Chemical defence against bacteria in the red alga Asparagopsis armata: linking structure with function

Although numerous algal products have antimicrobial activity, limited knowledge of metabolite localisation and presentation in algae has meant that ecological roles of algal natural products are not well understood. In this study, extracts of Asparagopsis armata had antibacterial activity against marine (Vibrio spp.) and biomedical (Escherichia coli, Pseudomonas aeruginosa and Staphylococcus spp.) strains. The major natural products in both life-history stages of A. armata (as determined by gas chromatography-mass spectrometry analysis GC-MS) were bromoform (0.58 to 4.3% of dry weight DW) and dibromoacetic acid DBA (0.02 to 2.6% DW), and each compound was active against these same bacteria. To resolve whether this antibiotic activity was ecologically relevant, we examined the localisation of metabolites in the specialised cells of A. armata and observed a delivery mechanism for the release of metabolites to the surface. Bromoform and DBA were subsequently quantified in the surrounding medium of laboratory cultures, establishing their release from the alga. In a novel ecological test of algal natural products, halogenated metabolites in A. armata were manipulated by omitting bromine from an artificial seawater medium. Significantly higher densities of epiphytic bacteria occurred on algae that no longer produced halogenated metabolites. Both bromoform and DBA were more active against bacteria isolated from algae lacking brominated metabolites than algae producing normal amounts of these compounds. Taken together, these results indicate that halogenated metabolites of A. armata may be important in reducing epiphytic bacterial densities.