Seaweed defence against bacteria: a poly-brominated 2-heptanone from the red alga Bonnemaisonia hamifera inhibits bacterial colonisation

It has previously been shown that the red alga Bonnemaisonia hamifera is less fouled by bacteria relative to co-occurring seaweeds and that surface extracts of B. hamifera inhibit bacterial growth at natural concentrations. In the present study, we isolated the antibacterial metabolite by bioassay-guided fractionation of extracts of B. hamifera using standard chromatographic methods. Nuclear magnetic resonance (NMR) spectroscopy and mass spectrometry were used for molecular identification. The antibacterial activity in the extracts was caused by a previously described poly-halogenated 2-heptanone: 1,1,3,3-tetrabromo-2-heptanone. To further investigate the role of this compound as an ecologically relevant antifoulant against bacterial colonisation, we quantified it on the surface of B. hamifera specimens collected in the field. Levels of 1,1,3,3-tetrabromo-2-heptanone on the surface of the algae were on average 3.6 \ub5g cm–2. Natural surface concentrations of this secondary metabolite were used to test for growth-inhibiting effects against 18 bacterial strains isolated from red algae co-occurring with B. hamifera. The test indicated a phylogenetic specificity of the metabolite, and gram-positive bacteria and flavobacteria proved to be particularly sensitive. In a further test, natural surface concentrations of 1,1,3,3-tetrabromo-2-heptanone were applied to artificial panels and incubated in the sea. After 4 and 7 d, the number of settled bacteria was significantly lower on all treated panels compared to controls. Thus, this study shows that 1,1,3,3-tetrabromo-2-heptanone has an ecologically relevant role as an antifoulant against bacterial colonisation on the surface of B. hamifera. This study is also one of only a few to quantify natural surface concentrations of a seaweed secondary metabolite