Waterborne polar macromolecules act as algal antifoulants in the seaweed Ulva reticulata

Repetitive field observations in Hong Kong waters revealed that the green macroalga Ulva reticulata was conspicuously free of epibiotic macroorganisms. This phenomenon was indicative of some sort of natural antifouling control in a macroalga that has been typified as undefended thus far. In previous studies, we identified the alga itself and a thallus-associated epibiotic Vibrio sp. as sources of waterborne, macromolecular substances with an inhibitory effect on the larval attachment of the polychaete Hydroides elegans. Employing a less artifactual water collection technique under field conditions in this study, we demonstrated that naturally obtained algal exudates evoked a similar inhibitory effect as those obtained artificially under laboratory conditions. In order to interpret the chemical defense system of U. reticulata with respect to the production and nature of bioactive metabolites, the allelopathic activity of secondary metabolites obtained from Vibrio sp. and the macroalga was bioassayed and biochemically analyzed in parallel. In addition to H. elegans, bioassays were performed with larvae of the bryozoan Bugula neritina. Different enzyme probes revealed chemical and structural features of the factor inhibitory to larval attachment in both samples, and pointed to large polysaccharide units and sulfate esters as structural parts of the bioactive moiety. Specific susceptibilities of the algal and bacterial samples to proteolytic enzymes indicated that (a) different macromolecular constituents were causative of the inhibitory effect, and (b) that the bioactive moiety in the alga-derived sample comprised a structural connection between carbohydrate(s) and proteinaceous molecular domains. The results indicated an antifouling concept in U. reticulata that was presumably driven by at least 2 different waterborne macromolecules, and revealed that highly water soluble macromolecules may play an essential chemical ecological role in a marine system. This exemplifies a distinct concept in comparison to other prominent examples of algal antifoulants, which exclusively comprise comparatively small and non-polar algal metabolites.