Surface chemistry versus whole-cell extracts: antifouling tests with seaweed metabolites

Development of research in the field of chemical inhibition of colonisation of seaweed surfaces has been constrained by the lack of appropriate methods for testing realistic concentrations of potentially deterrent compounds. Here we extend earlier work (de Nys et al. 1998) on the red alga Delisea pulchra to 6 other Australian seaweed species to investigate whether these methods could be used more generally in studies of natural defences against biofouling. We compared the effects of surface extracts of D. pulchra, Caulerpa filiformis, Dictyopteris acrostichoides, Dilophus marginatus, Laurencia rigida, Solieria robusta and Pterocladia capillacea on the settlement of 2 ecologically rele- vant fouling species, and further compared the effects of surface extracts to those of non-polar, whole-cell extracts of the 7 seaweeds. We also measured the natural biofouling cover of these sea- weeds in a field survey and examined whether levels of biofouling on the seaweeds in the field are predicted by the activity of either the surface extracts or the whole-cell extracts of these species. The results from settlement tests with surface extracts at natural concentrations showed that 2 species, D. pulchra and C. filiformis, had non-polar metabolites on their surfaces in sufficient quantities to significantly inhibit settlement. These species also had significantly lower biofouling cover in the field compared to the other seaweeds. The results of the settlement tests with whole-cell extracts, however, demonstrated that all the seaweeds contain non-polar metabolites that inhibit settlement at concentrations lower than total whole tissue content and that no individual whole-cell extract was generally more inhibitory than the others. Therefore, we conclude that results from settlement assays with whole-cell extracts are poor predictors of natural antifouling roles of seaweed metabolites, and that such bioassays are of little use if the objective is to explore the chemical mediation of interactions between seaweeds and fouling organisms. We also conclude, that with careful choice of solvent and extraction time, the surface extraction procedure described here may be broadly useful for investi- gating the deterrent effects of seaweed surface metabolites against fouling organisms.