A new method for determining surface concentrations of marine natural products on seaweeds

To understand the role of marine natural products in surface mediated ecological interactions-such as biofouling-we need to be able to quantify metabolites at or near the relevant surfaces We describe a new technique for determining the concentration of natural products on the surface of marine algae. Surface metabolltes were quantified for 2 red algae, Delisea pulchra and Laurencia obtusa, by dipplng the algae in hexane for 20 to 40 s at room temperature. This allowed for quantitative extraction of non-polar natural products from the surface of the thalli without disrupting cells (as determined by epifluorescence nucroscopy) More str~ngent extraction procedures using other solvents, or longer extraction times in hexane (>50 S ) , caused significant quantifiable cell damage. Natural products In the surface extracts were then measured uslng gas chromatography-mass spectroscopy (gc-ms). Mean total surface concentration of natural products from D. pulchra were 250 ng cm-" but < l ng cm-2 for L. obtusa. These results contrast to whole plant levels of total secondary metabolites in the 2 algae, w h ~c h W C ~C significantly higher in L. obtusa (7 pg mg-2 dry weight) than in D. pulchra (3.4 1-19 mg-2 dry weight) Dipping thalli in hexane for 30 S also caused no cell lysis in 8 other species of macroalgae This suggests that the procedure is more broadly applicable for the quantification of non-polar surface metabolites on seaweeds, and other organisms with resistant surface cells Our results h~ghlight the need to deternune where compounds occur, and at what concentration, if w e are to understand their ecological roles.