Isolation and purification of antialgal compounds from the red alga Gracilaria lemaneiformis for activity against common harmful red tide microalgae.

Seven antialgal compounds (1-7) were successfully isolated from the red alga Gracilaria lemaneiformis through a combination of silica gel column chromatography and repeated preparative thin-layer chromatography. On the basis of the spectral data, the compounds were identified as gossonorol (1), 7,10-epoxy-ar-bisabol-11-ol (2), glycerol monopalmitate (3), stigmasterol (4), 15-hydroxymethyl-2, 6, 10, 18, 22, 26, 30-heptamethyl-14-methylene-17-hentriacontene (5), 4-hydroxyphenethyl alcohol (6), and margaric acid (7). These seven compounds were isolated from G. lemaneiformis for the first time, while the compounds 4, 6, and 7 were isolated from marine macroalgae for the first time. Furthermore, a quantitative relationship between the inhibition of algal growth and the concentration of each antialgal compound was determined and important parameters for future practical HAB control, e.g., EC50-96h, were also obtained. The results indicated that isolated compounds 1-7 possess selective antialgal activity against the growth of several red tide microalgae (including Amphidinium carterae, Heterosigma akashiwo, Karenia mikimitoi, Phaeocystis globsa, Prorocentrum donghaiense, and Skeletonema costatum). Their antialgal activity against test red tide microalgae has not been previously reported. Furthermore, the EC50-96h of one or more of the compounds towards the tested red microalgae was not only significantly less than 10 μg/mL but also was smaller than that of the characteristic antialgal agent potassium dichromate. The study demonstrates that compounds 1-7 possess significant application potential as antialgal agents against several harmful red tide microalgae.