The polysaccharide- alginic acid was extracted from brown seaweed Sargassum wightii and the yield observed was 3.932%. The purity of alginic acid was determined by preliminary phytochemical analysis and the result indicated that it has only the presence of carbohydrates and its derivative saponins. The physicochemical properties of alginic acid were analyzed. The FT-IR, 13C and 1H NMR analysis indicated the presence of carbons and anomeric protons of guluronic acid and manuronic acid in purified and hydrolyzed alginic acid, respectively. The characterized alginic acid was enriched with instar II stage Artemia nauplii at 100, 200, 300 and 400 mg L-1 concentrations for 12 h and they were fed to Penaeus monodon postlarvae (PL15) for 20 days. After feeding experiment, the P. monodon PL35 were challenged with WSSV. The control group of shrimp PL fed with unenriched Artemia nauplii showed 100% mortality within 8 days, but the alginic acid (100-400 mg L-1) enriched Artemia nauplii fed shrimp PL showed less mortality (79 – 100 %) within 21 days of WSSV post challenge. The reduction in mortality percentage of alginic acid enriched Artemia nauplii fed groups over control group was ranged between 17.15 and 49.99 %. The RT-PCR analysis confirmed the considerable reduction of WSSV DNA copy numbers (71757 – 5.73 WSSV DNA copies) in shrimp postlarvae with respect to the concentration of alginic acid. The present result concluded that P. monodon PL fed with alginic acid of S. wightii enriched Artemia nauplii has increased the resistance against WSSV infection.
