ABSTRACT
Seaweeds or macroalgae are the primary producers of an oceanic food source, widely distributed across the globe andknown for their excellent defensive properties against numerous biotic and abiotic factors. These defensive traits comefrom their secondary metabolites that act as protective barriers against pathogens and harmful organisms where amongthese compounds some have been found possessing the antifouling characteristic. In this study, Dictyota dichotomaand Sargassum granuliferum collected from Pulau Nunuyan Laut, Sabah, Malaysia, were studied to determine its sterolcomposition, and isolation was carried out to isolate their pure sterol compounds. Two assays consist of disk diffusionmethod for antibacterial activity and crystal violet assay were carried out to study its antifouling activity. Campesterol,stigmasterol, and β-sitosterol were the dominant sterol compound detected in both samples and six pure sterols wereisolated (compounds 1–6). Results from the antibacterial and antifouling analysis showed better inhibition for Gramnegative compared to Gram-positive bacteria. Fucosterol (4) and epicoprostanol (5) gave the best antibacterial activitywith two bacteria inhibited compared to other compound. Meanwhile, coprostanol (1), campesterol (2), stigmasterol(3), epicoprostanol (5), and 5β-cholestan-3-one (6) showed strong antifouling activity towards the selected bacterialstrains with IC50 values ranges from 266.3 to 425.8 µg/ml.