The brown alga Sargassum horneri produces chemicals that can stop the growth of red tide microalgae like Skeletonema costatum, Chattonella antiqua, and Heterosigma akashiwo. The extracts from S. horneri were most effective at inhibiting S. costatum and C. antiqua, while H. akashiwo needed more extract to have the same effect. The chemicals changed the shape of the microalgae, causing them to grow less.