Brown algae are a group of marine and freshwater organisms belonging to the phylum Phaeophyta. They are characterized by their brown or olive-green color, which comes from the pigment fucoxanthin. Brown algae can vary in size, ranging from small filaments to large seaweeds like kelp, which can grow up to 60 meters long. These organisms play a crucial role in marine ecosystems, providing habitat and food for various marine life. Brown algae are also important for humans, as they are used in products like agar, a thickening agent in food, and alginates, which are used in pharmaceuticals and cosmetics.