Looks like a plant but is really a protist… • Most seaweed is photosynthetic. • Some are not producers but parasites of other seaweeds. • Seaweeds transform solar energy into chemical energy in the form of organic matter. • This organic matter is available to many other organisms. • Seaweed is a type of marine algae, it can also be referred to as a macrophyte or macroalgae. • Seaweed is multicellular and eukaryotic. • Seaweeds lack highly specialized structures and reproductive structures that are characteristic of terrestrial plants. • Seaweeds show a wide range of growth forms and complexity of structures. • Seaweeds lack true leaves and roots. • The complete body is called the thallus. It can be a filament, thin leafy sheet or giant kelp. • The leaf like, flattened portion of the thallus is known as the blade, these have no veins. • Gas-filled bladders, called pneumatocysts, sometimes keep the blades close to the sea surface. • Some have a distant stem-like structure that provides support, this is called the stipe. • Blades originate from the stipe. • A structure that looks like roots attach the thallus to the bottom. This is called the holdfast. • These help anchor the seaweed. They cannot usually cannot anchor in soft sediments and are restricted to hard bottoms. • These do not penetrate through sand or mud like true roots do, so they are not involved in the absorption of water and nutrients. • They also usually lack tissues specialized in the transport of water and nutrients. Water and nutrients are picked up across the surface of the thallus.