Multicellular algae are algae that have more than one cell in their makeup. Most algae are unicellular, which means that they are made up of a single cell. Red, green and brown algae, commonly referred to as seaweeds, are multicellular algae.
The classification system of algae changes as scientists learn more about them. Marine algae come in many shapes and sizes. The red algae are in the protist kingdom; the green algae belong to the chromist kingdom; and the brown algae are part of the plantae kingdom. These types of algae all have cell-wall structures and are capable of photosynthesis.