Seaweed does not belong in a single kingdom but in several different ones due to different evolutionary processes. These kingdoms include Kingdom Plantae, Kingdom Chromista and Kingdom Bacteria.
There are three phylums of seaweed that can be distinguished by the color of the thallus. Seaweed in phylum Ochrophyta are brown, ones in phylum Rhodophyta are red and those in phylum Chlorophyta are green. Besides color, these three groups have substantial differences in their structures and biochemical compositions, such as their cell walls, flagella, storage compounds, chloroplasts, photosynthetic pigments, mitosis and cell connections. About 11,500 species of seaweed have been discovered so far.