Where should seaweeds and kelps be classified? Give characteristics of these organisms which you used to place them in a taxonomic unit and the source you used to find their characteristics. Solution Seaweeds are classified into three phylum\'s based on the colour of the thallus, biochemical compositions, cell wall structures, photosynthetic pigments and chloroplasts. The three kingdoms of seaweeds are, Kingdom bacteria, Kindom Plantae and Kingdom Chromista. The three phylum\'s are, Phylum Rhodophyta (red alage), Phylum Chlorophyta (green algae) and Phylum Ochrophyta (brown algae). Kelps are the brown algae that belong to the order Laminariales, genus Laminaria..