The chromobionts encompass all brown, golden-brown, and yellow-green algae, as well as a few groups of the so-called lower fungi, in total comprising 30,000 confirmed and up to 50,000 recorded species, half of which are fossils. The chromobionts all contain photosynthetic components with the pigments chlorophyll a (found in plants), c and d. Their cells have tubular mitochondrial cristae, and chloroplasts within the rough endoplasmic reticulum. Food reserves are stored outside plastids, and most cells have flagella.

The chromobionts include the diatoms and the brown algae, which are closely related to each other; these groups contain the pigment fucoxanthin in addition to the others described above. The diatoms are all unicellular photosynthesizers that serve as major food sources in all aquatic habitats. Diatom cells have cell walls made of two plates (top and bottom) of the mineral silica (the same mineral in glass). Diatoms store food reserves as oils in their bodies, which helps them maintain buoyancy near the surface of the ocean, and the sun.

The brown algae are closely related to diatoms, though they are multicellular seaweeds. Kelp forests, made up of brown algae, are important marine habitats in and of themselves, providing shelter and food for animal that live in them. Like plants, brown algae have a life cycle including an alternation of generations; however, their cells lack plasmodesmata, as nutrients can be absorbed through all parts of the plant, and they do not produce starch. This shows that the chromobionts are not closely related to plants, or to other algae (green and red types).

Here are the phyla of the chromobionts

• Phylum Chrysophyta
• Phylum Synurophyta
• Phylum Haptophyta (Prymnesiophyta)
• Phylum Xanthophyta
• Phylum Pedinellophyta
• Phylum Chlorarachniophyta
• Phylum Eustigmatophyta
• Phylum Bacillariophyta (diatoms)
• Phylum Phaeophyta (brown algae)
• Phylum Oomycota
• Phylum Hyphochytridiomycota
• Phylum Proteromonadea
• Phylum Opalinata