The chlorobionts are the green algae, comprising 10,000 described species, most extant. These all photosynthesize, and their cells contain chloroplasts with the pigments chlorophyll a and b (except for the glaucophytes). Much like plants, their chloroplasts store starch as food and their cell walls contain cellulose.

There are unicellular (some colonial) and multicellular green algae. The unicellular species can be flagellated or unflagellated, but the flagella do not bear tubular hairs. Multicellular green algae have life cycles with an alternation of generations, much like plants; however, the sporophyte and gametophyte generations are identical. These seaweeds can absorb nutrients and water through all parts of the body, and are supported well by the water, so they do not have or need distinct organs or vascular tissue.

Multicellular green algae, probably the charophytes, gave rise to the first land plants. This, and the numerous similarities to plants (photosynthetic pigments, starch as food-storage compound, cellulosic cell walls, and alternation of generations), lead many biologists to classify green algae as plants.

Here are the phyla of the chlorobionts

• Phylum Chlorophyta
• Phylum Charophyta
• Phylum Micromonadophyta
• Phylum Pleurastrophyta
• Phylum Ulvophyta
• Phylum Glaucophyta controversial