Algae (singular: alga) are a vastly diverse group of simple autotrophs. Undifferentiated tissue is what renders algae as protists rather than plants. Like plants, most algae are photosynthetic.
Algae are categorized by color: green, yellow-green, red, and brown. Algae range from unicellular microbes, such as diatoms, to giant kelp, which may grow to 65 meters. Seaweed – including kelp (a brown algae) – are a colonial form; the largest and most complex marine algae.
Chlamydomonas reinhardtii is a tiny (10 µm) single-cell photosynthetic green alga, found worldwide in soil and fresh water. When carbon dioxide is in short supply for photosynthesis, C. reinhardtii resorts to consuming cellulose. It is the only known vegetative organism capable of herbivory.