Some marine algae—the kelps—look like plants with stem‐like stalks supporting “leaf” blades and a holdfast that anchors the kelp to rocks on the sea bottom. Growing offshore in colder northern waters they sometimes attain a height of 50–60 meters—the largest protists. Most of the algae are constructed more simply and are smaller—many are unicellular and microscopic in size. The algae, like the rest of the protists, have evolved a wondrous array of body forms and lifestyles—unicellular, motile free‐living; complex aggregations of cells in colonies; filaments, nets, coenocytic tubes; microscopic, slimy green threads—all are common algal forms.

The variable body structure is not useful taxonomically to separate the major groups, nor are the reproductive structures particularly helpful. Instead, the following five features are more definitive and used in algal classification:

  • The photosynthetic pigments they contain.
  • Kind of stored foods.
  • Materials composing the wall.
  • Number, kind, and position of the flagella.
  • Details of cell structure, such as the shape of the chloroplasts or the presence of pyrenoids or eye spots.
  • Today, a sixth characteristic is added:
  • The molecular sequences of their DNA and RNA.

Data obtained by using this newest approach are shaking the algal phylogenetic tree and revising many long-held ideas of relationships within the algae. Newly acquired data, however, have not changed the belief that members of the green algae orders Coleochaetales and Charales of the Class Charophyceae are the closest living relatives of the plants.

The green algae are green because they contain chlorophylls a and b and carotenoids in the same proportions as the green plants. Like the green plants, they store starch inside the plastids and most have cellulose cell walls. Some have reproductive cells that are structured with two whiplash flagella—like green plant sperm cells. These common features led biologists in the past to the green algae as the protistan ancestors of green plants—if one defines “plants” as “land plants” (embryophytes). With today's available data, some biologists say that algae are green plants of the aquatic kind and suggest a general category of Chlorobiota to include all green plants, aquatic and terrestrial. There are strong advocates for both approaches, but no clear majority for one over the other.