"Algae" is a term that is used for any living beings that have chloroplasts, thus they are able to capture solar light and produce free oxygen, except for the originally terrestrial plants (which descend from a certain group of green algae).

Most unicellular algae have flagella.

By phylogeny, there are several separate kinds of algae, which are not closely related and which have appeared as a consequence of separate symbiosis events.

After diatoms, dinoflagellates are among the most abundant unicellular algae. The chloroplasts of both diatoms and dinoflagellates have their origin in red algae that had been incorporated as intracellular symbionts in a distant past.