A few algae are prokaryotic, and belong among the bacteria. The most notably of these are the blue-green algae, or cyanobacteria?. The Prochlorophyta? are similar but have different pigmentation.
The remaining algae are eukaryotes with chloroplasts?. These are symbionts that perform photosynthesis, and have been incorporated several times into different lineages, originally from the prokaryotic Cyanobacteria? and related groups, but later from other algae as well. The pigmentation and membrane structure of the chloroplasts are useful for tracing their origin.
Most of the simpler algae are flagellates, but there is a tendency towards non-motile forms in most of the groups, simply because the cells don't need to find food anymore. Some of the more common organizational levels, more than one of which may occur in the life cycle of an alga, are:
The following groups are generally considered algal, or at least are primarily so, even though they may have a few colorless forms: