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The algae comprise those living things which are plant-like but are not actually true plants. They do not form a true taxonomic group, but the label is a handy description for various forms. Traditionally they are distinguished from bacteria and protozoa mainly in that they are autotrophic and produce food by photosynthesis.

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. These are among the first living things in the fossil record, dating back to c. 3800 million years ago.

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:

  • Colonial - small, regular groups of motile cells
  • Coccoid - individual non-motile cells
  • Palmelloid - non-motile cells in a gelatinous matrix
  • Filamentous - a string of non-motile cells connected together, sometimes branching

In three lines - the brown algae (Phaeophyta), red algae (Rhodophyta), and green algae - even higher levels of organization have developed, leading to organisms with tissue differentiation, and the case of the last to land plants. None of them have sex organs that have protective cell layers, though, as land plants do. The largest algae belong here, among the brown algae, which may reach 70 m in length.

The following groups are generally considered algal, or at least are primarily so, even though they may have a few colorless forms:

The study of algae is called phycology.