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Alga

The algae are those [[Protista]] 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 [[flagellate]]s, 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 ([[Plantae]]) - even higher levels of organization have developed, leading to organisms with tissue differentiation, and the case of the last to land plants.

The following groups are generally considered algal, even though they may have a few colorless forms:
* Basal [[Plantae]] (green algae)
* Most [[Stramenopiles]] (golden algae, brown algae, diatoms, etc)
* [[Haptophytes]]
* [[Dinoflagellates]]
* [[Chlorarchniophytes]]
* [[Rhodophyta]] (red algae)
* [[Cryptomonads]]
* [[Euglenids]]