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Chlorarachniophyte

Chlorarachniophytes are a small group of algae occasionally found in tropical oceans. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally they have the form of small amoeboids, with branching cytoplasmic extensions that capture prey and connect the cells together, forming a net. They may also form flagellate zoospores, which characteristically have a single subapical flagellum that spirals backwards around the cell body, and walled coccoid cells.

The chloroplasts were presumably acquired by ingesting some green alga. They are surrounded by four membranes, the outermost of which is continuous with the endoplasmic reticulum, and contain a small nucleomorph between the middle two, which is a remnant of the alga's nucleus. This contains a small amount of DNA and divides without forming a mitotic spindle. The origin of the chloroplasts from green algae is supported by their pigmentation, which includes chlorophylls a and b, and by genetic similarities. The only other group of algae that contain nucleomorphs are the cryptomonads, but their chloroplasts seem to be derived from a red alga.

The chlorarachniophytes only include four genera: Chlorarachnion, Gymnochlora, Lotharella, and Cryptochlora. These show some variation in their life-cycles and may lack one or two of the stages described above. They have mitochondria with tubular cristae, and a few other such groups have been considered possible relatives, notably the cercomonads[?]. The euglenids have also been considered possible relatives, but this is based entirely on the presence of secondary green chloroplasts and is contradicted by structural and genetic studies.



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