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Planctonema
Planctonema is a genus of green algae in the class Trebouxiophyceae.[2] It is found in freshwater habitats as plankton,[1] and is found worldwide. It is dominant in lakes during the summer and autumn months.[3] The classification of Planctonema has varied historically. It was originally described by Wilhelm Schmidle as a heterokont, and has also been placed in the family Ulotrichaceae. Molecular phylogenetic evidence has shown that Planctonema is closely related to members of the family Oocystaceae,[3] and AlgaeBase places Planctonema in Oocystaceae.[1] However, it lacks the cell wall ultrastructure characteristic of the family.[3] DescriptionPlanctonema consists of short, uniseriate filaments surrounded in a common mucilaginous sheath. Individual cells. Individual cells are spaced apart equidistantly, or end-to-end in separate pairs; the space between cells is filled with mucilage. Cells are cylindrical, with a parietal, plate-like chloroplast without pyrenoids or starch. The nucleus is located in the median constriction of the chloroplast. At the ends of the cell poles are vacuoles containing conspicuous oil globules.[1] Vegetative reproduction occurs by the fragmentation of filaments. Sexual reproduction is unknown.[1] Similar genera include Psephonema (formerly considered to be synonymous with Planctonema), and Planctonemopsis, described in 2017. Psephonema has shorter cells than Planctonema (with a length–width ratio of 1.5–2, while Planctonemopsis has chloroplasts with pyrenoids).[3] References
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