''Actidesmium'' is a small

genus Genus (; : genera ) is a taxonomic rank above species and below family (taxonomy), family as used in the biological classification of extant taxon, living and fossil organisms as well as Virus classification#ICTV classification, viruses. In bino ...

green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...

in the family

Characiaceae Characiaceae is a family of green algae in the order Sphaeropleales. It contains epiphytic or planktonic algae that are unicellular or colonial. The cells are heteropolar, with basal and apical ends having different shapes. The daughter cells are ...

. It was described by the German phycologist Paul Friedrich Reinsch in 1875. ''Actidesmium'' consists of microscopic colonies of numerous cells. Cells are in clusters of four, eight, or 16 at the end of long gelatinous strands, which are the remains of the old cell wall. One

chloroplast A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...

is present, lacking a

pyrenoid Pyrenoids are sub-cellular phase-separated micro-compartments found in chloroplasts of many algae,Giordano, M., Beardall, J., & Raven, J. A. (2005). CO2 concentrating mechanisms in algae: mechanisms, environmental modulation, and evolution. ''An ...

. ''Actidesmium'' reproduces by

zoospore A zoospore is a motile asexual spore that uses a flagellum for locomotion in aqueous or moist environments. Also called a swarm spore, these spores are created by some protists, bacteria, and fungi to propagate themselves. Certain zoospores are ...

s formed by successive division. Zoospores each have two

flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...

, and are released at the apex of the cell. After release, the mother cell gelatinizes and the zoospores mature into new vegetative cells. Two species are known: '' Actidesmium hookeri'' and '' Actidesmium sphaericum'' (formerly known as ''Actidesmium globosum''). The two differ in the size and shape of their cells, the latter being smaller and globose. ''Actidesmium'' has been reported from freshwater habitats in

North America North America is a continent in the Northern Hemisphere, Northern and Western Hemisphere, Western hemispheres. North America is bordered to the north by the Arctic Ocean, to the east by the Atlantic Ocean, to the southeast by South Ameri ...

and

Europe Europe is a continent located entirely in the Northern Hemisphere and mostly in the Eastern Hemisphere. It is bordered by the Arctic Ocean to the north, the Atlantic Ocean to the west, the Mediterranean Sea to the south, and Asia to the east ...

, and is rare.

References

Sphaeropleales Sphaeropleales genera Freshwater algae {{Chlorophyceae-stub