Etymology
A labyrinth is a synonym of a maze, reflecting the ectoplasmic net, which gives the protist a net-like shape which resembles a maze. The word comes from Ancient Greek λαβύρινθος (labúrinthos, “maze”).History of Knowledge
Cienkowski was the first person who published on ''Labyrinthula'' in 1867. He studied primarily the morphology, and he identified two species: ''Labyrinthula vitelli'' and ''Labyrinthula macrocystis''. In 1967, Pokorny published the first review over ''Labyrinthula'' and counted ten marine species, two freshwater species and one terrestrial species. Before being considered as protists and Stramenopiles, ''Labyrinthula'' had been classified in the Mycetozoa, the slime molds and the Rhizopodia. Interest in ''Labyrinthula'' started when it was defined as the cause of the "wasting disease". An epidemic happened in the 1930s in the North American and European coasts, killing over 90% of the seagrass (''Habitat and Ecology
The habitat of ''Labyrinthula'' is very diverse, they are found in marine and freshwater environments as well as in terrestrial environments all over the world. ''Labyrinthula'' have been found to live as pathogens, commensals or mutualists on several host organisms. One example is the endosymbiont ''Labyrinthula sp.'' inside ''Thecamoeba hilla'', which seem to live as mutualists symbionts.Description of the Organism
Morphology and Anatomy
The morphology of a single cell of ''Labyrinthula'' is not unique and varies a lot between the different species. The cells can be spindle-shaped like ''L. macrocystis'', spherical or ovoid to name only a few examples. The spindle shape is due to the microtubules of the cytoskeleton. There is no mean size of one cell, because of the diversity between the species. An approximation can still be made at 8-30 γm in length and 1.5-8 γm in width. The general morphology of the cell contain one central nucleus, the smooth endoplasmatic reticulum, mitochondria with tubular cristae, numerous lipid-composed granules and two large Golgi complexes. ''Labyrinthula'' are non-photosynthetic and are usually translucent white, but some yellow strains have been observed.Ectoplasmic Net
The defining characteristic of the genera ''Labyrinthula'' is the formation of an ectoplasmic net around the cells and embedding the whole colony. The ectoplasmic net is secreted and attached to the cell by specialized organelles called segenetosome or bothrosomes. A bothrosome is an electron-opaque organelle, which prevents the leaking of the organelles into the net. The etymology of "bothrosome" and "sagenetosome" originated from ''Cell Wall
Despite the lack of cell wall of the ectoplasmic net, each individual cell is surrounded by a cell wall located close to the cell membrane and composed of a single layer of Golgi-derived circular scales, which overlap over a few nanometers, but do not fuse. The main components of the cell wall are fucose or galactose-derived substances.Life Cycle
The reproduction of ''Labyrinthula'' occurs by zoosporulation, no sexual reproductive cells or structures have been identified yet. First, vegetative cells aggregate inside the ectoplasmic net and form yellow to orange networks in different areas of the net. The cells in these aggregates are compressed together and the boundaries blur, which leads to a plasmodium-like appearance. The vegetative cells then round up and enlarge to form a presporangium. A presporangium is delimited by an envelope of scales, which build a rather thick membrane. The presporangium is further divided into sporangia, and in some species into a sori, until each of them contain eight zoospores. The zoospores are released directly into the ectoplasmic net, where they swim away from the sporangium. The morphology of the zoospore is similar to a "standard" stramenopile cell. Zoospores contain an eyespot and two flagella. Only the anterior flagellum is covered with mastigonemes and the posterior flagellum propels the cell through the ectoplasmic net. Both flagella are inserted laterally and medially. After about 24 hours, the zoospores lose their flagella and round up to finally differentiate into vegetative spindle cells. Vegetative cells stop gliding through the net in older cultures or under bad conditions. Only few of the older cells autolyze, but most of them round up. Vegetative cells multiply mostly by mitosis. Some features of their binary fission are the ''de novo'' synthesis of the bothrosome and the cytokinesis, that occurs by vesicle accumulation and fusion.Practical Importance
''Labyrinthula'' have awoken the interest of scientists by being the cause of the "wasting disease" of the seagrass on the North American and European coasts in the 1930s. Since then, several pathogenic species have been identified which mostly live in marine water. Studies testing the virulence of the protist in seagrasses showed a low virulence of ''Labyrinthula''. The protist often lives inside the multicellular organism, but does not initiate any pathogenic event. Pathogenesis may be triggered by biotic or abiotic stress of the higher organism, like a large increase in temperature or a decrease in light exposure for example. The most famous parasiticList of Species
References
External links
* https://microbewiki.kenyon.edu/index.php/Labyrinthula * http://www.mycobank.org/ {{Taxonbar, from=Q6467745 Heterokont genera