Desmidiales, commonly called the desmids (Gr.desmos, bond or chain), are an order in the Charophyta, a division of green algae in which the land plants (Embryophyta) emerged.[2] Desmids consist of single-celled (sometimes filamentous or colonial) microscopic green algae. Because desmids are highly symmetrical, attractive, and come in a diversity of forms, they are popular subjects for microscopists, both amateur and professional.[3]
The desmids belong to the class Zygnematophyceae. Although they are sometimes grouped together as a single family Desmidiaceae,[4] most classifications recognize three to five families, usually within their own order, Desmidiales.[5]
The Desmidiales comprise around 40 genera and 5,000[6] to 6,000[7]species, found mostly but not exclusively in fresh water. In general, desmids prefer acidic waters (pH between 4.8 and 7.0), so many species may be found in the fissures between patches of sphagnum moss in marshes.[3] As desmids are sensitive to changes in their environments, they are useful as bioindicators for water and habitat quality.[8]
Nomenclature
The term "desmid" typically refers to a group of microscopic, mostly single-celled algae in the class Zygnematophyceae. Within the desmids, a distinction is typically made between "saccoderm" and "placoderm" desmids. Saccoderm desmids, corresponding to the family Mesotaeniaceae in the order Zygnematales, consist of cells that are unconstricted at the middle, lack median suture lines, and do not have mucilage-secreting pores in the cell wall. Meanwhile, placoderm desmids, corresponding to the order Desmidiales, consist of cells with two symmetrical halves, and mucilage-secreting pores in the cell wall.[9] Here, the term "desmids" and "placoderm desmids" will be used interchangeably to refer to the Desmidiales.
The structure of these algae is unicellular, and lacks flagella. Although most desmid species are unicellular, some genera form chains of cells, called filaments. A few genera form non-filamentous colonies, with individual cells connected by threads or remnants of parent cell walls.[10]
The cell of a desmid is often divided into two symmetricalcompartments separated by a narrow bridge or isthmus, wherein the spherical nucleus is located. Each semi-cell houses a large, often folded chloroplast for photosynthesizing. One or more pyrenoids can be found. These form carbohydrates for energy storage. The cell-wall, of two halves (termed semicells), which, in a few species of Closterium and Penium, are of more than one piece, has two distinct layers, the inner composed mainly of cellulose, the outer is stronger and thicker, often furnished with spines, granules, warts et cetera. It is made up of a base of cellulose impregnated with other substances including iron compounds, which are especially prominent in some species of Closterium and Penium and is not soluble in an ammoniacal solution of copper oxide.
Desmids assume a variety of highly symmetrical and generally attractive shapes, among those elongated, star-shaped and rotund configurations, which provide the basis for their classification.[11] The largest among them may be visible to the unaided eye.[7]
Desmids possess characteristic crystals of barium sulphate at either end of the cell[12] which exhibit continuous Brownian motion. The function of these crystals is completely unknown.[13]
Many desmids also secrete translucent, gelatinous mucilage from pores in the cell wall that acts as a protecting agent. These pores are either, as in Micrasterias, uniformly distributed across the cell-wall but always appear to be absent in the region of the isthmus, or, in highly ornamented forms, as many genera of Cosmarium, grouped symmetrically around the bases of the spines, warts and so on with which the cell is provided.
In the inner layer of the wall the pore is a simple canal, but in the outer, except in Closterium, the canal is surrounded by a specially differentiated cylindrical zone, not composed of cellulose, through which the canal passes. This is termed the pore-organ. The canals are no doubt in all cases occupied by threads of mucilage in process of excretion. At the inner surface of the wall they terminate in lens- or button-shaped swellings, while from the outer end of the pore-organ there sometimes arise delicate radiating or club-shaped masses of mucilage through which the canal passes and which appear to be more or less permanent in character. In most cases, however, these are absent or only represented by small perforated buttons.
Reproduction
Desmids most commonly reproduce by asexual fission. During cell division, the two halves of a cell separate, and each half develops into a new cell. After division, a cell may be asymmetric since the recently formed half is smaller than the original half.[9]
In adverse conditions, desmids may reproduce sexually through a process of conjugation, which are also found among other closely related taxa in the Zygnematophyceae.[14] Sexual reproduction is rare, and many species have never been observed sexually reproducing.[10]
Classification
Classification of the families and genera in the Desmidiales:[1]
The family Gonatozygaceae is sometimes included within the Peniaceae, reducing the number of families from four to three.[5] A fifth family Mesotaeniaceae was formerly included in the Desmidiales,[15] but analysis of cell wall structure and DNA sequences show that the group is more closely related to the Zygnemataceae, and so is now placed together with that family in the order Zygnematales.[5] However, the Zygnemataceae may have emerged in the Mesotaeniaceae.[16]
Desmids are found in freshwater habitats all over the world, but strongly prefer bogs, mires, and other nutrient-poor wetlands.[3] They generally have strict ecological requirements: most species prefer waters with low amounts of dissolved calcium and magnesium, low salinity levels, and somewhat acidic pH.[9][3] In waters with higher amounts of nutrients, desmids rapidly become outcompeted.[8] Desmid species are generally found attached to aquatic vegetation, such as Utricularia, or tychoplanktonic; that is, free-floating in the water column after being disturbed.[10]
Although the Desmidiales are cosmopolitan, a number of species appear to be restricted to continents or biogeographical realms; this is likely because desmids have strict ecological requirements and do not produce resting spores, making successful dispersal less likely.[17] Therefore, they can be grouped into several regions each with their own characteristic desmid floras. The Indo-Malayan to North Australian realm, for example, is characterized by species such as Micrasterias ceratofera, while equatorial Africa is characterized by species such as Allorgeia incredibilis.[17]
Ecology
Although desmids are incredibly diverse, with up to hundreds of them being found in a single site,[9] their interactions with the environment are relatively unknown.[7] Desmids are host to a wide array of parasites, particularly fungal parasites called chytrids.[7] They are also grazed by microscopic aquatic heterotrophs, such as crustaceans, rotifers, and ciliates.[7]
Related Research Articles
Chlorophyta is a taxon of green algae informally called chlorophytes. The name is used in two very different senses, so care is needed to determine the use by a particular author. In older classification systems, it is a highly paraphyletic group of all the green algae within the green plants (Viridiplantae) and thus includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. In newer classifications, it is the sister clade of the streptophytes/charophytes. The clade Streptophyta consists of the Charophyta in which the Embryophyta emerged. In this latter sense the Chlorophyta includes only about 4,300 species. About 90% of all known species live in freshwater. Like the land plants, green algae contain chlorophyll a and chlorophyll b and store food as starch in their plastids.
The Zygnematales, also called the Conjugatales, are an order of green algae, comprising several thousand different species in two families. The larger family Zygnemataceae, with well-known genera such as Zygnema and Spirogyra, includes members that grow as unbranched filaments, which grow longer through normal cell division. This group includes the desmids. Most members of both families live in freshwater, and form an important component of the algal scum that grows on or near plants, rocks, and various debris.
Streptophyta, informally the streptophytes, is a clade of plants. The composition of the clade varies considerably between authors, but the definition employed here includes land plants and all green algae except the Chlorophyta and the more basal Prasinodermophyta.
Charophyta is a group of freshwater green algae, called charophytes, sometimes treated as a division, yet also as a superdivision or an unranked clade. The terrestrial plants, the Embryophyta emerged deep within Charophyta, possibly from terrestrial unicellular charophytes, with the class Zygnematophyceae as a sister group.
The Zygnemataceae are a family of filamentous or unicellular, uniseriate (unbranched) green algae. The filaments are septated and reproduction is by conjugation; Spirogyra is commonly used in schools to demonstrate this kind of reproduction. The family is notable for its diversely shaped chloroplasts, such as stellate in Zygnema, helical in Spirogyra, and flat in Mougeotia. The Zygnemataceae are cosmopolitan, but though all generally occur in the same types of habitats, Mougeotia, Spirogyra, and Zygnema are by far the most common; in one study across North America, 95% of the Zygnemataceae collected were in these three genera. Classification and identification is primarily by the morphology of the conjugation, which is somewhat rare to find in natural populations of permanent water bodies; when in the vegetative state, the rarer genera resemble the three most common, and are often mistaken for them and catalogued as such. Conjugation can be induced in low-nitrogen culture. While they occupy many habitats, in North America all are found solely in freshwater or subaerial habitats. Species typically exist as floating mats in stagnant water in ditches and ponds, but some also grow in moving water, attaching themselves to a substrate by rhizoid-like projections of the basal cells of the filament. The mat species rise to the surface in early spring, grow rapidly through the summer, disappearing by late summer. Members of the Zygnemataceae, such as Spirogyra, fall prey to parasites, especially chytrids. Most genera previously assigned to Mesotaeniaceae as well as the Desmidiales actually emerged in the Zygnematacae.
Micrasterias is a unicellular green alga of the order Desmidiales. Its species vary in size reaching up to hundreds of microns.
The Closteriaceae are one of four families of Charophyte green algae in the order Desmidiales (desmids). It contains two genera, Closterium and Spinoclosterium.
The Desmidiaceae are one of four families of charophyte green algae in the order Desmidiales (desmids).
The Gonatozygaceae are one of four families of Charophyte green algae in the order Desmidiales (desmids).
Closterium is a genus of desmid, a group of charophyte green algae. It is placed in the family Closteriaceae. Species of Closterium are a common component of freshwater microalgae flora worldwide.
Cosmarium is a genus of freshwater organisms belonging to the Charophyta, a division of green algae from which the land plants (Embryophyta) emerged.
Desmidium is a genus of green algae, specifically of the Desmidiaceae.
Spinoclosterium is a genus of green algae, specifically of the Closteriaceae. It is rare, but widely distributed in freshwater regions throughout the world.
Staurodesmus is a genus of green algae, specifically of the Desmidiaceae.
Triploceras is a genus of desmid in the family Desmidiaceae.
Xanthidium is a genus of green algae, specifically of the Desmidiaceae.
Zygnematophyceae is a class of green algae in the paraphylum streptophyte algae, also referred to as Charophyta, consisting of more than 4000 described species. The Zygnematophyceae are the sister clade of the land plants.
The Klebsormidiaceae are a family containing five genera of charophyte green alga forming multicellular, non-branching filaments. The genus Chlorokybus was previously included as well, but this problematic and poorly known genus is now placed in a separate class Chlorokybophyceae.
Spirotaenia is a genus of basal unicellular green algae that may be sister to the Chlorokybophyceae. It was previously considered to be part of the Zygnemataceae. It is sexually conjugating, a mode of reproduction that was previously only known in the Zygnemataceae/Mesotaeniaceae, the sister groups to the land plants. This is surprising, as Spirotaenia is much more basal. The conjugating process is substantially aberrant. Spirotaenia may actually be more than one distinct lineage which may not be closely related.
Prescottiella is a genus of green algae in the family Desmidiaceae, containing the single species Prescottiella sudanensis. Originally classified as Micrasterias sudanensis in 1958, it was moved into its own genus by Carlos E. M. Bicudo in 1976, due to its asymmetric character. It is named after Gerald Webber Prescott, an American phycologist.
1 2 3 Brook, A. J.; Williamson, D. B. (2010). A Monograph on Some British Desmids. London: The Ray Society. ISBN978-0-903874-42-7.
↑ Van den Hoek, C., D. G. Mann, & H. M. Jahns, 1995. Algae:An Introduction to Phycology, page 468. (Cambridge: Cambridge University Press). ISBN0-521-30419-9
1 2 3 4 5 Brook, Alan J., 1981. The Biology of Desmids, page 1. (Berkeley: University of California Press). ISBN0-520-04281-6
1 2 Coesel, Peter F.M. (2001). "The significance of desmids as indicators of the trophic status of freshwaters". Biodiversity & Conservation. 10: 177–187.
1 2 3 Hall, John D.; McCourt, Richard M. (2014). "Chapter 9. Conjugating Green Algae Including Desmids". In Wehr, John D.; Sheath, Robert G.; Kociolek, J. Patrick (eds.). Freshwater Algae of North America: Ecology and Classification (2ed.). Elsevier Inc. ISBN978-0-12-385876-4.
↑ van Egmond, Wim. "Desmids". Micscape Microscopy and Microscope Magazine.
Survey of Clare Island 1990 - 2005, noting the Desmidiales recorded. Ed. Guiry, M.D., John, D.M., Rindi, F. and McCarthy, T.K. 2007. New Survey of Clare Island. Volume 6: The Freshwater and Terrestrial Algae. Royal Irish Academy. ISBN978-1-904890-31-7
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