Pediastrum is a genus of green algae, in the family Hydrodictyaceae. [1] It is a photoautotrophic, nonmotile coenobial green alga that inhabits freshwater environments. The name Pediastrum comes from the Greek root words pedion, meaning "plane", and astron, meaning "star", referring to its overall shape. [2]
Pediastrum develops colonies with a fixed number of cells, termed coenobia. In this case, the coenobia are composed of between 22 and 27 cells, which are orderly arranged in a flat disk. The diameter of a single coenobium ranges from 20 to 80 μm, making them microalgae. Cells in Pediastrum are dimorphic, consisting of interior cells and peripheral cells, distinguished by their position in the colony and by their shape. Some species have inter-cellular spaces between their interior cells. The peripheral cells surround the interior cells, and they usually possess bristles, V-like cutting edges, or wavy projections.
Pediastrum shows lots of morphological diversity, both between and within species. Although most species produce flat, circular coenobia, others produce coenobia that are more oval-shaped, or curved and bowl-shaped. For species identification, the shape of the cells (particularly marginal ones), presence or absence of intercellular gaps, and ornamentation of the cell wall are all important distinguishing characteristics. [3]
The genus of Pediastrum belongs to the Hydrodictyaceae family, along with Pseudopediastrum , Tetraedron, Hydrodictyon and so on. Pediastrum spp. on the phylogenetic tree can be divided into Group I, Group II, Group III. [4] Group I, which includes Pediastrum duplex, are mainly distributed in North America and Europe. Group II is mostly from Australia, containing among others P. angulosum and P. alternans. Group I, Group II form a monophyletic group (clade) in the phylogenetic tree together with Hydrodictyon spp. lastly, Group III forms a sister group with Monactinus spp.
For the morphological characteristics of these groups, Group I has intracellular spaces and V-like incisions in the cells along the outside, while some species in Group II lack intercellular spaces, and their peripheral cells are wavy. Although there are significant differences between the appearance of Pediastrum and Hydrodictyon , it is still related more closely to it than several genera that look more alike, such as Pseudopediastrum , Monactinus and Stauridium .
Cell walls of Pediastrum form a “globular network” with interconnected nodes. [5] These walls are difficult to break down due to the presence of sporopollenin [3] and silica, [6] which the algae uses to defend itself against infection. This also coincidentally makes it very resistant to diagenesis. Because of this, Pediastrum is known to have existed since the Early Cretaceous. [7] In 2005, several studies on the fossil record of Pediastrum were compiled, and 10 species in the genus are now recognized between the Late Cretaceous and the Quaternary in southern South America alone. [8]
The genus Pediastrum is present all around the world, and they are usually found in sediments of freshwater lakes or wetlands. The strong cell walls of Pediastrum make them preserve very well in sediments. The cell clusters retain their morphology, some organelles inside, and genetic materials for thousands of years. The cell walls can survive the harsh chemical treatments used to prepare samples in palynology, allowing them to still be identified to a species level in pollen samples, which is uncommon in that field of study. These special characteristics make Pediastrum useful as a bioindicator. Therefore, Pediastrum is very useful in the fields of paleoecology and paleolimnology. [9] [10] For example, Pediastrum is absent or decreased when less aquatic plants are present, allowing one to determine the health of an aquatic ecosystem even when no direct information about aquatic plant life is present in a sample. Dissolved organic carbon (DOC) and pH value are the main environmental variables that affect the distribution of Pediastrum. Because of this, sediment-preserved Pediastrum spp. helps correlate community changes and nutrient availability, and can be a useful indicator for the reconstruction of paleoenvironments. [11]
Pediastrum possesses a haplontic life cycle, and can be divided into three types. [12] The most commonly seen is the first kind of asexual life cycle (ALC1), which generates autocolonies. In ALC1 the zoospores aggregate into an orderly flat disk, with concentric rings (central cell, six cells, and then nine cells.) Afterwards,the peripheral cells grow in one or two spines from the external wall. The second asexual life cycle (ALC2,) is a single celled version of the former. The vesicle that contains the zoospores breaks down immediately after formation, and individual cells grow until maturity, forming motile zoospores. Each cell can then go on to generate a daughter colony with exactly the same number and arrangement of cells as the parent colony. As for the Sexual life cycle in Pediastrum, the cell forms two bi-flagellate gametes (isogamy), these then fuse into a zygote and develop further through the asexual lifecycle as described before.
The second asexual and sexual life cycles are rarely observed in Pediastrum, and only occur in very bright conditions or in high temperatures.
The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. They are usually green due to the dominance of pigments chlorophyll a and chlorophyll b. The chloroplast may be discoid, plate-like, reticulate, cup-shaped, spiral- or ribbon-shaped in different species. Most of the members have one or more storage bodies called pyrenoids located in the chloroplast. Pyrenoids contain protein besides starch. Some green algae may store food in the form of oil droplets. They usually have a cell wall made up of an inner layer of cellulose and outer layer of pectose.
Chlorophyta is a division of green algae informally called chlorophytes.
Sphaeropleales is an order of green algae that used to be called Chlorococcales. The order includes some of the most common freshwater planktonic algae such as Scenedesmus and Pediastrum. The Sphaeropleales includes vegetatively non-motile unicellular, colonial, or filamentous taxa. They have biflagellate zoospores with flagella that are directly opposed in direction : Sphaeroplea, Atractomorpha, Neochloris, Hydrodictyon, and Pediastrum. All of these taxa have basal body core connections. Motile cells generally lack cell walls or have only a very fine layer surrounding the cell membrane. Other common characteristics include a robust vegetative cell wall, cup-shaped chloroplasts with large pyrenoids, and relatively large nuclei.
The water net is a taxon of freshwater green algae in the family Hydrodictyaceae. Hydrodictyon does well in clean, eutrophic water, and has become a nuisance in New Zealand, where it has been recently introduced. The name water net comes from the mesh structure of their colonies, which can extend several decimeters; the scientific name also means "water net" in Greek.
Hydrodictyaceae is a family of green algae in the order Sphaeropleales. They are found in freshwater habitats worldwide.
Coelastrum is a genus of green algae in the Scenedesmaceae family. It is a common component of the phytoplankton in freshwater habitats such as ponds, lakes, waterfalls, and temporary pools of water, particularly eutrophic ones. The genus has a more or less cosmopolitan distribution, although some species appear to have more restricted geographical distributions.
Neochloridaceae is a family of green algae in the order Sphaeropleales.
Asterococcus is a genus of green algae in the order Chlamydomonadales. It is planktonic in freshwater ponds and lakes, or benthic within mires and swamps. It is a common and widespread genus, but is rarely abundant.
Atractomorpha is a genus in the Sphaeropleaceae, a family of green algae. The genus name is derived from Greek and means "spindle-shaped" or "arrow-shaped", and refers to the shape of the cells.
Enallax is a genus of green algae in the family Scenedesmaceae. It is found in freshwater habitats, such as peat bogs or wet rocks.
Lagerheimia is a genus of green algae in the family Oocystaceae. It is commonly found in freshwater habitats all over the world, although some species are rare and have only been recorded from Europe or the United States.
Monactinus is a genus of green algae in the family Hydrodictyaceae. It is very common in freshwater regions throughout the world.
Parapediastrum is a genus of green algae in the family Hydrodictyaceae. It is fairly common in freshwater regions throughout the world.
Pseudopediastrum is a genus of green algae in the family Hydrodictyaceae. It is very common in freshwater regions throughout the world.
Sorastrum is a genus of green algae in the family Hydrodictyaceae. It is a component of the phytoplankton of freshwater ponds, lakes, and ditches. Sorastrum is common in tropical to temperate regions of the world, but due to its small size it is often overlooked.
Stauridium is a genus of green algae in the family Hydrodictyaceae. It is very common in freshwater regions throughout the world.
Tetraedron is a genus of green algae in the family Hydrodictyaceae. It may also be spelled as Tetraëdron.
Follicularia is a genus of green algae, in the family Schizochlamydaceae. It is found in terrestrial habitats, mainly soil.
Korshikoviella is a genus of green algae in the family Characiaceae.
Pediastrum duplex is a species of fresh water green algae in the genus Pediastrum. It is the type species of the genus Pediastrum.
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