Pediastrum

''Pediastrum'' is a genus of green algae, in the family Hydrodictyaceae. It is a photoautotrophic, nonmotile coenobial (fixed number of cells) green algae that inhabits freshwater environments.

Morphology

''Pediastrum'' is a genus usually look like a green star in the pond or lake. They are coenobial green algae composed of 2² to 2⁷ cells, which are orderly arranged in a circular, flat disk. The diameter of a single coenobium can range from 20 to 80 μm, so they belong to the group of microalgae. Cells in ''Pediastrum'' can be divided into interior cells and peripheral cells by position in the coenobium or their shapes. Some of the species have intracellular cell spaces between interior cells, while some would closely aggregate together. The peripheral cells surround outside the interior cells, and they usually possess bristles, V-liked cutting edges, or wavy projections.

Phylogeny

The genus of ''Pediastrum'' belongs to the Hydrodictyaceae family, along with '' Psuedopediastrum'', '' Tetraedron, Hydrodictyon'' and so on. ''Pediastrum'' spp. on the phylogenetic tree could be divided into Group I, Group II, Group III. Group I includes ''P. duplex'' which are mainly distributed in North America and Europe. Group II consists of ''P. duplex'' mostly from Australia, ''P. angulosum'', ''P. alternans'' etc. Group I, Group II and ''Hydrodictyon'' spp. form a monophyletic group (clade) in the phylogenetic tree. As for the Group III, they form sister group with ''Monactinus'' spp.

For the morphological characteristic of these groups, Group I has intracellular spaces and V-liked incision peripheral cells, 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'', their phylogeny still related closer than genus '' Psuedopediastrum'', '' Monactinus'' and '' Stauridium'', which all look more similar to ''Pediastrum''. This indicates that morphology of green algae (Chlorophyceae) are various. And if we classified green algae by their morphological differences may not coincide with their placement on phylogenetic tree.
File:Pediastrum (cropped).jpg, ''Pediastrum''
File:Stauridium tetras 12083611.jpg, ''Stauridium tetras''
File:Tetraedron EPA.jpg, ''Tetraedron''
File:Pseudopediastrum boryanum as Pediastrum boryanum.JPG, ''Pseudopediastrum boryanum''
File:Pediastrum simplex EPA.jpg, ''Monactinus simplex''
File:Hydrodictyon reticulatum.jpg, ''Hydrodictyon reticulatum''

Fossil Records

Cell walls of ''Pediastrum'' form a “globular network” with interconnected nodes. These walls are difficult to breakdown due to the presence of silica,providing both defense of infection, and strong resistance to diagenesis. The species of ''Pediastrum'' are known to have existed since Early Cretaceous.In 2005, the studies on fossil records of ''Pediastrum'' are complied, and 10 species in the genus are recognized between the Late Cretaceous and the Quaternary age in southern South America.

Bioindicator

''Pediastrum'' is coenobial green algae which are globally distributed, and they are usually found in sediments of freshwater lakes or wetlands. The strong cell walls of ''Pediastrum'' make them preserve very well in the core of sediments for a long time. Besides, it could retain theirl morphology, some organelles inside, and genetic materials in harsh chemical treatments, allowing them be resolved to species level. These special characteristics lend ''Pediastrum'' potential as bioindicators, which can screen the health of ecosystem in the specific environment. Therefore, ''Pediastrum'' become great materials to study paleoecology and paleolimnology.''Pediastrum'' were absent or decreased when little macrophytes present. Moreover, dissolved organic carbon (DOC) and pH value were main environmental variables affect the distribution of Pediastrum. These sediment-preserved ''Pediastrum'' spp. helped link the community changes and nutrient availability, and could be useful indicators for the reconstruction of paleoenvironments.

Life Cycles

''Pediastrums'' posses a haplontic life cycles, and they can be divided into three types. The most commonly seen is the first kind of asexual life cycle (ALC1), which genergates autocolonies. In ALC1 the zoospores swarmed and the aggregates into flat disk orderly, making them a concentric ring (center, six, nine cells.) Afterwards,the peripheral cells grow one or two spines from external wall, just like a green star in the pond. The second asexual life cycle (ALC2,) this is a single cell version of asexual ALC1. The vesicle contains zoospores breakdown immediately after formation, a single cell grew itself until maturity, and forming motile zoospores. As for the Sexual life cycle in ''Pediastrum'', the reproductive cell would form two gametes with biflagellate (isogamy,) they fused into zygote and developed as ALCs.

Sexual life cycle and ALC2 are rarely observed in Pediastrum, only in some high light and high temperature conditions.

Reproduction

''Pediastrum'' reproduces asexually by producing autocolonies. The protoplast of each parent cell gives rise to a biflagellate zoospore for each cell in the parent colony. The zoospores are freed from the parent cell within a vesicle and then arrange themselves into the cellular arrangement for that particular species. The cells then enlarge until they reach full size. Each cell can generate a daughter autocolony with exactly the same number and arrangement of cells as the parent colony.

''Pediastrum'' reproduces sexually via the fusion of small, biflagellate gametes that are released from the parent cell. The zygotes germinate into zoospores, which turn into thick-walled polyeders that generate the new coenobia. These polyeders can also result from asexual reproduction.

File:Pediastrumboryanum.jpg, ''Pediastrum boryanum''
File:Колония водорослей Pediastrum. 1000х.jpg, Colony of ''Pediastrum'' algae

References

Sphaeropleales genera
Sphaeropleales
Freshwater algae

{{Chlorophyceae-stub