| Chlorophyceae | |
|---|---|
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| Light micrograph ofPediastrum sp. | |
Scientific classification | |
| Kingdom: | Plantae |
| Division: | Chlorophyta |
| Subphylum: | Chlorophytina |
| Class: | Chlorophyceae WilleinWarming, 1884[1] |
| Orders | |
TheChlorophyceae, also known aschlorophycean algae, are one of the classes ofgreen algae, within the phylumChlorophyta. They are a large assemblage of mostly freshwater and terrestrial organisms; many members are important primary producers in the ecosystems they inhabit. Their body plans are diverse and range from single flagellated or non-flagellated cells to colonies or filaments of cells.[2] The class Chlorophyceae has been distinguished on the basis of ultrastructural morphology;[3] molecular traits are also being used to classify taxa within the class.[2]
Chlorophycean algae areeukaryotic organisms composed ofcells which occur in a variety of forms. Depending on the species, Chlorophyceae can grow unicellular (e.g.Chlamydomonas), colonial (e.g.Volvox), coenocytic (e.g.Characiosiphon), or filamentous (e.g.Chaetophora). In their vegetative state, some members haveflagella while others produce them only in reproductive stages; still others never produce flagella.[2]
Chlorophycean algae havechloroplasts and nearly all members arephotosynthetic. There are a few exceptions, such asPolytoma, which have plastids that have lost the ability to photosynthesize.[4] They are usually green due to the presence ofchlorophylla andb; they can also contain the pigmentbeta-carotene. Chloroplasts are diverse in morphology and include many forms, including, cup-shaped (e.g.Chlamydomonas), or axial, or parietal and reticulate (e.g.Oedogonium).[2]
In many species, there may be one or more storage bodies calledpyrenoids (central proteinaceous body covered with a starch sheath) that are localised around the chloroplast.[5] Some algae may also store food in the form of oil droplets.[6] The inner cell wall layer is made of cellulose and the outer layer of pectose.
Cells of Chlorophyceae usually have two or fourflagella, but in some cases may have numerous flagella. The flagella emerge from the apex of the cell, and are connected to the nucleus viarhizoplasts.[2] The arrangement of flagella may be in one of two configurations, termed CW ("clockwise") or DO ("directly opposed"). In the CW configuration, thebasal bodies are arranged clockwise in the 1–7 o'clock position. In the DO configuration, the basal bodies are arranged in 12–6 o'clock. Taxa with the CW arrangement and DO arrangement correspond to two differentclades, roughly corresponding to the ordersChlamydomonadales andSphaeropleales, respectively.[7]
A combination ofultrastructural features are characteristic of the Chlorophyceae. These include: closedmitosis, thetelophase spindle collapsing beforecytokinesis, and a system ofmicrotubules called a phycoplast running parallel to the plane of cytokinesis.[2]
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Chlorophyceae can reproduce both asexually and sexually. Inasexual reproduction, cells may produceautospores,aplanospores orzoospores. Autospores (by definition) lack flagella and appear as smaller versions of vegetative cells. Zoospores typically have an elongate, hydrodynamic shape and often haveeyespots. Aplanospores are similar to zoospores in that they have characteristics typical of zoospores (such ascontractile vacuoles), but lack flagella.[8]
In addition to normal asexual reproduction, some genera such asChlamydomonas andDunaliella can go through a temporary phase known as the "palmella stage", in which flagella are absent and the cells divide vegetatively within a common mucilaginous envelope.[9] Algae enter the palmella stage in response to stressful conditions, such as changes in salinity or predation.[10] Additionally,Haematococcus produces resistant stages with thickcell walls, termedakinetes.[11]
Sexual reproduction shows considerable variation in the type and formation of sex cells; it may beisogamous (with two morphologically identical gamete types),anisogamous (with two morphologically distinct gamete types), andoogamous (with larger, nonmotile eggs and smaller motile sperm cells). Members of Chlorophyceae that undergo sexual reproduction have a zygotic life cycle, in which thezygotes are the onlydiploid stages. Zygotes may have thick and/or spiny cell walls; these are called hypnozygotes and they also function as resting stages.[2]
They share many similarities with higher plants, including the presence of asymmetrical flagellated cells, the breakdown of the nuclear envelope at mitosis, and the presence of phytochromes, flavonoids, and the chemical precursors to the cuticle.[12] However, unlike higher plants they do not go through a multicellularalternation of generations.[2]
The current taxonomy of algae is based onmolecular phylogenetics. Older classifications are simpler and more morphologically aligned; however, these classifications are recognized as artificial due to the extensivemorphological convergence present within the class (and more broadly within algae).[2] In even older, historical classifications, the term Chlorophyceae is sometimes used to apply to all the green algae except theCharales, and the internal division is considerably different.[citation needed]
As of May 2023[update],AlgaeBase accepted the following orders in the class Chlorophyceae:
Along with these genera, AlgaeBase recognizes several taxa that areincertae sedis (i.e. unplaced to an order):
Other orders that have been recognized include:
Current thinking of phylogenetic relationships are as follows:[17]
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