green alga
The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...
e, 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
A chloroplast () is a type of membrane-bound organelle known as a plastid that conducts photosynthesis mostly in plant and algal cells. The photosynthetic pigment chlorophyll captures the energy from sunlight, converts it, and stores it i ...
spiral
In mathematics, a spiral is a curve which emanates from a point, moving farther away as it revolves around the point.
Helices
Two major definitions of "spiral" in the American Heritage Dictionary are: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
Starch or amylum is a polymeric carbohydrate consisting of numerous glucose units joined by glycosidic bonds. This polysaccharide is produced by most green plants for energy storage. Worldwide, it is the most common carbohydrate in human die ...
. 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
Cellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of β(1→4) linked D-glucose units. Cellulose is an important structural component of the primary cell wa ...
* the body may be unicellular, colonial, filamentous or multicellular.
* They are usually green due to the presence of chlorophyll a, chlorophyll b and beta-carotene.
* The chloroplast may be discoid, cup-shaped (e.g. ''Chlamydomonas''), spiral or ribbon shaped
* Most chlorophytes have one or more storage bodies called pyrenoids (central proteinaceous body covered with a starch sheath) that are localised around the chloroplast.
* The inner cell wall layer is made of cellulose and the outer layer of pectose.
*
Asexual reproduction
Asexual reproduction is a type of reproduction that does not involve the fusion of gametes or change in the number of chromosomes. The offspring that arise by asexual reproduction from either unicellular or multicellular organisms inherit the ...
is by zoospores. They are flagellates produced from the parent cells by mitosis. Also by aplanospores, hypnospores, akinetes, Palmella stage, etc.
*
Sexual reproduction
Sexual reproduction is a type of reproduction that involves a complex life cycle in which a gamete ( haploid reproductive cells, such as a sperm or egg cell) with a single set of chromosomes combines with another gamete to produce a zygote th ...
of Chlorophyceae is isogamous, anisogamous or oogamous.
* The chlorophycean CW
clade
A clade (), also known as a monophyletic group or natural group, is a group of organisms that are monophyletic – that is, composed of a common ancestor and all its lineal descendants – on a phylogenetic tree. Rather than the English ter ...
, and chlorophycean DO clade, are defined by the arrangement of their
flagella
A flagellum (; ) is a hairlike appendage that protrudes from certain plant and animal sperm cells, and from a wide range of microorganisms to provide motility. Many protists with flagella are termed as flagellates.
A microorganism may have fro ...
. Members of the CW clade have flagella that are displaced in a "clockwise" (CW, 1–7 o'clock) direction e.g. Chlamydomonadales. Members of the DO clade have flagella that are "directly opposed" (DO, 12–6 o'clock) e.g.
Sphaeropleales
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 Spaeropleales includes vegetatively non-mot ...
.
Reproduction
Vegetative reproduction usually takes place by fragmentation. Asexual reproduction is by flagellated zoospores. And haplospore, perrination (akinate and palmellastage). Asexual reproduction by mitospore absent in spyrogyra.
Sexual reproduction shows considerable variation in the type and formation of sex cells and it may be isogamous e.g. ''Chlamydomonas, Ulothrix'', anisogamous e.g. ''Chlamydomonas, Eudorina'' or Oogamous e.g. ''Chlamydomonas, Volvox''. ''Chlamydomonas'' has all three types of sexual reproduction.
They share many similarities with the 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.Raven, Evert and Eichhorn. ''The Biology of Plants'' 7th edition, pg. 335. W. H. Freeman and Company, New York, 2005.
The sole method of reproduction in ''Chlorella'' is asexual and azosporic. The content of the cell divides into 2,4 (B), 8(C) sometimes daughter protoplasts. Each daughter protoplast rounds off to form a non-motile spore. These autospores (spores having the same distinctive shape as the parent cell) are liberated by the rupture of the parent cell wall (D). On release each autospore grows to become a new individual. The presence of sulphur in the culture medium is considered essential for cell division. It takes place even in the dark with sulphur alone as the source material but under light conditions nitrogen also required in addition. Pearsal and Loose (1937) reported the occurrence of motile cells in ''Chlorella''. Bendix (1964) also observed that ''Chlorella'' produces motile cells which might be gametes. These observations have an important bearing on the concept of the life cycle of ''Chlorella,'' which at present is considered to be strictly asexual in character.
Asexual reproduction in ''Chlorella ellipsoides'' has been studied in detail and the following four phases have been observed during the asexual reproduction.
(i) Growth Phase - During this phase the cells grow in size by utilizing the photosynthetic products.
(ii) Ripening phase - In this phase the cells mature and prepare themselves for division.
(iii) Post ripening phase - During this phase, each mature cell divides twice either in dark or in light. The cells formed in dark are known as dark to light phase, cells again grow in size.
(iv) Division Phase - During this phase the parent cell wall ruptures and unicells are released.
Orders
, AlgaeBase accepted the following orders in the class Chlorophyceae:
*
Chaetopeltidales
Chaetopeltidales are an order of green algae
The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants ( Embryo ...
C.J.O'Kelly, Shin Watanabe, & G.L.Floyd – 15 species
*
Chaetophorales
Chaetophorales is an order of green algae in the class Chlorophyceae.
Families
* '' Aphanochaetaceae'' Oltmanns
* '' Barrancaceae'' Caisová et al.
* ''Chaetophoraceae
Chaetophoraceae is a family of green algae in the order Chaetophora ...
Wille – 216 species
* Chlamydomonadales F.E.Fritsch – 1752 species
*
Oedogoniales
The Oedogoniales are an order of filamentous freshwater green algae of the class Chlorophyceae. The order is well-defined and has several unique features, including asexual reproduction with zoospores that possess stephanokont flagella: numero ...
Sphaeropleales
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 Spaeropleales includes vegetatively non-mot ...
Luerssen – 909 species
Other orders that have been recognized include:
* Dunaliellales – '' Dunaliella'' and
Dunaliellaceae
Dunaliellaceae is a family of algae in the order Chlamydomonadales.See the NCBIbr>webpage on Dunaliellaceae Data extracted from the
Genera
As accepted by GBIF;
* '' Aulacomonas'' (1)
* ''Dunaliella
''Dunaliella'' is a single-celled, photosy ...
are placed in Chlamydomonadales by AlgaeBase
* Chlorococcales – ''
Chlorococcum
''Chlorococcum'' is a genus of green algae, in the family Chlorococcaceae. The alga may be useful in the flocculation of lipids from wastewater.
Species
, AlgaeBase accepted the following species:
*'' Chlorococcum acidum'' P.A.Archibald & Bold
* ...
'' and Chlorococcaceae are placed in Chlamydomonadales by AlgaeBase
* Microsporales – '' Microspora'' and Microsporaceae are placed in Sphaeropleales by AlgaeBase
* Tetrasporales – ''
Tetraspora
''Tetraspora'' is a genus of green algae in the family Tetrasporaceae of the order Chlamydomonadales, division Chlorophyta. Species of ''Tetraspora'' are unicellular green algae that exist in arrangements of four and consist of cells being packa ...
'' and Tetrasporaceae are placed in Chlamydomonadales by AlgaeBase
In older classifications, the term Chlorophyceae is sometimes used to apply to all the green algae except the Charales, and the internal division is considerably different.
See also
*
Cladistics
Cladistics (; ) is an approach to biological classification in which organisms are categorized in groups (" clades") based on hypotheses of most recent common ancestry. The evidence for hypothesized relationships is typically shared derived cha ...
*
List of Chlorophyceae genera
The Chlorophyceae are a class 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 ...