Chlorophyceae (from the Greek word chloros, meaning “green”) make up an extremely large and important class of green algae. Members may be unicellular, colonial, or filamentous. Cells of unicellular and colonial chlorophyceans may have two or more flagella.
There are about 2,650 living species of chlorophyceans. The main features of the class (and most plants) are the use of starch as the principal food reserve and the green chloroplasts with chlorophylls a and b. In spite of plant characteristics, this algal group is not directly related to early land plants.
Chlorophyceans are almost entirely restricted to freshwater and terrestrial habitats. Some members of this class have adapted to life on snow as snow algae. Snow algae cause snow to appear red-burgundy or orange in color because of high levels of unusual carotenoid pigments within the algal cells.
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There are a variety of asexual and sexual reproduction modes among members of this class. Sexual reproduction is characterized by the formation of a zygote produced by gametic fusion.
Chlorophyceans show differences during cell division compared to other green algal groups. For example, they produce a set of microtubules, the phycoplast, that is parallel to the plane of cell division.
Diversity
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| Diversity |
Both are important research models in laboratories. Chlorophyceans fall into several orders, including Volvocales, Chlorococcales, Chaetophorales, and Oedogoniales.
Volvocales
Members of the order Volvocales include both unicellular organisms, such as those in the genus Chlamydomonas with their two equal flagella, and colonial forms. The Chlamydomonas are a large genus of chlorophyceans.
More than six hundred species have been described worldwide. The Chlamydomonas probably represent the most primitive structure among chlorophyceans. Nevertheless, their basic cell features may be found among other representatives of this order.
A cell wall made of glycoproteins, rather than cellulose, surrounds each Chlamydomonas cell. Inside the cell, there is a single large chloroplast and a pyrenoid, which forms starch.
Other cytoplasmic structures include the contractile vacuole rather than a central vacuole. The contractile vacuole is responsible for the removal of water from the cell. Cells of Chlamydomonas are capable of phototaxis: They swim toward moderate light but away from high-intensity light.
Rhodopsin-like pigment is their primary lightsensing photoreceptor. Under dry conditions, Chlamydomonas form a palmelloid stage, in which nonflagellate cells are held together by common mucilage.
Chlamydomonas reproduce asexually via cell division. Also, cells of this alga can become gametes. In most species of Chlamydomonas, the male and female gametes appear the same; they are designated (+) and (–).
Colonial flagellates of the order Volvocales range from simple colonies of Gonium to visible-without-magnification spheres of Volvox with up to several thousands of cells and some sort of cellular specialization.
Volvox are one of the most structurally advanced colonial forms of green algae.Only specialized cells participate in reproduction. During asexual reproduction, some cells of Volvox divide and bulge inward, forming new daughter colonies, which are held for some time within the parent colony. Volvox are also capable of sexual reproduction. They produce gametes that differentiate into sperm and eggs.
Chlorococcales
Members of the order Chlorococcales include nonmotile unicellular and colonial algae. Typical representatives of the unicellular nonmotile form are found in Chlorococcum. They occur as spherical single cells or cell aggregates and produce flagellated zoospores.
Examples of colonial representatives of Chlorococcales are Hydrodictyon, commonly known as the “water net”; Pediastrum, famous for their distinctive, starlike shape; and Scenedesmus, wide-spread inhabitants of the freshwater phytoplankton.
The order Chlorococcales has now been divided on the basis of small subunit ribosomal ribonucleic acid (RNA) sequence data into several groups, including the Sphaeropleales, Tetracystis clade, and Dunaliella clade.
Chaetophorales and Oedogoniales
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| Chaetophorales and Oedogoniales |
The Draparnaldia (named for Jacques Phillipe Raymond Draparnaud, a French naturalist) from order Chaetophorales have a main filamentous axis with relatively large cells, primary branches with smaller cells, and secondary branches with even smaller cells.
One representative of Oedogoniales, the green alga Oedogonium (from the Greek oidos, meaning “swelling”), has been a subject of intense study for its unusual cell division technique.
The entire contents of an Oedogonium cell may be used in the for mation of one large zoospore with multiple flagella. Members of Bulbochaete (from the Greek bolbos, meaning “bulb”) resemble Oedogonium in cell division but differ in being branched and having a distinctive hair cell at the end of each branch.
Technological Uses
A few chlorophycean green algae have commercial value. These algae are good candidates for the industrial production of hydrogen gas because they are able to release the gas from water using solar energy. Hydrogen gas is an environmentally desirable fuel because the burning of hydrogen produces water, and it can be converted effectively to electricity.
Selenastrumcapricornutum are the most widely used algal biomonitors in the detection of water pollution. Chlorophyceans are used in freshwater aquaculture systems as food for fish.One alga with possible potential for salmon feeds is Haematococcus.
Algae contain large amounts of the pigment astaxanthin, which is responsible for the red coloration typical of salmon flesh. Chlorella (formerly classified in the order Chlorococcales) are famous both as the experimental systems in the discovery of the photosynthetic Calvin cycle and as health food in Asia.
