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StreptophytaGreen algae; Stoneworts; Brittleworts; Chlorophyta; Charophyta |
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| Following modified from University of Western Cape, South Africa |
The Chlorophyta The Chlorophyta, or green algae, includes a around 7000 species of algae with representatives in almost every habitat on earth. They are characterized by the presence of chlorophyll- a and chlorophyll- b , the absence of phycobiliprotein accessory pigments, and the presence of carotenoid accessory pigments. The cell wall is composed of cellulose, hemicellulose and pectins. Food is stored as starch that is clustered around pyrenoids or scattered throughout chloroplast stroma. The chloroplast membrane is 2-layered, the thylakoids are stacked in groups of two or more that are organized into lamellae, pseudograna, or grana.
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| Following modified from University of Western Cape, South Africa |
The Chlorophyta are widespread in virtually any habitat on earth where there is light, including ice and snow and desert soils. Although they are called green algae, their colour varies from green through shades of yellow, purple, orange, red or even blackish. Most are, however, green though not all algae that are green are green algae. Over 10000 species of chlorophytes are known.
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| Following modified from University of Western Cape, South Africa |
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| Following modified from University of Western Cape, South Africa |
A number of classification systems have been used for the Chlorophyta, the most widely used being based on artificial criteria that are not supported by more recent ultrastructural and molecular biological evidence. The most common grouping of the green algae into orders is based largely on organizational level, something which shows only marginal relationship to phylogeny. There have been a lot of publications on the higher-level classification of the green algae in recent years, and there is as yet no general agreement on a system to use. It is beyond the scope of this module to delve deeply into green algal classification, but there are at least some groups of greens that any serious student of botany should be familiar with. The following links provide information on green algal classification, and should be explored further.
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| Following modified from University of Western Cape, South Africa |
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| Following modified from University of Western Cape, South Africa |
At least some biologists advocate elevating this whole group to the Kingdom level as the Chlorobionta. The 'land plants' that form a component of this branch are treated here as Divisions for consistency with most published undergraduate text books. It is important to realise that the higher-level classification of these organisms is still open to some debate, and will ultimately depend on further molecular studies.
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| Following modified from Museum of Paleontology, University of California, Berkeley |
Introduction to the "Green Algae"
The "green algae" is the most diverse group of algae, with more than 7000 species growing in a variety of habitats. The "green algae" is a paraphyletic group because it excludes the Plantae . Like the plants, the green algae contain two forms of chlorophyll , which they use to capture light energy to fuel the manufacture of sugars, but unlike plants they are primarily aquatic. Because they are aquatic and manufacture their own food, these organisms are called " algae ," along with certain members of the Chromista , the Rhodophyta , and photosynthetic bacteria , even though they do not share a close relationship with any of these groups. The above picture shows a dense growth of sea lettuce ( Ulva ), growing in a tide pool at the Berkeley Marina. This is a marine species of "green algae" often found attached to rocks, and exposed at low tide.
Visit Wisconsin for pictures of various green algae, or try the Protist Image Data Base for information about Chlamydomonas , Tetraselmis , Halosphaera , and Pyramimonas , which are all "green algae". Additional images of green algae are available from Tavole di Botanica sistematica . Algae: The Forgotten Treasure of Tidepools is an exhibit at Sonoma State, with images and information on California tidepool algae, including green algae . Duke Univeristy maintains the Chlamydomonas Genetics Center , which includes databases and educational material. A more general offering is the Seaweed Information Server at University College Galway. For a fuller listing of on-line phycological collections resources, try our Phycological Collection Catalogs Listings. 
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Chlorophytes range from tiny unicells, just big enough to see with the light microscope (one is even the smallest eukaryote known), to some quite large forms over a metre across. They include the largest known cells, a green alga called
Ventricaria ventricosa
that is one cell that may be several centimetres in diameter. It occurs on coral reefs, where the cell wall is often overgrown by thin, pink coralline red algae. It is able to grow to a large size and retain a rigorous shape purely because of turgor pressure. Any tiny hole in the cell, and it collapses like a wet balloon. 
The Chlorophyta is probably not a natural grouping of organisms because they are
paraphyletic
, that is the group does not contain all the descendants of the ancestral green alga. This is so because they exclude the so-called land plants, from mosses to angiosperms. According to most systems of classification, the mosses, gymnosperms, angiosperms and several other groups are treated as divisions. On a purely phylogenetic basis, all plants should be treated as Chlorophyta. That all plants are not included in the Chlorophyta is more a result of our terrestrial, macroscopic viewpoint in combination with traditional bias, rather than a result of modern scientific analysis.
The 
