Green Algae

Green Algae
Green Algae
The green algae are a diverse group of eukaryotic organisms classified in the phylum Chlorophyta. They are considered eukaryotic because individual cells possess a prominent structural feature known as a nucleus, which houses the chemicals responsible for heredity and metabolic regulation.

The phylum is one of several algal phyla in the kingdom Protista, where algae are grouped based upon pigmentation, carbohydrate storage reserves, and cell wall composition.

Green algae are found in moist soils and fresh-water and saltwater habitats; most are believed to be freshwater-dwelling. The phylum consists of at least eight thousand species. Some estimates place this number at seventeen thousand species. Several shared characteristics support the hypothesis that these organisms and terrestrial plants derived from a common ancestor.

General Characteristics

The green algae, or chlorophytes, may be unicellular, multicellular, colonial, or filamentous. Multicellular forms may demonstrate some tissue differentiation but not to the complexity displayed by terrestrial plants. Colonial algae tend to cluster in a pattern resembling a hollow sphere or disc. Some filamentous forms are coenocytic, meaning they have lost a portion or all of their cross walls.

The cell walls consist of cellulose. There are usually two layers of cellulose fortified by pectin. Some unicellular forms have a lorica (thin wall or cuticle), which is separated from the protoplast by a gelatinous matrix or water.

The phylum Chlorophyta is named for the prominent green chloroplast, a cell structure containing pigments that carry out photosynthesis, similar to that found in higher plants.

The chloroplasts are green because the accessory pigments, which include xanthophylls and various carotenoids, do not mask the chlorophylls, the principal photosynthetic pigments, present.

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All classes contain chlorophylls a and b. Chlorophyll c has been found in a few species of the class Prasinophyceae. The chloroplasts are double-membraned structures with thylakoids (membranous folds) stacked in groups of from two to six.

The storage carbohydrate is starch. Starch grains can be found clustered around pyrenoids (protein bodies), if they are present. However, they are found generally scattered throughout the fluid portion of the chloroplast.

Chlorophytes possess either two or four flagella (whiplike appendages for motility) at least once during their life cycle, although some forms have a single flagellum. In addition to providing motility, flagella may play a key role in the sexual process for some unicellular forms.

There is considerable debate over the classification of green algae. Most taxonomists currently classify Chlorophyta in the kingdom Protista. Because of the many similarities to terrestrial plants, many taxonomists feel that Chlorophyta should have its own kingdom. Living species of charophytes are grouped into three classes: Chlorophyceae, Charophyceae, and Ulvophyceae.

Chlorophyceae

Chlorophyceae
Chlorophyceae
The class Chlorophyceae is the largest in terms of the number of species listed. Members have two or more flagella; a diverse array of sexual and asexual reproductive strategies; production of a zygospore following sexual reproduction; and mitosis that involves phycoplasts (microtubules that separate daughter nuclei during division). Representative genera include Chlamydomonas, Pandorina, Volvox, and Gonium.

Chlamydomonas species are unicellular, with two apical flagella and a cup-shaped chloroplast. Gonium is a colonial species with four or more cells with no functional or morphological differentiation. Pandorina species form spherical colonies with limited differentiation and structural organization.

Colonies of Volvox can consist of up to sixty thousand cells and demonstrate some structural specialization. Portions of the colony have cells with large flagella and stigmata. These cells appear to be specialized for colony motility. The posterior region consists of cells with small flagella and no stigmata. These seem to be responsible for reproduction.

Charophyceae

Charophyceae contains asymmetrical cells that may or may not be motile. Motile cells have two apical flagella. Sexual reproduction is characterized by the formation of a zygospore and zygotic meiosis.

This class is similar to land plants in that nuclear envelopes dissolve during mitosis, which is not the case for the other two classes. The genus Chara includes members that resemble vascular plants. Chara species have a central axis and branchlike extensions.

These organisms demonstrate apical growth that begins with an apical cell, which is analogous to the apical meristems of terrestrial plants. Spirogyra is a well-known filamentous genus that is distinguished by spiral chloroplasts. Sexual reproduction is characterized by the formation of a conjugation filament between two cells that allows for gamete transfer.

Ulvophyceae

Ulvophyceae is a diverse class of primarily marine organisms that can consist of small colonial forms, filamentous forms, thin sheets of cells, or coenocytic complexes. Reproduction is by alternation of generations, with meiosis occurring in spores.

There may be two or more flagella, if flagella are present. The genus Ulva, also known as sea lettuce, displays a green sheet of cells that are found in intertidal waters. Reproduction involves an isomorphic alternation of generations.

Ulothrix contains freshwater filamentous algae that can attach to surfaces via a hold fast. Ulothrix asexually generates zoospores and aplanospores. Species are able to reproduce sexually by formation of a heterothallic zygote/zygospore from isogamous gametes.

Reproductive Strategies

Chlorophytes reproduce sexually, which involves alternating haploid (organisms with half the complete chromosome set) and diploid stages. Haplobiontic haploid organisms consist of mature haploid forms that produce gametes by mitosis (division resulting in offspring cells identical to the parent form). Compatible gametes fuse and form a diploid zygote, which divides by meiosis (division resulting in four haploid offspring cells) to form four spores.

A haplobiontic diploid organism consists of mature diploid forms that produce gametes by meiosis. Diplobiontic green algae are more complex, with a zygote under going mitosis. This results in the formation of a haploid and diploid thalli.

The haploid thallus is referred to as the gametophyte, and the diploid thallus is referred to as the sporophyte. Gametophytes generate gametes, while sporophytes produce spores. This pattern is referred to as an alternation of generations.

The thalli may be identical (isomorphic) or different (heteromorphic). If a thallus produces both sperm and eggs, it is considered homothallic. If the egg and sperm are produced on separate thalli, the organism is heterothallic.

Gametes may be isogamous (indistinguishable and motile) or heterogamous (two distinct types). Male gametes develop in gametangia known as antheridia. Female gametes develop in either oogonia (single-celled gametangia) or archegonia (multicelled gametangia). Zygotes often form thick-walled resting structures called zygospores.

The most common type of spore is the zoospore, which is a flagellated cell. Cells can form single zoospores or divide mitotically to produce many zoospores. Zoospores mature into vegetative cells within minutes or days, depending upon the species.

Vegetative cells may or may not keep their flagella. Zoospores are typically formed in compartments called sporangia but may be formed following meiosis in a zygote.

Most zoospores resemble members of the chlorophyte genus Chlamydomonas. Thick-walled, nonmotile spores called akinetes may be formed and can produce zoospores via mitosis or form filamentous structures. Some chlorophytes form aplanospores, which are nonmotile.

Ecology

Chlorophytes are found in diverse habitats all over the world. While most inhabit temperate, freshwater environments, marine and terrestrial forms also exist. Terrestrial forms include some living on moist soils, some on moist rocks, and some in snow-covered areas. Some terrestrial forms are specialized as lichens, a close association between an alga and a fungus, or living on animals such as turtles or sloths.

Because they are photoautotrophic, capable of making their own carbohydrates using sunlight energy, chlorophytes are critical to life on earth. Green algae are the planet’s largest food source. They fix approximately 1,010 tons of carbon per year. As a result, they also contribute significantly to oxygen production.

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