The green algae (singular: green alga) are the large group of algae Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms, such as the giant kelps that grow to 65 meters in length. The US Algal Collection is represented by almost 300,000 accessioned and inventoried herbarium specimens. The largest and most complex marine forms are called from which the embryophytes The embryophytes are the most familiar group of plants. They are often called land plants because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes (higher plants) emerged.^[1] As such, they form a paraphyletic A group of taxa is said to be paraphyletic if the group contains its last common ancestor but does not contain all the descendants of that ancestor. This term is used in both phylogenetics[note 1] and linguistics group, although the group including both green algae and embryophytes is monophyletic (and often just known as kingdom Plantae Plants are living organisms belonging to the kingdom Plantae. They include familiar organisms such as trees, herbs, bushes, grasses, vines, ferns, mosses, and green algae. The scientific study of plants, known as botany, has identified about 350,000 extant species of plants, defined as seed plants, bryophytes, ferns and fern allies. As of 2004,). The green algae include unicellular and colonial flagellates Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do. Many protists take the form of single-celled flagellates, usually but not always with two flagella A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion. An example of a flagellated bacterium is the ulcer- per cell, as well as various colonial, coccoid, and filamentous forms. In the Charales Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus in 1753, the closest relatives of higher plants, full differentiation of tissues occurs. There are about 6,000 species of green algae.^[2] Many species live most of their lives as single cells, while other species form colonies or long filaments.

A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in euglenids The euglenids are one of the best-known groups of flagellates, commonly found in freshwater especially when it is rich in organic materials, with a few marine and endosymbiotic members. Most euglenids are unicellular. Many euglenids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or strictly by and chlorarachniophytes Chlorarachniophytes are a small group of algae occasionally found in tropical oceans. They are typically mixotrophic, ingesting bacteria and smaller protists as well as conducting photosynthesis. Normally they have the form of small amoebae, with branching cytoplasmic extensions that capture prey and connect the cells together, forming a net. They were acquired from ingested green algae,^[1] and in the latter retain a vestigial nucleus (nucleomorph). Some species of green algae, particularly of genera Trebouxia In taxonomy, Trebouxia is a genus of algae, specifically of the Microthamniales and Pseudotrebouxia (Trebouxiophyceae), can be found in symbiotic associations with fungi A fungus ) is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds, as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi (pronounced /ˈfʌndʒaɪ/ or /ˈfʌŋɡaɪ/), that is separate from plants, animals and bacteria. One major difference is that fungal to form lichens Lichens are composite organisms consisting of a symbiotic association of a fungus (the mycobiont) with a photosynthetic partner (the photobiont or phycobiont), usually either a green alga (commonly Trebouxia) or cyanobacterium (commonly Nostoc). The morphology, physiology and biochemistry of lichens are very different from those of the isolated. In general the fungal species that partner in lichens cannot live on their own, while the algal species is often found living in nature without the fungus.

Contents 1 Cellular structure 2 Origins 3 Classification 4 Reproduction 5 Chemistry 6 References 7 External links 8 See also

Cellular structure

Almost all forms have chloroplasts. These contain chlorophylls Chlorophyll is a green pigment found in all plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός (chloros "green") and φύλλον (phyllon "leaf"). Chlorophyll absorbs light most strongly in the blue portion of the electromagnetic spectrum, followed by the red portion. However, it is a poor a and b, giving them a bright green colour (as well as the accessory pigments beta carotene β-Carotene is an organic compound and classified as a terpenoid. It is a strongly-coloured red-orange pigment abundant in plants and fruits. As a carotene with beta-rings at both ends, it is the most common form of carotene. It is a precursor of vitamin A and xanthophylls Xanthophylls are yellow pigments from the carotenoid group. The name is from Greek xanthos (ξανθος, "yellow") + phyllon (φύλλον, "leaf"), due to their contribution to the yellow band in early chromatography of leaf pigments. Their molecular structure is based on carotenes, with additional oxidation. Thus, they are),^[3] and have stacked thylakoids A thylakoid is a membrane-bound compartment inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. The word "thylakoid" is derived from the Greek thylakos, meaning "sac". Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids.^[4]

All green algae have mitochondria In cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5 to 10 micrometers (μm) in diameter. Mitochondria are sometimes described as "cellular power plants" because they generate most of the cell's supply of adenosine triphosphate (ATP), used as a source of with flat cristae. When present, flagella A flagellum is a tail-like projection that protrudes from the cell body of certain prokaryotic and eukaryotic cells, and functions in locomotion. There are some notable differences between prokaryotic and eukaryotic flagella, such as protein composition, structure, and mechanism of propulsion. An example of a flagellated bacterium is the ulcer- are typically anchored by a cross-shaped system of microtubules Microtubules are one of the components of the cytoskeleton. They have a diameter of 25 nm and length varying from 200 nanometers to 25 micrometers. Microtubules serve as structural components within cells and are involved in many cellular processes including mitosis, cytokinesis, and vesicular transport, but these are absent among the higher plants and charophytes. Flagella are used to move the organism. Green algae usually have cell walls containing cellulose Cellulose is an organic compound with the formula n, a polysaccharide consisting of a linear chain of several hundred to over ten thousand β(1→4) linked D-glucose units, and undergo open mitosis Mitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two cells containing roughly equal shares of these cellular components. Mitosis and without centrioles A centriole is a barrel-shaped cell structure found in most animal eukaryotic cells, though absent in higher plants and most fungi. The walls of each centriole are usually composed of nine triplets of microtubules . Deviations from this structure include Drosophila melanogaster embryos, with nine doublets, and Caenorhabditis elegans sperm cells.

Origins

The chloroplasts of green algae are bound by a double membrane, so presumably they were acquired by direct endosymbiosis The endosymbiotic theory concerns the origins of mitochondria and plastids , which are organelles of eukaryotic cells. According to this theory, these organelles originated as separate prokaryotic organisms that were taken inside the cell as endosymbionts. Mitochondria developed from proteobacteria (in particular, Rickettsiales or close relatives) of cyanobacteria Cyanobacteria is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria (Greek: κυανός (kyanós) = blue). A number of cyanobacteria show similar pigmentation, but this appears to have arisen more than once, and the chloroplasts of green algae are no longer considered closely related to such forms. Instead, the green algae probably share a common origin with the red algae The Red algae (or Rhodophyta, pronounced /roʊˈdɒfɨtə, ˌroʊdəˈfaɪtə/, from Greek: ῥόδον = rose + φυτόν (phyton) = plant, thus red plant) are one of the oldest groups of eukaryotic algae, and also one of the largest, with about 5,000–6,000 species of mostly multicellular, marine algae, including many notable seaweeds. Other.

Classification

A growth of the green seaweed, Enteromorpha on rock substratum at the ocean shore. Some green seaweeds, such as Enteromorpha and Ulva The sea lettuces comprise the genus Ulva, a group of edible green algae that are widely distributed along the coasts of the world's oceans. The type species within the genus Ulva is Ulva lactuca Linnaeus, "lactuca" meaning lettuce. Additionally, some species in the algae genus Monostroma are known as slender sea lettuces, are quick to utilize inorganic nutrients A nutrient is a chemical that an organism needs to live and grow or a substance used in an organism's metabolism which must be taken in from its environment. Nutrients are the substances that enrich the body. They build and repair tissues, give heat and energy, and regulate body processes. Methods for nutrient intake vary, with animals and from land runoff, and thus can be indicators of nutrient pollution.

Green algae are often classified with their embryophyte descendants in the green plant clade A clade[note 1] is a group consisting of an organism and all its descendants. In the terms of biological systematics, a clade is a single "branch" on the "tree of life". The idea that such a "natural group" of organisms should be grouped together and given a taxonomic name is central to biological classification. In Viridiplantae Viridiplantae are a clade comprising the green algae and land plants (or Chlorobionta). Viridiplantae, together with red algae and glaucophyte The glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and green algae plus land plants (Viridiplantae or Chloroplastida), they form the Archaeplastida. However, the relationships between the red algae, green algae and glaucophytes are unclear, in large algae, form the supergroup Primoplantae, also known as Archaeplastida The Archaeplastida are a major group of eukaryotes, comprising the red and green algae and the land plants, together with a small group called the glaucophytes. The plastids of all of these organisms are surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria. In all other groups, plastids are surrounded by or Plantae sensu lato.

Classification systems which have a kingdom of Protista Protists are a diverse group of eukaryotic microorganisms. which include mostly unicellular organisms that do not fit into the other kingdoms. Historically, protists were treated as the kingdom Protista, which includes mostly unicellular organisms that do not fit into the other kingdoms, but this group is contested in modern taxonomy. Instead, it may include green algae in the Protista or in the Plantae.^[5]

Phylogeny:^[6]

Chlorophyta Chlorophyta, a division of green algae, includes about 7,000 species of mostly aquatic photosynthetic eukaryotic organisms. Like the land plants , green algae contain chlorophylls a and b, and store food as starch in their plastids. They are related to the Charophyta and Embryophyta (land plants), together making up the Viridiplantae Prasinophyceae In taxonomy, Prasinophytes are a class of the Division Chlorophyta. These are primitive eukaryotic, marine green algae. Its best known genus is Ostreococcus , which is considered to be the smallest (ca. 0.95 μm) free-living eukaryote and which has been detected in marine samples around the world. Prasinophytes are thought to have low cellular (paraphyletic A group of taxa is said to be paraphyletic if the group contains its last common ancestor but does not contain all the descendants of that ancestor. This term is used in both phylogenetics[note 1] and linguistics) Ulvophyceae The Ulvophyceae or Ulvophytes are class of green algae, distinguished mainly on the basis of ultrastructural morphology. The sea lettuce, Ulva, belongs here. Other well-known members include Ulothrix and Acetabularia. The Ulvophytes are diverse in their morphology and their habitat. Most are seaweeds such as those listed above. Others, such as Trebouxiophyceae In taxonomy, the Trebouxiophyceae are a class of green algae, in the division Chlorophyta Chlorophyceae The Chlorophyceae are one of the classes of green algae, distinguished mainly on the basis of ultrastructural morphology. For example the chlorophycean CW clade, and chlorophycean DO clade, are defined by the arrangement of their flagella. Members of the CW clade have flagella that are displaced in a "clockwise" direction eg Mesostigma Streptophyta Streptophytina is formally a subdivision or subphylum which contains two classes: Charophyceae, containing the Charales order (Charophytes sensu stricto); and Embryophyceae, which contains the embryophytes (land plants, bryophytes, and vascular plants) Chlorokybales Klebsormidiales The Klebsormidiales are an order containing three genera of charophyte green alga forming multicellular, non-branching filaments Zygnematales The Zygnematales (Greek: ζυγός + νήμα (nēma) (nom.), νήματος (nēmatos) (gen.)), also called the Conjugales, are an order of green algae, comprising several thousand different species in genera such as the well-known Zygnema and Spirogyra. All the members of this group develop into unbranched filaments, one cell thick, which grow and Desmidiales Desmids are an order of green algae, comprising around 40 genera and 5,000 to 6,000 species, found mostly but not exclusively in fresh water. Most are unicellular, and are divided into two compartments separated by a narrow bridge or isthmus. Desmids assume a variety of highly symmetrical and generally attractive shapes, which provide the basis Coleochaetales The Coleochaetales are an order of parenchymous charophyte algae, so they represents one of the most primitive multicellular relatives of land plants. They questionably include the fossil genus Parka Charales Charales is an order of pondweeds, freshwater algae in the division Charophyta. They are green plants believed to be the closest relatives of the green land plants. Linnaeus established the genus in 1753 Embryophytes The embryophytes are the most familiar group of plants. They are often called land plants because they live primarily in terrestrial habitats, in contrast with the related green algae that are primarily aquatic. The embryophytes include trees, flowers, ferns, mosses, and various other green land plants. All are complex multicellular eukaryotes ("higher" plants)

The orders outside the Chlorophyta are often grouped as the division Charophyta The Charophyta are a division of green algae, including the closest relatives of the embryophyte plants. In some groups, such as conjugating green algae, flagellate cells do not occur. The latter group does engage in sexual reproduction, and motility does not involve flagella, since they are totally lacking. Flagellate cells in the form of sperm, which is paraphyletic A group of taxa is said to be paraphyletic if the group contains its last common ancestor but does not contain all the descendants of that ancestor. This term is used in both phylogenetics[note 1] and linguistics to higher plants, together comprising the Streptophyta Streptophytina is formally a subdivision or subphylum which contains two classes: Charophyceae, containing the Charales order (Charophytes sensu stricto); and Embryophyceae, which contains the embryophytes (land plants, bryophytes, and vascular plants). Sometimes the Charophyta is restricted to the Charales, and a division Gamophyta is introduced for the Zygnematales and Desmidiales. In older systems the Chlorophyta may be taken to include all the green algae, but taken as above they appear to form a monophyletic group.

One of the most basal green algae is the flagellate Flagellates are organisms with one or more whip-like organelles called flagella. Some cells in animals may be flagellate, for instance the spermatozoa of most phyla. Higher plants and fungi do not produce flagellate cells, but the closely related green algae and chytrids do. Many protists take the form of single-celled flagellates Mesostigma, although it is not yet clear whether it is sister to all other green algae, or whether it is one of the more basal members of the Streptophyta Streptophytina is formally a subdivision or subphylum which contains two classes: Charophyceae, containing the Charales order (Charophytes sensu stricto); and Embryophyceae, which contains the embryophytes (land plants, bryophytes, and vascular plants).^[1][7]

Green Algae conjugating

Reproduction

Green algae are eukaryotic organisms that follow a reproduction cycle called alternation of generations.

Reproduction varies from fusion of identical cells (isogamy) to fertilization of a large non-motile cell by a smaller motile one (oogamy). However, these traits show some variation, most notably among the basal green algae, called prasinophytes.

Haploid algae cells (containing only one copy of their DNA) can fuse with other haploid cells to form diploid zygotes. When filamentous algae do this, they form bridges between cells, and leave empty cell walls behind that can be easily distinguished under the light microscope. This process is called conjugation.

The species of Ulva are reproductively isomorphic, the diploid vegetative phase is the site of meiosis and releases haploid zoospores, which germinate and grow producing a haploid phase alternating with the vegetative phase. [1]

Chemistry

The green algae span a wide range of δ¹³C values, with different groups having different typical ranges.

Algal group	δ13C range[8]
HCO3-using red algae	−22.5‰ – −9.6‰
CO2-using red algae	−34.5‰ – −29.9‰
Brown algae	−20.8‰ – −10.5‰
Green algae	−20.3‰ – −8.8‰

References

1. ^ ^{a b c} Jeffrey D. Palmer, Douglas E. Soltis and Mark W. Chase (2004). "The plant tree of life: an overview and some points of view". American Journal of Botany 91: 1437–1445. doi:10.3732/ajb.91.10.1437. http://www.amjbot.org/cgi/content/full/91/10/1437.
2. ^ Thomas, D. 2002. Seaweeds. The Natural History Museum, London. ISBN 0 565 09175 1
3. ^ Burrows 1991. Seaweeds of the British Isles. Volume 2 Natural History Museum, London. ISBN 0 565 00981 8
4. ^ Hoek, C. van den, Mann, D.G. and Jahns, H.M. 1995. Algae An introduction to phycology. Cambridge University Press, Cambridge. ISBN 0 521 30419 9
5. ^ T Cavalier-Smith (1993 December). "Kingdom protozoa and its 18 phyla". Microbiol Rev. 57 (4): 953–994. PubMed Central PMC372943. PMID 8302218. PMC 372943. http://www.pubmedcentral.nih.gov/pagerender.fcgi?tool=pmcentrez&artid=372943&pageindex=1.
6. ^ Lewis, L. A & R. M. McCourt (2004). "Green algae and the origin of land plants". American Journal of Botany 91 (10): 1535–1556. doi:10.3732/ajb.91.10.1535. http://www.amjbot.org/cgi/content/full/91/10/1535.
7. ^ Andreas Simon, Gernot Glöckner, Marius Felder, Michael Melkonian and Burkhard Becker (2006). "EST analysis of the scaly green flagellate Mesostigma viride (Streptophyta): Implications for the evolution of green plants (Viridiplantae)". BMC Plant Biology 6 (2): 2. doi:10.1186/1471-2229-6-2. PMID 16476162. PMC 1413533. http://www.biomedcentral.com/1471-2229/6/2.
8. ^ Maberly, S. C.; Raven, J. A.; Johnston, A. M. (1992), "Discrimination between 12C and 13C by marine plants", Oecologia 91: 481, doi:10.1007/BF00650320

External links

• Green algae and cyanobacteria in lichens

• Green algae (UC Berkeley)

• Monterey Bay green algae

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