Raffi at 04:58, 8 April 2007

2007-04-08T04:58:39Z

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{{Taxobox
| color = lightgreen
| name = Green algae
| image = Green algae.jpg
| image_width = 240px
| image_caption =
| regnum =[[Protist]]a or [[Plant]]ae
| subdivision_ranks = Divisions
| subdivision =
[[Chlorophyta]]<br />
[[Charophyta]]<br />
}}

'''Green algae''' are microscopic protists; found in all aquatic environments, including marine, freshwater and brackish water.

The green algae (singular: ''green alga'') are the large group of [[alga]]e from which the [[embryophyte]]s (higher plants) emerged. As such, they form a [[paraphyletic]] group, variously included among the [[Plantae]] or with the [[Protist]]a. The green algae include unicellular and colonial [[flagellate]]s, usually but not always with two [[flagellum|flagella]] per cell, as well as various colonial, coccoid, and filamentous forms. In the [[Charales]], the closest relatives of higher plants, full differentiation of tissues occurs. There are about 1000 — 2000 species of green algae. <ref name="Thomas 02">'''Thomas, D.''' 2002. ''Seaweeds.'' The Natural History Museum, London. ISBN 0 565 09175 1</ref> Many species live most of their lives as single-cells, other species form colonies or long filaments.

A few other organisms rely on green algae to conduct photosynthesis for them. The chloroplasts in [[euglenid]]s and [[chlorarachniophyte]]s were presumably acquired from ingested green algae, and in the latter retain a vestigial nucleus (nucleomorph). Some species of green algae, particularly of genera ''[[Trebouxia]]'' or ''[[Pseudotrebouxia]]'' (Trebouxiophyceae), can be found in symbiotic associations with [[fungi]] to form [[lichen]]s. 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.

===Cellular structure===
Almost all forms have chloroplasts. These contain [[chlorophyll]]s ''a'' and ''b'', giving them a bright green colour (as well as the accessory pigments [[beta carotene]] and [[xanthophyll]]s), and have stacked [[thylakoid]]s.

All green algae have [[mitochondrion|mitochondria]] with flat cristae. When present, [[flagellum|flagella]] are typically anchored by a cross-shaped system of [[microtubule]]s, but these are absent among the higher plants and charophytes. They usually have cell walls containing [[cellulose]], and undergo open [[mitosis]] without [[centrioles]].

===Origins===

The chloroplasts of green algae are bound by a double membrane, so presumably they were acquired by direct [[endosymbiotic theory|endosymbiosis]] of [[cyanobacteria]]. 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]].

===Classification===

Green algae are often classified with their embryophyte descendants in the green plant [[clade]] [[Viridiplantae]] (or [[Chlorobionta]]). Viridiplantae, together with red algae and [[glaucophyte]] algae, form the supergroup [[Primoplantae]], also known as [[Archaeplastida]] or Plantae ''sensu lato''.

[[Image:Intertidal greenalgae.jpg|280px|thumb|right|A growth of the green seaweed, ''[[Enteromorpha]]'' on rock substratum at the ocean shore. Some green seaweeds, such as ''Enteromorpha'' and ''[[Sea lettuce|Ulva]]'', are quick to utilize inorganic [[nutrient]]s from land runoff, and thus can be indicators of nutrient pollution.]]

* [[Chlorophyta]]
** [[Chlorophyceae]]
** [[Ulvophyceae]]
** [[Trebouxiophyceae]]
* [[Chlorokybales]]
* [[Klebsormidiales]]
* [[Zygnematales]]
* [[Desmid]]iales
* [[Coleochaetales]]
* [[Charales]] (stoneworts)

The orders outside the Chlorophyta are often grouped as the division [[Charophyta]], which is [[paraphyletic]] to higher plants, together comprising the Streptophyta. 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 g
reen algae, but taken as above they appear to form a monophyletic group.

[[Image:Algaeconjugate.jpg|thumb|right|Green Algae conjugating]]
===Reproduction===

Green algae are eukaryotic organisms. Sexual reproduction varies from fusion of identical cells (isogamy) to [[fertilisation|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 [[prasinophyte]]s.

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''.

== References ==
<references/>

==External links==

*{{cite journal |quotes=no |author=Lewis, L. A & R. M. McCourt |year=2004 |title=Green algae and the origin of land plants|

*journal=[[American Journal of Botany]] |volume=91 |issue=10 |pages=1535–1556 |url=http://www.amjbot.org/cgi/reprint/91/10/1535?maxtoshow=&HITS=10&hits=10&RESULTFORMAT=&searchid=1&FIRSTINDEX=0&sortspec=relevance&volume=91&firstpage=1535&resourcetype=HWCIT}}

*[http://home.arcor.de/stefan.wic/diplomarbeit.pdf#search=%22%22stefan%20wic%22%22/ Green algae and cyanobacteria in lichens]

*[http://www.ucmp.berkeley.edu/greenalgae/greenalgae.html Green algae (UC Berkeley)]

*[http://www.mbari.org/staff/conn/botany/flora/green.htm Monterey Bay green algae]

==See also==
*''[[Codium]]''
*[[Red algae]]
*[[Brown algae]]

[[Category:Green algae|*]]
[[Category:Non-vascular plants]]

Green algae - Revision history

Raffi at 04:58, 8 April 2007