Everything about The Green Algae totally explained
The
green algae (singular:
green alga) are the large group of
algae from which the
embryophytes (higher plants) emerged. As such, they form a
paraphyletic group, although the group including both green algae and embryophytes is monophyletic (and often just known as kingdom
Plantae). The green algae include unicellular and colonial
flagellates, usually but not always with two
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 6000 species of green algae. 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
euglenids and
chlorarachniophytes were acquired from ingested green algae, and have stacked
thylakoids.
All green algae have
mitochondria with flat cristae. When present,
flagella are typically anchored by a cross-shaped system of
microtubules, 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, 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
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.
Classification systems which have a kingdom of
Protista may include green algae in the Protista or in the Plantae.
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 green algae, but taken as above they appear to form a monophyletic group.
One of the most basal green algae is the
flagellate Mesostigma, although it isn't yet clear whether it's sister to all other green algae, or whether it's one of the more basal members of the
Streptophyta.
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.
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