Green algae
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Green algae
Stigeoclonium, a chlorophyte green alga
genus
Scientific classification
(unranked): Archaeplastida
Kingdom: Plantae
Groups included
Mesostigmatophyceae
Spirotaenia
Chlorokybophyceae
Chlorophyta
Charophyta
Cladistically included but
traditionally excluded taxa
Embryophyta
The green algae (singular: green alga) are a large, informal grouping
of algae consisting of the Chlorophyta and Charophyta/Streptophyta, which are now
placed in separate divisions, together with the more
basal Mesostigmatophyceae, Chlorokybophyceae and Spirotaenia.[1][2]
The land plants, or embryophytes, are thought to have emerged from the charophytes.
[3]
Therefore, cladistically, embryophytes belong to green algae as well. However,
because the embryophytes are traditionally classified as neither algae nor green algae,
green algae are a paraphyletic group. Since the realization that the embryophytes
emerged from within the green algae, some authors are starting to include them. [4][5][6][7]
[8]
The clade that includes both green algae and embryophytes is monophyletic and is
referred to as the clade Viridiplantae and as the kingdom Plantae. The green algae
include unicellular and colonial flagellates, most with two flagella per cell, as well as
various colonial, coccoid and filamentous forms, and macroscopic,
multicellular seaweeds. There are about 8,000 species of green algae. [9] Many species
live most of their lives as single cells, while other species form coenobia (colonies), long
filaments, or highly differentiated macroscopic seaweeds.
A few other organisms rely on green algae to conduct photosynthesis for them.
The chloroplasts in dinoflagellates of the
genus Lepidodinium, euglenids and chlorarachniophytes were acquired from ingested
green algae,[10] and in the latter retain a nucleomorph (vestigial nucleus). Green algae
are also found symbiotically in the ciliate Paramecium, and in Hydra viridissima and
in flatworms. Some species of green algae, particularly of genera Trebouxia of the
class Trebouxiophyceae and Trentepohlia (class Ulvophyceae), can be found in
symbiotic associations with fungi to form lichens. 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. Trentepohlia is a filamentous green alga that can live
independently on humid soil, rocks or tree bark or form the photosymbiont in lichens of
the family Graphidaceae. Also the macroalga Prasiola calophylla (Trebouxiophyceae) is
terrestrial,[11] and Prasiola crispa, which live in the supralittoral zone, is terrestrial and can
in the Antarctic form large carpets on humid soil, especially near bird colonies. [12]
Contents
1Cellular structure
2Origins
3Evolution and classification
4Reproduction
o 4.1Sex pheromone
5Physiology
6References
7External links
Cellular structure[edit]
Green algae have chloroplasts that contain chlorophyll a and b, giving them a bright
green color, as well as the accessory pigments beta carotene (red-orange)
and xanthophylls (yellow) in stacked thylakoids.[13][14] The cell walls of green algae usually
contain cellulose, and they store carbohydrate in the form of starch.[15]
All green algae have mitochondria with flat cristae. When present, paired flagella are
used to move the cell. They are anchored by a cross-shaped system
of microtubules and fibrous strands. Flagella are only present in the motile male
gametes of charophytes[16] bryophytes, pteridophytes, cycads and Ginkgo, but are
absent from the gametes of Pinophyta and flowering plants.
Members of the class Chlorophyceae undergo closed mitosis in the most common form
of cell division among the green algae, which occurs via a phycoplast.[17] By
contrast, charophyte green algae and land plants (embryophytes) undergo
open mitosis without centrioles. Instead, a 'raft' of microtubules, the phragmoplast, is
formed from the mitotic spindle and cell division involves the use of this phragmoplast in
the production of a cell plate.[18]
Origins[edit]
Photosynthetic eukaryotes originated following a primary endosymbiotic event, where a
heterotrophic eukaryotic cell engulfed a photosynthetic cyanobacterium-like prokaryote
that became stably integrated and eventually evolved into a membrane-
bound organelle: the plastid.[19] This primary endosymbiosis event gave rise to
three autotrophic clades with primary plastids: the green plants, the red algae and
the glaucophytes.[20]
Evolution and classification[edit]
A growth of the green seaweed Ulva on rock substratum at the ocean shore. Some green seaweeds, such
as Ulva, are quick to utilize inorganic nutrients 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 Viridiplantae (or Chlorobionta). Viridiplantae, together with red algae
and glaucophyte algae, form the supergroup Primoplantae, also known
as Archaeplastida or Plantae sensu lato. The ancestral green alga was a unicellular
flagellate.[21]
The Viridiplantae diverged into two clades. The Chlorophyta include the early
diverging prasinophyte lineages and the core Chlorophyta, which contain the majority of
described species of green algae. The Streptophyta include charophytes and land
plants. Below is a consensus reconstruction of green algal relationships, mainly based
on molecular data. [22][23][24][25][26][27][28][29][2][30][30][31][32]
Viridiplantae/
