The green algae (: green alga) are a group of
chlorophyll
Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words (, "pale green") and (, "leaf"). Chlorophyll allows plants to absorb energy ...
-containing
autotrophic
eukaryote
The eukaryotes ( ) constitute the Domain (biology), domain of Eukaryota or Eukarya, organisms whose Cell (biology), cells have a membrane-bound cell nucleus, nucleus. All animals, plants, Fungus, fungi, seaweeds, and many unicellular organisms ...
s consisting of the phylum
Prasinodermophyta and its unnamed
sister group that contains the
Chlorophyta
Chlorophyta is a division of green algae informally called chlorophytes.
Description
Chlorophytes are eukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single green chloroplast in each cell. They are ...
and
Charophyta/
Streptophyta. The
land plants (
Embryophytes) have emerged deep within the charophytes as a sister of the
Zygnematophyceae.
Since the realization that the Embryophytes emerged within the green algae, some authors are starting to include them.
The completed
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
that includes both green algae and embryophytes is
monophyletic
In biological cladistics for the classification of organisms, monophyly is the condition of a taxonomic grouping being a clade – that is, a grouping of organisms which meets these criteria:
# the grouping contains its own most recent co ...
and is referred to as the clade
Viridiplantae and as the kingdom
Plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
ae. The green algae include unicellular and colonial
flagellates, most with two
flagella per cell, as well as various colonial, coccoid (spherical), and filamentous forms, and macroscopic, multicellular
seaweeds. There are about 22,000 species of green algae, many of which 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
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s in
dinoflagellates of the genus ''
Lepidodinium'',
euglenids and
chlorarachniophytes were acquired from
ingested endosymbiont green algae, 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
A fungus (: fungi , , , or ; or funguses) is any member of the group of eukaryotic organisms that includes microorganisms such as yeasts and mold (fungus), molds, as well as the more familiar mushrooms. These organisms are classified as one ...
to form
lichen
A lichen ( , ) is a hybrid colony (biology), colony of algae or cyanobacteria living symbiotically among hypha, filaments of multiple fungus species, along with yeasts and bacteria embedded in the cortex or "skin", in a mutualism (biology), m ...
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. ''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, 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.
Cellular structure
Green algae have chloroplasts that contain
chlorophyll ''a'' and
''b'', giving them a bright green colour, as well as the accessory pigments
beta carotene (red-orange) and
xanthophylls (yellow) in stacked
thylakoids.
[Burrows 1991. ''Seaweeds of the British Isles.'' Volume 2 Natural History Museum, London. ] The
cell walls of green algae usually contain
cellulose
Cellulose is an organic compound with the chemical formula, formula , a polysaccharide consisting of a linear chain of several hundred to many thousands of glycosidic bond, β(1→4) linked glucose, D-glucose units. Cellulose is an important s ...
, and they store carbohydrate in the form of
starch.
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 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.
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.
[P.H. Raven, R.F. Evert, S.E. Eichhorn (2005): ''Biology of Plants'', 7th Edition, W.H. Freeman and Company Publishers, New York, ]
Origins
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
In cell biology, an organelle is a specialized subunit, usually within a cell (biology), cell, that has a specific function. The name ''organelle'' comes from the idea that these structures are parts of cells, as Organ (anatomy), organs are to th ...
: the
plastid.
This primary endosymbiosis event gave rise to three
autotrophic clades with primary plastids: the
(green) plants (with
chloroplast
A chloroplast () is a type of membrane-bound organelle, organelle known as a plastid that conducts photosynthesis mostly in plant cell, plant and algae, algal cells. Chloroplasts have a high concentration of chlorophyll pigments which captur ...
s) the
red algae (with rhodoplasts) and the
glaucophytes (with muroplasts).
Evolution and classification

Green algae are often classified with their embryophyte descendants in the green plant
clade
In biology, a clade (), also known as a Monophyly, monophyletic group or natural group, is a group of organisms that is composed of a common ancestor and all of its descendants. Clades are the fundamental unit of cladistics, a modern approach t ...
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.
The Viridiplantae diverged into two clades. The
Chlorophyta
Chlorophyta is a division of green algae informally called chlorophytes.
Description
Chlorophytes are eukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single green chloroplast in each cell. They are ...
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.
The basal character of the Mesostigmatophyceae, Chlorokybophyceae and ''spirotaenia'' are only more conventionally basal Streptophytes.
The algae of this paraphyletic group "Charophyta" were previously included in Chlorophyta, so green algae and Chlorophyta in this definition were synonyms. As the green algae clades get further resolved, the embryophytes, which are a deep charophyte branch, are included in "
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
", "green algae" and "
Charophytes", or these terms are replaced by cladistic terminology such as
Archaeplastida,
Plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
ae/
Viridiplantae, and
streptophytes, respectively.
Reproduction

Green algae are a group of photosynthetic, eukaryotic organisms that include species with haplobiontic and diplobiontic life cycles. The diplobiontic species, such as ''
Ulva'', follow a reproductive cycle called
alternation of generations in which two multicellular forms, haploid and diploid, alternate, and these may or may not be isomorphic (having the same morphology). In haplobiontic species only the haploid generation, the
gametophyte is multicellular. The fertilized egg cell, the diploid
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
, undergoes
meiosis, giving rise to haploid cells which will become new gametophytes. The diplobiontic forms, which evolved from haplobiontic ancestors, have both a multicellular haploid generation and a multicellular diploid generation. Here the zygote divides repeatedly by
mitosis and grows into a multicellular diploid
sporophyte
A sporophyte () is one of the two alternation of generations, alternating multicellular organism, multicellular phases in the biological life cycle, life cycles of plants and algae. It is a diploid multicellular organism which produces asexual Spo ...
. The sporophyte produces haploid spores by meiosis that germinate to produce a multicellular gametophyte. All
land plants have a diplobiontic common ancestor, and diplobiontic forms have also evolved independently within
Ulvophyceae more than once (as has also occurred in the red and brown algae).
Diplobiontic green algae include isomorphic and heteromorphic forms. In isomorphic algae, the morphology is identical in the haploid and diploid generations. In heteromorphic algae, the morphology and size are different in the gametophyte and sporophyte.
Reproduction varies from fusion of identical cells (
isogamy
Isogamy is a form of sexual reproduction that involves Gamete, gametes of the same Morphology (biology), morphology (indistinguishable in shape and size), and is found in most Unicellular organism, unicellular eukaryotes. Because both gametes lo ...
) to
fertilization of a large non-motile cell by a smaller motile one (
oogamy
Oogamy is a form of anisogamy where the gametes differ in both size and form.
In oogamy the large female gamete (also known as ovum) is immotile, while the small male gamete (also known as spermatozoon) is mobile. Oogamy is a common form of an ...
). However, these traits show some variation, most notably among the basal green algae called
prasinophytes.
Haploid algal 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'' and occurs for example in ''
Spirogyra
''Spirogyra'' (common names include water silk, mermaid's tresses, and blanket weed) is a genus of filamentous charophyte green algae of the order Zygnematales, named for the helical or spiral arrangement of the chloroplasts that is charact ...
''.
Sex pheromone
Sex pheromone production is likely a common feature of green algae, although only studied in detail in a few model organisms. ''
Volvox'' is a genus of
chlorophytes. Different species form spherical colonies of up to 50,000 cells. One well-studied species, ''
Volvox carteri'' (2,000 – 6,000 cells) occupies temporary pools of water that tend to dry out in the heat of late summer. As their environment dries out, asexual ''V. carteri'' quickly die. However, they are able to escape death by switching, shortly before drying is complete, to the sexual phase of their life cycle that leads to production of dormant desiccation-resistant
zygote
A zygote (; , ) is a eukaryote, eukaryotic cell (biology), cell formed by a fertilization event between two gametes.
The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individ ...
s. Sexual development is initiated by a
glycoprotein pheromone (Hallmann et al., 1998). This pheromone is one of the most potent known biological effector molecules. It can trigger sexual development at concentrations as low as 10
−16M.
Kirk and Kirk
showed that sex-inducing pheromone production can be triggered experimentally in
somatic cells by
heat shock. Thus heat shock may be a condition that ordinarily triggers sex-inducing pheromone in nature.
The ''Closterium peracerosum-strigosum-littorale (C. psl)'' complex is a unicellular, isogamous
charophycean alga group that is the closest unicellular relative to land plants.
Heterothallic strains of different
mating type can conjugate to form
zygospores. Sex pheromones termed protoplast-release inducing proteins (glycopolypeptides) produced by mating-type (-) and mating-type (+) cells facilitate this process.
Physiology
The green algae, including the characean algae, have served as
model experimental organisms to understand the mechanisms of the ionic and water permeability of membranes,
osmoregulation,
turgor regulation,
salt tolerance,
cytoplasmic streaming, and the generation of
action potentials.
References
External links
Green algae and cyanobacteria in lichens
{{Authority control
Green algae
Paraphyletic groups