The
green algae (singular:
green alga) are the large group of
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
from which the
embryophyteThe 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...
s (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
flagellateFlagellates are cells 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...
s, usually but not always with two
flagellaA flagellum is a tail-like structure that projects 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...
per cell, as well as various colonial, coccoid, and filamentous forms. In the
CharalesCharales 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.-Description:...
, the closest relatives of higher plants, full differentiation of tissues occurs.
The
green algae (singular:
green alga) are the large group of
algaeAlgae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
from which the
embryophyteThe 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...
s (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
flagellateFlagellates are cells 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...
s, usually but not always with two
flagellaA flagellum is a tail-like structure that projects 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...
per cell, as well as various colonial, coccoid, and filamentous forms. In the
CharalesCharales 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.-Description:...
, the closest relatives of higher plants, full differentiation of tissues occurs. There are about 6,000 species of green algae. 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
euglenidThe 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. Many euglenids have chloroplasts and produce energy through photosynthesis, but others feed by phagocytosis or...
s and
chlorarachniophyteChlorarachniophytes 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...
s were acquired from ingested green algae, and in the latter retain a vestigial nucleus (nucleomorph). Some species of green algae, particularly of genera
TrebouxiaIn taxonomy, Trebouxia is a genus of algae, specifically of the Microthamniales.-Scientific databases:* * *...
and
Pseudotrebouxia (Trebouxiophyceae), can be found in symbiotic associations with fungi to form
lichenLichens are composite organisms consisting of a symbiotic association of a fungus with a photosynthetic partner , usually either a green alga or cyanobacterium...
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
chlorophyllChlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
s
a and
b, giving them a bright green colour (as well as the accessory pigments beta carotene and
xanthophyllXanthophylls are yellow pigments from the carotenoid group. Their molecular structure is based on carotenes; contrary to the carotenes, some hydrogen atoms are substituted by hydroxyl groups and/or some pairs of hydrogen atoms are substituted by oxygen atoms...
s), and have stacked
thylakoidA 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 consists of a thylakoid membrane surrounding a thylakoid lumen...
s.
All green algae have
mitochondriaIn cell biology, a mitochondrion is a membrane-enclosed organelle found in most eukaryotic cells. These organelles range from 0.5–10 micrometers in diameter...
with flat cristae. When present,
flagellaA flagellum is a tail-like structure that projects 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...
are typically anchored by a cross-shaped system of
microtubuleMicrotubules 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...
s, but these are absent among the higher plants and charophytes. Flagella are used to move the organism. Green algae usually have cell walls containing
celluloseCellulose is an organic compound with the formula , a polysaccharide consisting of a linear chain of several hundred to over ten thousand β linked D-glucose units....
, and undergo open
mitosisMitosis is the process by which a eukaryotic cell separates the chromosomes in its cell nucleus into two identical sets in two daughter nuclei. It is generally followed immediately by cytokinesis, which divides the nuclei, cytoplasm, organelles and cell membrane into two daughter cells containing...
without centrioles.
Origins
The chloroplasts of green algae are bound by a double membrane, so presumably they were acquired by direct
endosymbiosisThe 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...
of
cyanobacteriaCyanobacteria, also known as blue-green algae, blue-green bacteria or Cyanophyta, is a phylum of bacteria that obtain their energy through photosynthesis. The name "cyanobacteria" comes from the color of the bacteria = 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 algaeThe red algae 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...
.
Classification
Green algae are often classified with their embryophyte descendants in the green plant
cladeA clade is a term used in modern alpha taxonomy, the scientific classification of living and fossil organisms, to describe a monophyletic group, defined as a group consisting of a single common ancestor and all its descendants.[The term "monophyletic group" is used in this article...]
ViridiplantaeViridiplantae are a clade comprising the green algae and land plants.In some classification systems they have been treated as a kingdom, under various names, e.g...
(or Chlorobionta). Viridiplantae, together with red algae and
glaucophyteThe glaucophytes, also known as glaucocystophytes or glaucocystids, are a small group of freshwater microscopic algae. Together with the red algae and Viridiplantae they form the Archaeplastida...
algae, form the supergroup Primoplantae, also known as
ArchaeplastidaThe Archaeplastida are a major line of eukaryotes, comprising the land plants, green and red algae, and a small group called the glaucophytes. All of these organisms have plastids surrounded by two membranes, suggesting they developed directly from endosymbiotic cyanobacteria...
or Plantae
sensu lato.
Classification systems which have a kingdom of Protista may include green algae in the Protista or in the Plantae.
Phylogeny:
The orders outside the Chlorophyta are often grouped as the division
CharophytaThe 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...
, 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
flagellateFlagellates are cells 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...
MesostigmaMesostigma is a species of freshwater green algae which is one of the most basal of the green algae, branching from the others near the point of the split between the Streptophyta and the Chlorophyta....
, 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.
Reproduction
Green algae are eukaryotic organisms that follow a reproduction cycle called
alternation of generationsThe Alternation of phases describes the life cycle of plants, fungi and protists. A multicellular diploid phase alternates with a multicellular haploid phase. The term can be confusing for people familiar only with the life cycle of a typical animal...
.
Reproduction varies from fusion of identical cells (
isogamyIsogamy refers to a form of sexual reproduction involving gametes of similar morphology, differing only in allele expression in one or more mating-type regions. Since both gametes look alike, they cannot be classified as "male" or "female." Instead, organisms undergoing isogamy are said to have...
) to
fertilizationFertilisation , is the fusion of gametes to produce a new organism. In animals, the process involves a sperm fusing with an ovum, which eventually leads to the development of an embryo...
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
meiosisIn biology, meiosis is a process of reductional division in which the number of chromosomes per cell is cut in half. In animals, meiosis always results in the formation of gametes, while in other organisms it can give rise to spores. As with mitosis, before meiosis begins, the DNA in the original...
and releases haploid zoospores, which germinate and grow producing a haploid phase alternating with the vegetative phase.
http://www.mbari.org/staff/conn/botany/greens/anna/frontpages/lifecyc.htm
Chemistry
The green algae span a wide range of δ
13C values, with different groups having different typical ranges.
External links
See also
- Algae
Algae are a large and diverse group of simple, typically autotrophic organisms, ranging from unicellular to multicellular forms. The largest and most complex marine forms are called seaweeds. They are photosynthetic, like plants, and "simple" because they lack the many distinct organs found in...
- Brown algae
The Phaeophyceae or brown algae, is a large group of mostly marine multicellular algae, including many seaweeds of colder Northern Hemisphere waters. They play an important role in marine environments both as food, and for the habitats they form...
- Chlorophyll
Chlorophyll is a green pigment found in most plants, algae, and cyanobacteria. Its name is derived from the Greek χλωρός and φύλλον...
- Codium
Codium is a genus of seaweed in the Chlorophyta of the Order Bryopsidales. There are about 50 species world-wide. -Description:The genus has thalli of two forms, either erect or prostrate. The erect plants are dichotomously branched to 40 cm long with branches forming a compact spongy...
- Plant
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,...
s
- Prasiola
In taxonomy, Prasiola is a genus of algae, specifically of the Prasiolaceae.Prasiola is a genus of fresh water and marine green algae...
- Red algae
The red algae 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...
- Sea lettuce
The sea lettuces comprise the genus Ulva, a group of edible green algae widely distributed along the coasts of the world's oceans....
- Seaweed
SeaweedSeaweed has antioxidents. Is a loose colloquial term encompassing macroscopic, multicellular, benthic marine algae. The term includes some members of the red, brown and green algae...