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 structurally diverse: most groups of chlorophytes are
unicellular, such as the earliest-diverging
, but in two major classes (
Chlorophyceae and
Ulvophyceae) there is an evolutionary trend toward various types of complex colonies and even
multicellularity.
Chloroplasts
Chlorophyte cells contain green chloroplasts surrounded by a double-membrane envelope. These contain
chlorophyll a and
chlorophyll b, and the
carotin,
lutein,
zeaxanthin,
antheraxanthin,
violaxanthin, and
neoxanthin, which are also present in the
leaves of
. Some special carotenoids are present in certain groups, or are synthesized under specific environmental factors, such as
siphonaxanthin,
prasinoxanthin,
echinenone,
canthaxanthin,
loroxanthin, and
astaxanthin. They accumulate carotenoids under nitrogen deficiency, high irradiance of sunlight, or high salinity. In addition, they store
starch inside the chloroplast as
carbohydrate reserves. The
can appear single or in stacks. In contrast to other divisions of algae such as
Ochrophyta, chlorophytes lack a chloroplast endoplasmic reticulum.
Flagellar apparatus
Chlorophytes often form
flagellate cells that generally have two or four
flagella of equal length, although in prasinophytes heteromorphic (i.e. differently shaped) flagella are common because different stages of flagellar maturation are displayed in the same cell. Flagella have been independently lost in some groups, such as the
Chlorococcales. Flagellate chlorophyte cells have symmetrical cross-shaped ('cruciate') root systems, in which
with a variable high number of
alternate with rootlets composed of just two microtubules; this forms an arrangement known as the "X-2-X-2" arrangement, unique to chlorophytes. They are also distinguished from
by the place where their flagella are inserted: directly at the cell apex, whereas streptophyte flagella are inserted at the sides of the cell apex (sub-apically).
Below the flagellar apparatus of prasinophytes are , contractile muscle-like structures that sometimes connect with the chloroplast or the cell membrane. In core chlorophytes, this structure connects directly with the surface of the nucleus.
The surface of flagella lacks microtubular hairs, but some genera present scales or fibrillar hairs. The earliest-branching groups have flagella often covered in at least one layer of scales, if not naked.
Metabolism
Chlorophytes and streptophytes differ in the enzymes and organelles involved in
photorespiration. Chlorophyte algae use a
dehydrogenase inside the
mitochondria to process
glycolate during photorespiration. In contrast, streptophytes (including land plants) use
that contain glycolate oxidase, which converts glycolate to
glycoxylate, and the hydrogen peroxide created as a subproduct is reduced by
located in the same organelles.
Reproduction and life cycle
Asexual reproduction is widely observed in chlorophytes. Among core chlorophytes, both unicellular groups can reproduce asexually through
, wall-less zoospores, fragmentation, plain cell division, and exceptionally budding. Multicellular thalli can reproduce asexually through motile zoospores, non-motile
, autospores, filament fragmentation, differentiated resting cells, and even unmated gametes. Colonial groups can reproduce asexually through the formation of autocolonies, where each cell divides to form a colony with the same number and arrangement of cells as the parent colony.
Many chlorophytes exclusively conduct asexual reproduction, but some display sexual reproduction, which may be isogamy (i.e., of both sexes are identical), anisogamy (gametes are different) or oogamy (gametes are sperm and egg cells), with an evolutionary tendency towards oogamy. Their gametes are usually specialized cells differentiated from , although in unicellular Volvocales the vegetative cells can function simultaneously as gametes. Most chlorophytes have a diplontic life cycle (also known as zygotic), where the gametes fuse into a zygote which germinates, grows and eventually undergoes meiosis to produce haploid (gametes), similarly to and . Some exceptions display a haplodiplontic life cycle, where there is an alternation of generations, similarly to land plants. These generations can be isomorphic (i.e., of similar shape and size) or heteromorphic. The formation of reproductive cells usually does not occur in specialized cells, but some Ulvophyceae have specialized reproductive structures: gametangia, to produce gametes, and sporangia, to produce spores.
The earliest-diverging unicellular chlorophytes (prasinophytes) produce walled resistant stages called microbial cyst or 'phycoma' stages before reproduction; in some groups the cysts are as large as 230 μm in diameter. To develop them, the flagellate cells form an inner wall by discharging mucilage vesicles to the outside, increase the level of lipids in the cytoplasm to enhance buoyancy, and finally develop an outer wall. Inside the cysts, the nucleus and cytoplasm undergo cell division into numerous flagellate cells that are released by rupturing the wall. In some species these daughter cells have been confirmed to be gametes; otherwise, sexual reproduction is unknown in prasinophytes.
Ecology
Free-living
Chlorophytes are an important portion of the
phytoplankton in both freshwater and marine habitats,
carbon fixation more than a billion tons of carbon every year. They also live as multicellular
macroalgae, or
, settled along rocky ocean shores. Most species of Chlorophyta are aquatic, prevalent in both marine and freshwater environments. About 90% of all known species live in freshwater. Some species have adapted to a wide range of terrestrial environments. For example,
Chlamydomonas nivalis lives on summer alpine snowfields, and
Trentepohlia species, live attached to rocks or woody parts of trees. Several species have adapted to specialised and extreme environments, such as deserts, arctic environments,
hypersaline habitats, marine deep waters, deep-sea hydrothermal vents and habitats that experience extreme changes in temperature, light and salinity. Some groups, such as the
Trentepohliales, are exclusively found on land.
Symbionts
Several species of Chlorophyta live in
symbiosis with a diverse range of
, including
fungi (to form
),
,
Foraminifera,
and
. Some species of Chlorophyta are
heterotrophic, either free-living or
parasitic. Others are
through
phagocytosis. Two common species of the heterotrophic green alga
Prototheca are
pathogenic and can cause the disease
protothecosis in humans and animals.
With the exception of the three classes Ulvophyceae, Trebouxiophyceae and Chlorophyceae in the UTC clade, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular. Some members of the group form symbiosis relationships with protozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to form , but the majority of species are free-living. All members of the clade have motile flagellated swimming cells. Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.
Systematics
Taxonomic history
The first mention of Chlorophyta belongs to German botanist Heinrich Gottlieb Ludwig Reichenbach in his 1828 work
Conspectus regni vegetabilis. Under this name, he grouped all algae,
mosses ('musci') and
ferns ('filices'), as well as some
seed plants (
Zamia and
Cycas). This usage did not gain popularity. In 1914, Bohemian botanist
Adolf Pascher modified the name to encompass exclusively
green algae, that is, algae which contain
chlorophyll a and
chlorophyll b and store
starch in their
. Pascher established a scheme where Chlorophyta was composed of two groups: Chlorophyceae, which included algae now known as Chlorophyta, and
Conjugatae, which are now known as Zygnematales and belong to the
Streptophyta clade from which land plants evolved.
During the 20th century, many different classification schemes for the Chlorophyta arose. The Smith system, published in 1938 by American botanist Gilbert Morgan Smith, distinguished two classes: Chlorophyceae, which contained all green algae (unicellular and multicellular) that did not grow through an apical cell; and Charophyceae, which contained only multicellular green algae that grew via an apical cell and had special sterile envelopes to protect the sex organs.
With the advent of electron microscopy studies, botanists published various classification proposals based on finer cellular structures and phenomena, such as mitosis, cytokinesis, cytoskeleton, flagella and cell wall polysaccharides. British botanist proposed in 1971 a scheme which distinguishes Chlorophyta from other green algal divisions Charophyta, Prasinophyta and Euglenophyta. He included four classes of chlorophytes: Zygnemaphyceae, Oedogoniophyceae, Chlorophyceae and Bryopsidophyceae. Other proposals retained the Chlorophyta as containing all green algae, and varied from one another in the number of classes. For example, the 1984 proposal by Mattox & Stewart included five classes, while the 1985 proposal by Bold & Wynne included only two, and the 1995 proposal by Christiaan van den Hoek and coauthors included up to eleven classes.
The modern usage of the name 'Chlorophyta' was established in 2004, when phycologists Lewis & McCourt firmly separated the chlorophytes from the streptophytes on the basis of molecular phylogenetics. All green algae that were more closely related to land plants than to chlorophytes were grouped as a paraphyletic division Charophyta.
Within the green algae, the earliest-branching lineages were grouped under the informal name of "", and they were all believed to belong to the Chlorophyta clade. However, in 2020 a study recovered a new clade and division known as Prasinodermophyta, which contains two prasinophyte lineages previously considered chlorophytes. Below is a cladogram representing the current state of green algal classification:
Classification
Currently eleven chlorophyte classes are accepted, here presented in alphabetical order with some of their characteristics and
biodiversity:
-
Chlorodendrophyceae (60 species, 15 extinct): unicellular (monadoids) surrounded by an outer cell covering or theca of organic extracellular scales composed of and ketosugars. Some of these scales make up hair-like structures. Capable of asexual reproduction through cell division inside the theca. No sexual reproduction has been described. Each cell contains a single chloroplast and exhibits two flagella. Present in marine and freshwater habitats.
-
Chlorophyceae (3,974 species): either unicellular monadoids (flagellated) or (without flagella) living solitary or in varied colonial forms (including coenocyte), or multicellular filamentous (branch-like) thalli that may be ramified, or foliose (leaf-like) thalli. Cells are surrounded by a crystalline covering composed of abundant in glycine and hydroxyproline, as well as , arabinogalactan proteins, and extensin. They exhibit a haplontic life cycle with isogamy, anisogamy or oogamy. They are capable of asexual reproduction through flagellated , , or . Each cell contains a single chloroplast, a variable number of (including lack thereof), and from one to hundreds of flagella without . Present in marine, freshwater and terrestrial habitats.
-
Chloropicophyceae (8 species): unicellular solitary coccoids. Cells are surrounded by a multi-layered cell wall. No sexual or asexual reproduction has been described. Each cell contains a single chloroplast with astaxanthin and loroxanthin, and lacks pyrenoids or flagella. They are exclusively marine.
-
Mamiellophyceae (25 species): unicellular solitary monadoids. Cells are naked or covered by one or two layers of flat scales, mainly with spiderweb-like or reticulate ornamentation. Each cell contains one or rarely two chloroplasts, almost always with prasinoxanthin; two equal or unequal flagella, or just one flagellum, or lacking any flagella. If flagella are present, they can be either smooth or covered in scales in the same manner as the cells. Present in marine and freshwater habitats.
-
Nephroselmidophyceae (29 species): unicellular monadoids. Cells are covered by scales. They are capable of sexual reproduction through hologamy (fusion of entire cells), and of asexual reproduction through binary fission. Each cell contains a single cloroplast, a pyrenoid, and two flagella covered by scales. Present in marine and freshwater habitats.
-
Pedinophyceae (24 species): unicellular asymmetrical monadoids that undergo a coccoid palmelloid phase covered by mucilage. Cells lack extracellular scales, but in rare cases are covered on the posterior side by a theca. Each cell contains a single chloroplast, a pyrenoid, and a single flagellum usually covered in mastigonemes. Present in marine, freshwater and terrestrial habitats.
-
Picocystophyceae (1 species): unicellular coccoids, ovoid and trilobed in shape. Cells are surrounded by a multi-layered cell wall of poly-arabinose, mannose, galactose and glucose. No sexual reproduction has been described. They are capable of asexual reproduction through autosporulation, resulting in two or rarely four daughter cells. Each cell contains a single bilobed chloroplast with diatoxanthin and monadoxanthin, without any pyrenoid or flagella. Present in .
-
Pyramimonadophyceae (166 species, 59 extinct): unicellular monadoids or coccoids. Cells are covered by two or more layers of organic scales. No sexual reproduction has been described, but some cells with only one flagellum have been interpreted as potential gametes. Asexual reproduction has only been observed in the coccoid forms, via zoospores. Each cell contains a single chloroplast, a pyrenoid, and between 4 and 16 flagella. The flagella are covered in at least two layers of organic scales: a bottom layer of pentagonal scales organized in 24 rows, and a top layer of limuloid scales distributed in 11 rows. They are exclusively marine.
-
Trebouxiophyceae (926 species, 1 extinct): unicellular monadoids occasionally without flagella, or colonial, or ramified filamentous thalli, or living as the of lichen. Cells are covered by a cell wall of cellulose, , and β-galactofuranane. No sexual reproduction has been described with the exception of some observations of gamete fusion and presence of meiosis . They are capable of asexual reproduction through autospores or zoospores. Each cell contains a single chloroplast, a pyrenoid, and one or two pairs of smooth flagella. They are present in marine, freshwater and terrestrial habitats.
-
Ulvophyceae (2,695 species, 990 extinct): macroscopic thalli, either filamentous (which may be ramified) or foliose (composed of monostromatic or distromatic layers) or even compact tubular forms, generally multinucleate. Cells surrounded by a cell wall that may be calcified, composed of cellulose, β-manane, β-xilane, sulphated or piruvilated polysaccharides or sulphated ramnogalacturonanes, arabinogalactan proteins, and extensin. They exhibit a haplodiplontic life cycle where the alternating generations can be isomorphic or heteromorphic. They reproduce asexually via zoospores that may be covered in scales. Each cell contains a single chloroplast, and one or two pairs of flagella without mastigonemes but covered in scales. They are present in marine, freshwater and terrestrial habitats.
Evolution
In February 2020, the fossilized remains of a green alga, named
Proterocladus antiquus were discovered in the northern province of
Liaoning,
China. At around a billion years old, it is believed to be one of the oldest examples of a multicellular chlorophyte. It is currently classified as a member of order
Cladophorales, class
Ulvophyceae. In 2023, a study calculated the
molecular clock of green algae as calibrated by this fossil. The study estimated the origin of Chlorophyta within the
Mesoproterozoic era, at around 2.04–1.23 billion years ago.
Usage
Model organisms
Among chlorophytes, a small group known as the volvocine green algae is being researched to understand the origins of cell differentiation and
multicellularity. In particular, the unicellular flagellate
Chlamydomonas reinhardtii and the colonial organism
Volvox carteri are object of interest due to sharing homologous
genes that in
Volvox are directly involved in the development of two different cell types with full division of labor between swimming and reproduction, whereas in
Chlamydomonas only one cell type exists that can function as a
gamete. Other volvocine species, with intermediate characters between these two, are studied to further understand the transition towards the cellular division of labor, namely
Gonium pectorale,
Pandorina morum,
Eudorina elegans and
Pleodorina starrii.
Industrial uses
Chlorophyte
microalgae are a valuable source of
biofuel and various chemicals and products in industrial amounts, such as
,
and unsaturated fatty acids. The genus
Botryococcus is an efficient producer of hydrocarbons, which are converted into
biodiesel. Various genera (
Chlorella,
Scenedesmus,
Haematococcus,
Dunaliella and
Tetraselmis) are used as cellular factories of biomass, lipids and different vitamins for either human or animal consumption, and even for usage as pharmaceuticals. Some of their pigments are employed for cosmetics.
Footnotes
Citations
Cited literature
Further reading