Chlorophytes areeukaryotic organisms composed of cells with a variety of coverings or walls, and usually a single greenchloroplast in each cell.[4] They are structurally diverse: most groups of chlorophytes areunicellular, such as the earliest-divergingprasinophytes, but in two major classes (Chlorophyceae andUlvophyceae) there is an evolutionary trend toward various types of complexcolonies and evenmulticellularity.[8]
Chlorophytes often formflagellate cells that generally have two or fourflagella 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.[13] Flagella have been independently lost in some groups, such as theChlorococcales.[8] Flagellate chlorophyte cells have symmetrical cross-shaped ('cruciate') root systems, in whichciliary rootlets with a variable high number ofmicrotubules alternate with rootlets composed of just two microtubules; this forms an arrangement known as the "X-2-X-2" arrangement, unique to chlorophytes.[14] They are also distinguished fromstreptophytes 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).[15]
Below the flagellar apparatus of prasinophytes arerhizoplasts, contractile muscle-like structures that sometimes connect with the chloroplast or the cell membrane.[13] In core chlorophytes, this structure connects directly with the surface of the nucleus.[16]
The surface of flagella lacks microtubular hairs, but some genera present scales or fibrillar hairs.[11] The earliest-branching groups have flagella often covered in at least one layer of scales, if not naked.[13]
Chlorophytes and streptophytes differ in the enzymes and organelles involved inphotorespiration. Chlorophyte algae use adehydrogenase inside themitochondria to processglycolate during photorespiration. In contrast, streptophytes (including land plants) useperoxisomes that containglycolate oxidase, which converts glycolate toglycoxylate, and the hydrogen peroxide created as a subproduct is reduced bycatalases located in the same organelles.[17]
Asexual reproduction is widely observed in chlorophytes. Among core chlorophytes, both unicellular groups can reproduce asexually throughautospores,[18] wall-less zoospores,[19] fragmentation, plain cell division, and exceptionally budding.[20] Multicellular thalli can reproduce asexually through motile zoospores,[21] non-motileaplanospores, autospores, filament fragmentation,[22] differentiated resting cells,[23] and even unmated gametes.[24] 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.[25]
Many chlorophytes exclusively conduct asexual reproduction, but some displaysexual reproduction, which may beisogamous (i.e.,gametes of both sexes are identical),anisogamous (gametes are different) oroogamous (gametes are sperm and egg cells), with an evolutionary tendency towards oogamy. Their gametes are usually specialized cells differentiated fromvegetative cells, although in unicellularVolvocales the vegetative cells can function simultaneously as gametes. Most chlorophytes have adiplontic life cycle (also known as zygotic), where the gametes fuse into azygote which germinates, grows and eventually undergoesmeiosis to producehaploidspores (gametes), similarly toochrophytes andanimals. Some exceptions display ahaplodiplontic life cycle, where there is an alternation of generations, similarly to land plants.[26] These generations can be isomorphic (i.e., of similar shape and size) or heteromorphic.[27] The formation of reproductive cells usually does not occur in specialized cells,[28] but someUlvophyceae have specialized reproductive structures: gametangia, to produce gametes, and sporangia, to produce spores.[27]
The earliest-diverging unicellular chlorophytes (prasinophytes) produce walled resistant stages calledcysts 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 enhancebuoyancy, and finally develop an outer wall. Inside the cysts, the nucleus and cytoplasm undergodivision 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.[29]
Chlorophytes are an important portion of thephytoplankton in both freshwater and marine habitats,fixating more than a billion tons of carbon every year. They also live as multicellularmacroalgae, orseaweeds, settled along rocky ocean shores.[8] Most species of Chlorophyta are aquatic, prevalent in both marine and freshwater environments. About 90% of all known species live in freshwater.[30] Some species have adapted to a wide range of terrestrial environments. For example,Chlamydomonas nivalis lives on summer alpine snowfields, andTrentepohlia species, live attached to rocks or woody parts of trees.[31][32] Several species have adapted to specialised and extreme environments, such as deserts, arctic environments,hypersaline habitats, marine deep waters, deep-seahydrothermal vents and habitats that experience extreme changes in temperature, light and salinity.[33][34][35] Some groups, such as theTrentepohliales, are exclusively found on land.[36][37]
With the exception of the three classesUlvophyceae,Trebouxiophyceae andChlorophyceae in theUTC clade, which show various degrees of multicellularity, all the Chlorophyta lineages are unicellular.[42] Some members of the group formsymbiotic relationships withprotozoa, sponges, and cnidarians. Others form symbiotic relationships with fungi to formlichens, but the majority of species are free-living. All members of the clade have motile flagellated swimming cells.[43]Monostroma kuroshiense, an edible green alga cultivated worldwide and most expensive among green algae, belongs to this group.
The first mention of Chlorophyta belongs to German botanistHeinrich Gottlieb Ludwig Reichenbach in his 1828 workConspectus regni vegetabilis. Under this name, he grouped all algae,mosses ('musci') andferns ('filices'), as well as someseed plants (Zamia andCycas).[44] This usage did not gain popularity. In 1914, Bohemian botanistAdolf Pascher modified the name to encompass exclusivelygreen algae, that is, algae which containchlorophyllsa andb and storestarch in theirchloroplasts.[45] Pascher established a scheme where Chlorophyta was composed of two groups: Chlorophyceae, which included algae now known as Chlorophyta, andConjugatae, which are now known as Zygnematales and belong to theStreptophyta clade from which land plants evolved.[3][46]
During the 20th century, many different classification schemes for the Chlorophyta arose. TheSmith system, published in 1938 by American botanistGilbert Morgan Smith, distinguished two classes: Chlorophyceae, which contained all green algae (unicellular and multicellular) that did not grow through anapical 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.[47]
With the advent ofelectron microscopy studies, botanists published various classification proposals based on finer cellular structures and phenomena, such asmitosis,cytokinesis,cytoskeleton,flagella andcell wallpolysaccharides.[48][49] British botanistFrank Eric Round [nl] 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.[50] 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,[48] while the 1985 proposal by Bold & Wynne included only two,[51] and the1995 proposal by Christiaan van den Hoek and coauthors included up to eleven classes.[45]
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 ofmolecular phylogenetics. All green algae that were more closely related to land plants than to chlorophytes were grouped as aparaphyleticdivisionCharophyta.[46]
Within the green algae, the earliest-branching lineages were grouped under the informal name of "prasinophytes", and they were all believed to belong to the Chlorophyta clade.[46] However, in 2020 a study recovered a new clade and division known asPrasinodermophyta, which contains two prasinophyte lineages previously considered chlorophytes.[52] Below is a cladogram representing the current state of green algal classification:[53][52][54][55]
Representatives of all living classes of chlorophytes
Tetraselmis suecica (Chlorodendrophyceae)
Volvox aureus (Chlorophyceae)
Chloropicon sieburthii (Chloropicophyceae)
Micromonas pusilla (Mamiellophyceae)
Nephroselmis olivacea (Nephroselmidophyceae)
Mantoniella tinhauana (Pedinophyceae)
Picocystis salinarum (Picocystophyceae)
Pyramimonas longicauda (Pyramimonadophyceae)
Coccomyxa polymorpha (Trebouxiophyceae)
Ulva lactuca (Ulvophyceae)
Currently eleven chlorophyte classes are accepted, here presented in alphabetical order with some of their characteristics andbiodiversity:
Chlorodendrophyceae (60 species, 15 extinct):[6] unicellularflagellates (monadoids) surrounded by an outer cell covering ortheca of organic extracellular scales composed ofproteins and ketosugars. Some of these scales make up hair-like structures. Capable ofasexual reproduction through cell division inside the theca. Nosexual reproduction has been described. Each cell contains a single chloroplast and exhibits twoflagella. Present in marine and freshwater habitats.[56][57][58]
Chloropicophyceae (8 species):[6] unicellular solitary coccoids. Cells are surrounded by a multi-layeredcell wall. No sexual or asexual reproduction has been described. Each cell contains a single chloroplast withastaxanthin andloroxanthin, and lacks pyrenoids or flagella. They are exclusively marine.[53]
Mamiellophyceae (25 species):[6] 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 withprasinoxanthin; 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.[63][57]
Nephroselmidophyceae (29 species):[6] unicellular monadoids. Cells are covered by scales. They are capable of sexual reproduction throughhologamy (fusion of entire cells), and of asexual reproduction throughbinary fission. Each cell contains a single chloroplast, a pyrenoid, and two flagella covered by scales. Present in marine and freshwater habitats.[64][65][57]
Pedinophyceae (24 species):[6] unicellular asymmetrical monadoids that undergo a coccoid palmelloid phase covered bymucilage. 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.[66][57][67]
Picocystophyceae (1 species):[6] unicellular coccoids, ovoid and trilobed in shape. Cells are surrounded by a multi-layered cell wall of poly-arabinose,mannose,galactose andglucose. 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 insaline lakes.[68][53][57]
Pyramimonadophyceae (166 species, 59 extinct):[6] 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.[57][69]
Trebouxiophyceae (926 species, 1 extinct):[6] unicellular monadoids occasionally without flagella, or colonial, or ramified filamentous thalli, or living as thephotobionts oflichen. Cells are covered by a cell wall ofcellulose,algaenans, and β-galactofuranane. No sexual reproduction has been described with the exception of some observations of gamete fusion and presence ofmeioticgenes. 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.[59][70][4][71]
Ulvophyceae (2,695 species, 990 extinct):[6] macroscopic thalli, either filamentous (which may be ramified) or foliose (composed of monostromatic or distromatic layers) or even compact tubular forms, generallymultinucleate. Cells surrounded by a cell wall that may becalcified, composed of cellulose, β-manane, β-xilane, sulphated or piruvilated polysaccharides or sulphated ramnogalacturonanes, arabinogalactan proteins, and extensin. They exhibit ahaplodiplontic 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.[59][4][72]
In February 2020, the fossilized remains of a green alga, namedProterocladus antiquus were discovered in the northern province ofLiaoning,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 orderSiphonocladales, classUlvophyceae.[1] In 2023, a study calculated themolecular age of green algae ascalibrated by this fossil. The study estimated the origin of Chlorophyta within theMesoproterozoic era, at around 2.04–1.23 billion years ago.[55]
Among chlorophytes, a small group known as the volvocine green algae is being researched to understand the origins ofcell differentiation andmulticellularity. In particular, the unicellular flagellateChlamydomonas reinhardtii and the colonial organismVolvox carteri are object of interest due to sharinghomologousgenes that inVolvox are directly involved in the development of two different cell types with full division of labor between swimming and reproduction, whereas inChlamydomonas only one cell type exists that can function as agamete. Other volvocine species, with intermediate characters between these two, are studied to further understand the transition towards the cellular division of labor, namelyGonium pectorale,Pandorina morum,Eudorina elegans andPleodorina starrii.[73]
Chlorophytemicroalgae are a valuable source ofbiofuel and various chemicals and products in industrial amounts, such ascarotenoids,vitamins andunsaturated fatty acids. The genusBotryococcus is an efficient producer of hydrocarbons, which are converted intobiodiesel. Various genera (Chlorella,Scenedesmus,Haematococcus,Dunaliella andTetraselmis) 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.[74]
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