Trebouxiophyceae
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The Trebouxiophyceae, also known as trebouxiophytes, are a
class Class, Classes, or The Class may refer to: Common uses not otherwise categorized * Class (biology), a taxonomic rank * Class (knowledge representation), a collection of individuals or objects * Class (philosophy), an analytical concept used d ...
of
green algae The green algae (: green alga) are a group of chlorophyll-containing autotrophic eukaryotes consisting of the phylum Prasinodermophyta and its unnamed sister group that contains the Chlorophyta and Charophyta/ Streptophyta. The land plants ...
, in the division
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 ...
. Members of this class are single-celled, colonial, or multicellular and are found in freshwater, terrestrial or marine habitats worldwide. Many taxa in the Trebouxiophyceae form
symbiotic relationship Symbiosis (Ancient Greek : living with, companionship < : together; and ''bíōsis'': living) is any type of a close and long-term biolo ...
s with other organisms; in particular, the majority of phycobionts within
lichens 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 ...
are trebouxiophytes. A number of taxa have also lost the ability to photosynthesize, and have evolved to become parasitic; examples include '' Prototheca'' and '' Helicosporidium''. Trebouxiophyceae was originally defined by ultrastructural characteristics, but is now generally circumscribed based on
phylogenetic In biology, phylogenetics () is the study of the evolutionary history of life using observable characteristics of organisms (or genes), which is known as phylogenetic inference. It infers the relationship among organisms based on empirical dat ...
s, particularly based on the 18S rDNA locus. As of 2024, Trebouxiophyceae contains 211 genera and about 925 species.


Morphology

Members of the Trebouxiophyceae are microscopic or macroscopic organisms which exist in a variety of forms: non-flagellate coccoid or elliptical single cells, unbranched filaments, blades, or colonies of cells. Although most species are
photosynthetic Photosynthesis ( ) is a Biological system, system of biological processes by which Photoautotrophism, photosynthetic organisms, such as most plants, algae, and cyanobacteria, convert light energy, typically from sunlight, into the chemical ener ...
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, a few genera ('' Helicosporidium'' and '' Prototheca'') lack them. Chloroplasts are diverse in morphology and placement and may be axial and stellate (e.g. '' Prasiola''), numerous and discoid (e.g. '' Eremosphaera''), or parietal. Pyrenoids may be present or absent. In most species, reproduction occurs exclusively asexual, and occurs via the formation of autospores or zoospores. Sexual reproduction is rare in the class, with reports from only a few taxa;
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 ...
,
anisogamy Different forms of anisogamy: A) anisogamy of motile cells, B) 283x283px Anisogamy is a form of sexual reproduction">egg cell">oogamy (egg cell and sperm cell), C) anisogamy of non-motile cells (egg cell and spermatia).">283x283px Anisogamy is ...
and
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 ...
have all been reported. However, many more taxa possess the genes for sexual reproduction, suggesting it is more common than previously thought.


Ultrastructure

Trebouxiophyceaen algae are characterized by a combination of plesiomorphic traits. It has basal bodies of its
flagella A flagellum (; : flagella) (Latin for 'whip' or 'scourge') is a hair-like appendage that protrudes from certain plant and animal sperm cells, from fungal spores ( zoospores), and from a wide range of microorganisms to provide motility. Many pr ...
in a counterclockwise orientation, non-persistent, metacentric spindles during telophase, and phycoplast-mediated cytokinesis. The presence of counterclockwise basal bodies are shared with
Ulvophyceae The Ulvophyceae or ulvophytes are a class of green algae, distinguished mainly on the basis of Ultrastructure, ultrastructural morphology, Biological life cycle, life cycle and molecular phylogenetics, molecular phylogenetic data. The sea lettu ...
, the metacentric spindles are shared with walled prasinophytes, and the non-persistent spindles and phycoplasts are also present in Chlorophyceae. Some members do not produce flagellated stages at all.


Ecology

Trebouxiophycean algae are common and widespread, and found in a variety of habitats. Terrestrial species live most commonly in places such rocks, soils or tree bark. However, some genera, such as '' Dictyosphaerium'', are
planktonic Plankton are the diverse collection of organisms that drift in water (or air) but are unable to actively propel themselves against currents (or wind). The individual organisms constituting plankton are called plankters. In the ocean, they pro ...
within
fresh water Fresh water or freshwater is any naturally occurring liquid or frozen water containing low concentrations of dissolved salt (chemistry), salts and other total dissolved solids. The term excludes seawater and brackish water, but it does include ...
, and may dominate the water column from early spring to late summer. Marine trebouxiophycean algae such as '' Prasiola'' typically occupy the shoreline, and may exhibit tolerance to freezing. In these harsh environments,
ultraviolet Ultraviolet radiation, also known as simply UV, is electromagnetic radiation of wavelengths of 10–400 nanometers, shorter than that of visible light, but longer than X-rays. UV radiation is present in sunlight and constitutes about 10% of ...
-absorbing compounds or cryoprotectants such as proline are often present.


Symbioses

Trebouxiophyceae is most well known for the ability to form symbiotic relationships 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 ...
, forming lichens. When associated with a lichen-forming fungus, the alga is termed a phycobiont. The fungus benefits from the
carbohydrate A carbohydrate () is a biomolecule composed of carbon (C), hydrogen (H), and oxygen (O) atoms. The typical hydrogen-to-oxygen atomic ratio is 2:1, analogous to that of water, and is represented by the empirical formula (where ''m'' and ''n'' ...
s provided by the algae, while the algae inhabit a matrix of
hyphae A hypha (; ) is a long, branching, filamentous structure of a fungus, oomycete, or actinobacterium. In most fungi, hyphae are the main mode of vegetative growth, and are collectively called a mycelium. Structure A hypha consists of one o ...
which provide protection. ''Trebouxia'' (also the namesake of the class) and '' Asterochloris'' are the most common and widespread phycobionts of lichens; it is estimated that ''Trebouxia'' is associated with over 20% of all lichen-forming fungi worldwide. Research on lichens has historically focused more on the fungal partner (the mycobiont) over the phycobionts. Research on trebouxiophycean phycobionts has been hampered by the time-consuming nature of identification. Because the morphology of these algae is often heavily influenced by the environment, reliable identification relies on molecular techniques such as DNA barcoding. However, recent research has discovered considerable diversity. It was previously thought that a specific mycobiont associates with only one type of phycobiont; however, it is now accepted that multiple trebouxiophycean algae can associate with a single species or even a single thallus. In addition to lichens, single-celled microalgae (called zoochlorellae) are commonly found as
endosymbiont An endosymbiont or endobiont is an organism that lives within the body or cells of another organism. Typically the two organisms are in a mutualism (biology), mutualistic relationship. Examples are nitrogen-fixing bacteria (called rhizobia), whi ...
s within a variety of freshwater and marine organisms. Endosymbionts come from several genera, mainly '' Micractinium'' and ''Chlorella''. The host organisms are diverse and include ciliates (e.g. '' Paramecium''),
sea anemone Sea anemones ( ) are a group of predation, predatory marine invertebrates constituting the order (biology), order Actiniaria. Because of their colourful appearance, they are named after the ''Anemone'', a terrestrial flowering plant. Sea anemone ...
s, '' Hydra'' and freshwater sponges. '' Paramecium bursaria'' is a well-studied example and
model organism A model organism is a non-human species that is extensively studied to understand particular biological phenomena, with the expectation that discoveries made in the model organism will provide insight into the workings of other organisms. Mo ...
for endosymbiosis. The hosts may have an obligate or a facultative relationship with the symbionts.


Parasitism

''Prototheca'' and ''Helicosporidium'' are two unusual examples of once-algal organisms which have lost their chloroplasts and become parasitic. ''Prototheca'' infects vertebrates (including humans), and induces protothecosis; ''Helicosporidium'' infects a wide array of
invertebrate Invertebrates are animals that neither develop nor retain a vertebral column (commonly known as a ''spine'' or ''backbone''), which evolved from the notochord. It is a paraphyletic grouping including all animals excluding the chordata, chordate s ...
s. '' Phyllosiphon'' is an unusual genus, consisting of branched siphonous filaments with chloroplasts. It is an endophytic parasite occurring in leaves, particularly the family Araceae. Desmococcus on beech 02.jpg, '' Desmococcus'', a terrestrial alga growing on the bark of a beech tree Eremosphaera viridis 361793586.jpg, '' Eremosphaera'', found in freshwater plankton Prasiola stipitata Suhr (AM AK307212-6).jpg, '' Prasiola stipitata'' growing on a rocky shore Trebouxia 2 - Miguel Varona - Cuaderno de Campo del Treparriscos.jpg, ''Trebouxia'', a common phycobiont of lichens Paramecium bursaria.jpg, The ciliate ''Paramecium bursaria'' with endosymbiotic trebouxiophycean algae Anthopleura xanthogrammica 1.jpg, '' Anthopleura xanthogrammica''; the green color is due to endosymbiotic '' Elliptochloris marina'' Prototheca wickerhamii.GMS.jpg, Photomicrograph (stained) of '' Prototheca wickerhamii'' infection in a human Phyllosiphon arisari 244841554.jpg, Parasitic alga '' Phyllosiphon arisari'' causing yellow spots on '' Arisarum''


Evolution and phylogeny

The clade Trebouxiophyceae likely originated about 600–800 million years ago, and fossils from trebouxiophycean algae have appeared in sediments dating back to at least the
Precambrian The Precambrian ( ; or pre-Cambrian, sometimes abbreviated pC, or Cryptozoic) is the earliest part of Earth's history, set before the current Phanerozoic Eon. The Precambrian is so named because it preceded the Cambrian, the first period of t ...
. Fossil deposits of '' Botryococcus'' are a major component of oil shales. The closest relatives of Trebouxiophyceae are Chlorophyceae and Ulvophyceae; together, they form a monophyletic group termed the UTC clade (also known as core Chlorophyta). The ancestral trebouxiophycean alga was likely a sexual organism; later lineages appear to have independently lost the ability to reproduce sexually many times. It is hypothesized that the production of autospores became advantageous in terrestrial environments, since flagellated cells require water for movement.


Taxonomy

The taxonomy of algae has traditionally been based on morphological characters; however, microalgae typically have few morphological characters, and therefore morphological classifications are limited by
convergent evolution Convergent evolution is the independent evolution of similar features in species of different periods or epochs in time. Convergent evolution creates analogous structures that have similar form or function but were not present in the last comm ...
and cryptic diversity. Therefore, modern taxonomic classifications involve an integrative species concept combining morphological and molecular data. Higher-level relationships within Trebouxiophyceae are not yet fully resolved. As of 2025, AlgaeBase accepts the following orders: * Chlorellales * Microthamniales * Phyllosiphonales * Prasiolales * Trebouxiales * Watanabeales Genera and families without intervening taxonomy include: * Coccomyxaceae * DictyosphaeriaceaeIn AlgaeBase the type genus, ''Dictyosphaerium'', is placed within Chlorellaceae. * MicractiniaceaeIn AlgaeBase the type genus, ''Micractinium'', is placed within Chlorellaceae. * Ragelichloridaceae * '' Apatococcus'' * '' Autumnella'' * '' Chlorolobion'' * '' Chloropyrula'' * ''
Elliptochloris ''Elliptochloris'' is a genus of green algae in the order Prasiolales. Species of this genus are common and found in a variety of terrestrial habitats such as soils. Some species in the genus are partners in lichens. One species, '' E. marina ...
'' * '' Eremochloris'' * '' Glaphyrella'' * '' Koliellopsis'' * '' Lemmermannia'' * '' Leptochlorella'' * '' Leptosira'' * '' Lunachloris'' * '' Neochlorella'' * '' Rhopalosolen'' * '' Xerochlorella'' * '' Xylochlorella''


Usage

The microalga ''Chlorella'' has been used by researchers to study basic elements of
biochemistry Biochemistry, or biological chemistry, is the study of chemical processes within and relating to living organisms. A sub-discipline of both chemistry and biology, biochemistry may be divided into three fields: structural biology, enzymology, a ...
and
physiology Physiology (; ) is the science, scientific study of function (biology), functions and mechanism (biology), mechanisms in a life, living system. As a branches of science, subdiscipline of biology, physiology focuses on how organisms, organ syst ...
, as an simpler analog of
land plant The embryophytes () are a clade of plants, also known as Embryophyta (Plantae ''sensu strictissimo'') () or land plants. They are the most familiar group of photoautotrophs that make up the vegetation on Earth's dry lands and wetlands. Embryophyt ...
s. Biochemical research involving ''Chlorella'' has resulted in two Nobel Prizes: in 1931, Otto Heinrich Warburg was awarded the Nobel Prize in Physiology or Medicine for his research on
cellular respiration Cellular respiration is the process of oxidizing biological fuels using an inorganic electron acceptor, such as oxygen, to drive production of adenosine triphosphate (ATP), which stores chemical energy in a biologically accessible form. Cell ...
, and in 1961, Melvin Calvin was awarded the Nobel Prize in Chemistry for studying
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
assimilation in
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 ...
s, using ''Chlorella'' as a model. Some trebouxiophycean microalgae are under interest as potential sources of
biofuel Biofuel is a fuel that is produced over a short time span from Biomass (energy), biomass, rather than by the very slow natural processes involved in the formation of fossil fuels such as oil. Biofuel can be produced from plants or from agricu ...
s or other products, such as proteins and lipids. ''Botryococcus'' is of particular interest as a producer for biodiesel, since it produces a high amount of
lipid Lipids are a broad group of organic compounds which include fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The functions of lipids include storing ...
s. A few algae in Trebouxiophyceae are edible, such as '' Prasiola'', which is edible and locally harvested for food
Japan Japan is an island country in East Asia. Located in the Pacific Ocean off the northeast coast of the Asia, Asian mainland, it is bordered on the west by the Sea of Japan and extends from the Sea of Okhotsk in the north to the East China Sea ...
and
Myanmar Myanmar, officially the Republic of the Union of Myanmar; and also referred to as Burma (the official English name until 1989), is a country in northwest Southeast Asia. It is the largest country by area in Mainland Southeast Asia and has ...
. In the mid-twentieth century, ''Chlorella'' was seen as an economical source of food and an answer to the global food crises of the time. Since then, improvements in crop yield have caused a decline in interest in ''Chlorella'' as food; however, it has a small market as a niche nutritional supplement.


See also

* List of Trebouxiophyceae genera


Notes


References

{{Taxonbar, from=Q144474 Green algae classes