Trebouxia is a unicellular green alga. It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and temperate regions. It can either exist in a symbiotic relationship with fungi in the form of lichen or it can survive independently as a free-living organism alone or in colonies. Trebouxia is the most common photobiont in extant lichens. It is a primary producer of marine, freshwater and terrestrial ecosystems. It uses carotenoids and chlorophyll a and b to harvest energy from the sun and provide nutrients to various animals and insects. An ancestor of Trebouxia may have introduced photosynthesis into terrestrial habitats approximately 450 million years ago. It is also a bioindicator of habitat disturbances, freshwater quality, air pollution, carbon dioxide concentration, and climate change. Furthermore, its life cycle is complex and much research needs to be done to characterize it more completely. For decades, the presence of sexual reproduction was unknown. However, recent (2000s) molecular evidence of recombination and the observation of sexual fusions of gametes to form zygotes suggest that sexual reproduction occurs. Trebouxia (as circumscribed in 1994) is a paraphyletic group; the issue was resolved by moving some members to Asterochloris. Horizontal gene transfer of protein encoding genes between fungi and Trebouxia is known to have occurred. There is also evidence of intron horizontal gene transfer among different strains of Trebouxia in lichen thalli. The presence of globose cells in fossil lichens from the Lower Devonian period (415 million years ago) that look similar to Trebouxia indicate the significance of Trebouxia-like fungal symbiosis throughout the terrestrial history of Earth.
History of knowledge The genus Trebouxia was initially circumscribed by Puymaly in 1924. The type species of the genus is Trebouxia arboricola. The genus name of Trebouxia honours Octave Treboux (1876–ca. 1940), who was an Estonian botanist and plant physiologist, from the National University of Kharkiv and Riga. The genus was divided into two genera Trebouxia and Pseudotrebouxia. Some recent (2000s) studies imply that the differences between two groups are invalid. Trebouxia should instead be divided in different ways such as splitting Trebouxia into two genera, Asterochloris (including photobionts of suborder Cladoniinae) and Trebouxia (including photobionts of suborder Lecanorineae). The split to Asterochloris was formally done in 2010. The remaining species of Trebouxia are known to occur in four clades in molecular analysis, termed "A", "C", "I", and "S". A new "D" clade was found in 2020. Trebouxia’s systematic location and taxonomy has been uncertain for decades. Initially, in 1995, the group was placed in the order Pleurastrales and then in Microthamniales. Later in 2002, it was part of the order Chlorococcales and now it is placed in the order Trebouxiales. It is unknown whether all photobionts described as “trebouxioid” belong to a single genus. Also, it is also unclear how many and which species should be accepted and recognized. Furthermore, in earlier years, classification and nomenclature of species was based on organism’s color, size, growth and shape of colonies, texture, and the lichen it was isolated from. It was believed that each algae species belonged to a specific lichen species. However, since the 1960s, each Trebouxia species has been treated independently from lichen species since the same species of Trebouxia can be associated with many lichens. Later, classification and nomenclature of species was based on morphological characteristics such as chloroplast shape and pyrenoid structure. Currently Trebouxia species are delimited based on a combination of different characteristics, such as morphological, physiological, and molecular data. As of 2020, most of the diversity within Trebouxia has yet to be formally described.
Habitat and ecology Trebouxia is a photosynthetic autotrophic genus that can exist in almost every environmental condition in nature. It can be found in the tropics, Arctic, Antarctic, boreal forest, fresh water, marine, bare rocks, wood debris, tree bark, sandstone, soil, hot and semi-arid deserts. Some species can live in extreme conditions such as dry valleys of Antarctica with less than 5% soil moisture or habitats that are rich in iron and metals. It can tolerate a wide range of temperatures and prolonged periods of desiccation;. Carotenoids such as xanthophyll astaxanthin allow Trebouxia to tolerate high irradiance. Furthermore, Trebouxia can exist in its free-living form or in a lichen thallus as a photobiont partner with its fungi mycobiont. The release or escape of alga zoospores from intact lichens is a source of free-living algae colonies or single free-living cells. Moreover, the same Trebouxia species can be associated with many mycobiont species or many Trebouxia strains can inhabit single lichen. However, the maturation of the lichen could lead to the elimination of all Trebouxia strains except one. Also, Trebouxia species are not selective towards their fungal symbionts while fungal species are very selective regarding their algae partners. In areas where algae species are scarce, fungi are less selective and forms a symbiotic relationship with any Trebouxia species and later on switch to a more suitable algae species. Some Trebouxia species are highly dependent on their fungal partners and cannot exist as independent organisms. Fungi obtain nutrients through self parasitism or selectively harvesting old Trebouxia cells. Trebouxia, on the other hand, provides 90% of its photosynthetic products to the mycoboint. Pyrenoglobuli (lipid rich stores in the pyrenoid of Trebouxia) are used by the mycoboint for energy and water. Trebouxia acts as an important primary producer in freshwater, marine, and terrestrial ecosystems. Trebouxia uses carotenoids and chlorophyll a and b to harvest energy from the sun and synthesize organic compounds that serve as a substantial food source for a wide range of heterotrophs including animals, invertebrates and insects.
Description of the organism
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