''Trebouxia'' is a unicellular

green alga The green algae (singular: green alga) are a group consisting of the Prasinodermophyta and its unnamed sister which contains the Chlorophyta and Charophyta/Streptophyta. The land plants (Embryophytes) have emerged deep in the Charophyte alga as ...

.Silverside, A. J. (2009). Retrieved from http://www.bioref.lastdragon.org/Chlorophyta/''Trebouxia''.html It is a

photosynthetic Photosynthesis is a process used by plants and other organisms to convert light energy into chemical energy that, through cellular respiration, can later be released to fuel the organism's activities. Some of this chemical energy is stored in c ...

organism that can exist in almost all habitats found in

polar Polar may refer to: Geography Polar may refer to: * Geographical pole, either of two fixed points on the surface of a rotating body or planet, at 90 degrees from the equator, based on the axis around which a body rotates * Polar climate, the c ...

tropical The tropics are the regions of Earth surrounding the Equator. They are defined in latitude by the Tropic of Cancer in the Northern Hemisphere at N and the Tropic of Capricorn in the Southern Hemisphere at S. The tropics are also referred to ...

, and

temperate In geography, the temperate climates of Earth occur in the middle latitudes (23.5° to 66.5° N/S of Equator), which span between the tropics and the polar regions of Earth. These zones generally have wider temperature ranges throughout t ...

regions.Erokhina, L. G., Shatilovich, A. V., Kaminskaya, O. P., & Gilichinskii, D. A. (2004). Spectral Properties of the Green Alga ''Trebouxia'', a Phycobiont of Cryptoendolithic Lichens in the Antarctic Dry Valley. Microbiology,73(4), 420-424. doi:10.1023/b:mici.0000036987.18559Lukesova, A., & Frouz, J. (2007). Soil and Freshwater Micro-Algae as a Food Source for Invertebrates in Extreme Environments. Cellular Origin, Life in Extreme Habitats and Astrobiology Algae and Cyanobacteria in Extreme Environments,265-284. doi:10.1007/978-1-4020-6112-7_14Seckbach, J. (2007). Algae and cyanobacteria in extreme environments. Dordrecht: Springer. doi:https://doi.org/10.1007/978-1-4020-6112-7Seckbach, J. (2002). Symbiosis: Mechanisms and model systems. Dordrecht: Kluwer Academic.John, D. M., Whitton, B. A., & Brook, A. J. (2002). The freshwater algal flora of the British Isles: An identification guide to freshwater and terrestrial algae. Cambridge: Cambridge University Press. 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.Bubrick, P., Galun, M., & Frensdorff, A. (1984). Observations On Free-Living ''Trebouxia'' De Puymalyand ''Pseudotrebouxia'' Archibald, And Evidence That Both Symbionts From Xanthoria Parietina (L.) Th. Fr. Can Be Found Free-Living In Nature. New Phytologist,97(3), 455-462. doi:10.1111/j.1469-8137.1984.tb03611.x ''Trebouxia'' is the most common photobiont in extant

lichens A lichen ( , ) is a composite organism that arises from algae or cyanobacteria living among hypha, filaments of multiple fungus, fungi species in a mutualism (biology), mutualistic relationship.Honegger, R. (2018). Fossil lichens from the

Lower Devonian The Early Devonian is the first of three epochs comprising the Devonian period, corresponding to the Lower Devonian series. It lasted from and began with the Lochkovian Stage , which was followed by the Pragian from and then by the Emsian, w ...

and their bacterial and fungal epi- and endobionts. In Biodiversity and Ecology of Fungi, Lichens, and Mosses.(Vol. 34, Biosystematics and Ecology Series, pp. 547-563). Vienna: Austrian Academy of Sciences. Retrieved March 2, 2019, from https://www.researchgate.net/publication/326113052_Fossil_lichens_from_the_Lower_Devonian_and_their_bacterial_and_fungal_epi-and_endobionts It is a primary producer of marine, freshwater and terrestrial ecosystems. It uses

carotenoids Carotenoids (), also called tetraterpenoids, are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, ...

and

chlorophyll Chlorophyll (also chlorophyl) is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...

a and b to harvest energy from the sun and provide nutrients to various animals and insects. ''Trebouxia'' may have introduced photosynthesis into the terrestrial habitats approximately 450 million years ago.Horwath, W. R. (2017). The Role of the Soil Microbial Biomass in Cycling Nutrients. In T. K. Russel (Author), Microbial Biomass: A Paradigm Shift In Terrestrial Biogeochemistry(pp. 1-348). World Scientific. doi:https://doi.org/10.1142/q0038 It is also a

bioindicator A bioindicator is any species (an indicator species) or group of species whose function, population, or status can reveal the qualitative status of the environment. The most common indicator species are animals. For example, copepods and other sma ...

of habitat disturbances, freshwater quality, air pollution,

carbon dioxide Carbon dioxide (chemical formula ) is a chemical compound made up of molecules that each have one carbon atom covalently double bonded to two oxygen atoms. It is found in the gas state at room temperature. In the air, carbon dioxide is transpar ...

concentration, and climate change.Ismail, A., Marzuki, S., Yusof, N. M., Buyong, F., Said, M. M., Sigh, H., & Zulkifli, A. (2017). Epiphytic Terrestrial Algae (''Trebouxia'' sp.) as a Biomarker Using the Free-Air-Carbon Dioxide-Enrichment (FACE) System. Biology,6(4), 19. doi:10.3390/biology6010019Taylor, T. N., Krings, M., & Taylor, E. L. (2015). Lichens. Fossil Fungi,201-220. doi:10.1016/b978-0-12-387731-4.00010-4 Furthermore, its life cycle is complex and much research needs to be done to characterize it more completely.Ahmadjian, V. (1960). Some New and Interesting Species of ''Trebouxia'', a Genus of Lichenized Algae. American Journal of Botany,47(8), 677. doi:10.2307/2439519Friedl, T. (1993). New Aspects of the Reproduction by Autospores in the Lichen Alga ''Trebouxia'' (Microthamniales, Chlorophyta). Archiv Für Protistenkunde,143(1-3), 153-161. doi:10.1016/s0003-9365(11)80284-8Melkonian, M., & Peveling, E. (1987). Zoospore ultrastructure in species of''Trebouxia'' and''Pseudotrebouxia'' (Chlorophyta). Plant Systematics and Evolution,158(2-4), 183-210. doi:10.1007/bf00936344Sanders, W. B. (2005). Observing microscopic phases of lichen life cycles on transparent substrata placed in situ. The Lichenologist,37(05), 373-382. doi:10.1017/s0024282905015070 For decades, the presence of sexual reproduction was unknown.Kroken, S., & Taylor, J. W. (2000). Phylogenetic Species, Reproductive Mode, and Specificity of the Green Alga''Trebouxia''Forming Lichens with the Fungal GenusLetharia. The Bryologist,103(4), 645-660. doi:10.1639/0007-2745(2000)103 645:psrmas.0.co;2 However, recent molecular evidence of recombination and the observation of sexual fusions of

gametes A gamete (; , ultimately ) is a haploid cell that fuses with another haploid cell during fertilization in organisms that reproduce sexually. Gametes are an organism's reproductive cells, also referred to as sex cells. In species that produce t ...

to form

zygotes A zygote (, ) is a eukaryotic cell formed by a fertilization event between two gametes. The zygote's genome is a combination of the DNA in each gamete, and contains all of the genetic information of a new individual organism. In multicellula ...

suggest that sexual reproduction occurs. ''Trebouxia'' is a

paraphyletic In taxonomy (general), taxonomy, a group is paraphyletic if it consists of the group's most recent common ancestor, last common ancestor and most of its descendants, excluding a few Monophyly, monophyletic subgroups. The group is said to be pa ...

group.Friedl, T., & Zeltner, C. (1994). Assessing The Relationships Of Some Coccoid Green Lichen Algae And The Microthamniales (Chlorophyta) With 18S Ribosomal Rna Gene Sequence Comparisons1. Journal of Phycology,30(3), 500-506. doi:10.1111/j.0022-3646.1994.00500.x The

horizontal gene transfer Horizontal gene transfer (HGT) or lateral gene transfer (LGT) is the movement of genetic material between Unicellular organism, unicellular and/or multicellular organisms other than by the ("vertical") transmission of DNA from parent to offsprin ...

of protein encoding genes occurred between fungi and ''Trebouxia''.Beck, A., Divakar, P. K., Zhang, N., Molina, M. C., & Struwe, L. (2014). Evidence of ancient horizontal gene transfer between fungi and the terrestrial alga ''Trebouxia''. Organisms Diversity & Evolution,15(2), 235-248. doi:10.1007/s13127-014-0199-x There is also evidence of

intron An intron is any nucleotide sequence within a gene that is not expressed or operative in the final RNA product. The word ''intron'' is derived from the term ''intragenic region'', i.e. a region inside a gene."The notion of the cistron .e., gene. ...

among different strains of ''Trebouxia'' in lichen thalli. The presence of

globose A sphere () is a geometrical object that is a three-dimensional analogue to a two-dimensional circle. A sphere is the set of points that are all at the same distance from a given point in three-dimensional space.. That given point is the ce ...

cells in fossil lichens from the Lower Devonian period (415 million years ago) that look similar to ''Trebouxia'' indicate the significance of ''Trebouxia'' fungal

symbiosis Symbiosis (from Greek , , "living together", from , , "together", and , bíōsis, "living") is any type of a close and long-term biological interaction between two different biological organisms, be it mutualistic, commensalistic, or parasit ...

throughout the history of Earth.

History of knowledge

The genus ''Trebouxia'' was initially described by Puymaly in 1924.Puymaly, A. D. (1924). Le Chlorococcum humicola (Nag.) Rabenh. Revue Algologique, 1, 107-114 The genus name of ''Trebouxia'' is in honour of Octave Treboux (1876–ca. 1940), who was an Estonian botanist (Mykology and Algology),

plant physiologist Plant physiology is a subdiscipline of botany concerned with the functioning, or physiology, of plants. Closely related fields include plant morphology (structure of plants), plant ecology (interactions with the environment), phytochemistry (bio ...

, from the

National University of Kharkiv The Kharkiv University or Karazin University ( uk, Каразінський університет), or officially V. N. Karazin Kharkiv National University ( uk, Харківський національний університет імені ...

and Riga. The genus was divided into two genera ''Trebouxia'' and ''Pseudotrebouxia''.Nash, T. H., III. (1996). Lichen biology. Cambridge;New York, NY, USA;: Cambridge University Press Some recent studies imply that the differences between two groups are invalid and the genus should be divided in different ways such as splitting ''Trebouxia'' into two genera, ''Asterochloris'' (including photobionts of suborder Cladoniinae) and ''Trebouxia'' (including photoboints of suborder Lecanorineae). Trebouxia’s systematic location and taxonomy has been uncertain for decades. Initially, in 1995, the group was placed in the order Pleurastrales Hoek, C. v. d., Mann, D. G., & Jahns, H. M. (1995). Algae: An introduction to phycology. Cambridge;New York;: Cambridge University Press 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, in recent years, each ''Trebouxia'' species has been treated independently from lichen species since the same species of ''Trebouxia'' can be associated with many lichens. Now, the classification and nomenclature of species is based on pyrenoid structure, electron dense vesicles, chloroplast and grana shape, vegetative cell size and thickness of cell wall.Casano, L. M., Campo, E. M., García-Breijo, F. J., Reig-Armiñana, J., Gasulla, F., Hoyo, A. D., . . . Barreno, E. (2010). Two ''Trebouxia'' algae with different physiological performances are ever-present in lichen thalli of Ramalina farinacea. Coexistence versus Competition? Environmental Microbiology,13(3), 806-818. doi:10.1111/j.1462-2920.2010.02386.x

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.Anderson, O. R. (2014). Microbial Communities Associated with Tree Bark Foliose Lichens: A Perspective on their Microecology. Journal of Eukaryotic Microbiology,61(4), 364-370. doi:10.1111/jeu.12116Rascio, N., & Rocca, N. L. (2013). Biological Nitrogen Fixation. Reference Module in Earth Systems and Environmental Sciences. doi:10.1016/b978-0-12-409548-9.00685-0 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.Fell, J. W., Scorzetti, G., Connell, L., & Craig, S. (2006). Biodiversity of micro-eukaryotes in Antarctic Dry Valley soils with. Soil Biology and Biochemistry,38(10), 3107-3119. doi:10.1016/j.soilbio.2006.01.014 It can tolerate a wide range of temperatures and prolonged periods of desiccation;.Determeyer-Wiedmann, N., Sadowsky, A., Convey, P., & Ott, S. (2018). Physiological life history strategies of photobionts of lichen species from Antarctic and moderate European habitats in response to stressful conditions. Polar Biology,42(2), 395-405. doi:10.1007/s00300-018-2430-2Sadowsky, A., Mettler-Altmann, T., & Ott, S. (2016). Metabolic response to desiccation stress in strains of green algal photobionts (''Trebouxia'') from two Antarctic lichens of southern habitats. Phycologia,55(6), 703-714. doi:10.2216/15-127.1 Carotenoids such as xanthophyll astaxanthin allow ''Trebouxia'' to tolerate high irradiance.Bidigare, R. R., Ondrusek, M. E., Kennicutt, M. C., Iturriaga, R., Harvey, H. R., Hoham, R. W., & Macko, S. A. (1993). Evidence A Photoprotective For Secondary Carotenoids Of Snow Algae1. Journal of Phycology,29(4), 427-434. doi:10.1111/j.1529-8817.1