Trebouxia is a unicellular green alga.[1] It is a photosynthetic organism that can exist in almost all habitats found in polar, tropical, and temperate regions.[2][3][4][5][6] 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.[7] Trebouxia is the most common photobiont in extant lichens.[8] It is a primary producer of marine, freshwater and terrestrial ecosystems.[3] It uses carotenoids and chlorophyll a and b to harvest energy from the sun and provide nutrients to various animals and insects.[2][4]
Trebouxia may have introduced photosynthesis into the terrestrial habitats approximately 450 million years ago.[9] It is also a bioindicator of habitat disturbances, freshwater quality, air pollution, carbon dioxide concentration, and climate change.[10][11] Furthermore, its life cycle is complex and much research needs to be done to characterize it more completely.[12][13][14][15][1][5] For decades, the presence of sexual reproduction was unknown.[16] However, recent molecular evidence of recombination and the observation of sexual fusions of gametes to form zygotes suggest that sexual reproduction occurs.[5]
Trebouxia is a paraphyletic group.[17] The horizontal gene transfer of protein encoding genes occurred between fungi and Trebouxia.[18][5] There is also evidence of intron horizontal gene transfer among different strains of Trebouxia in lichen thalli.[5] 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 fungal symbiosis throughout the history of Earth.[8]
The genus Trebouxia was initially described by Puymaly in 1924.[19] The genus was divided into two genera Trebouxia and Pseudotrebouxia.[20][1] 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).[20][1][5]
Trebouxia’s systematic location and taxonomy has been uncertain for decades. Initially, in 1995, the group was placed in the order Pleurastrales [21] and then in Microthamniales.[1] Later in 2002, it was part of the order Chlorococcales [6] and now it is placed in the order Trebouxiales.[1] It is unknown whether all photobionts described as “trebouxioid” belong to a single genus.[1] Also, it is also unclear how many and which species should be accepted and recognized.[1]
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.[12] It was believed that each algae species belonged to a specific lichen species.[12] 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.[12] 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.[22]