[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.

[2][4] An ancestor of Trebouxia may have introduced photosynthesis into 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.

[19][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-like fungal symbiosis throughout the terrestrial history of Earth.

[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] 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.

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.

[27][2][28][4][5] 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.

[32][4] Furthermore, Trebouxia can exist in its free-living form or in a lichen thallus as a photobiont partner with its fungi mycobiont.

[3] 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.

[12][14] They contain one contractile vacuole, nucleus, dictyosome, chloroplast, and single mitochondrial reticulum or branched mitochondria linked to microbody.

Zoospores are flagellated motile stages within lichens that migrate and settle near fungal spores when liquid water is present.

These differentiated vegetative cells are later transformed into zoosporangium/ autosporangia with numerous small autospores, but without adhering packages or tetrad formation.

[5] Initially, it was thought that the fungus suppresses sexual reproduction in Trebouxia to inhibit the formation of novel genotypes that could be less suitable for symbiosis.

[5] Viruses that infect Trebouxia and fungi also aid in horizontal gene transfer among different phycobiont species.

Its rDNA (internal transcribed spacer regions recombinant DNA) sequence comparisons with the aid of Polymerase chain reaction (PCR) are easy and fast ways to identify different Trebouxia species that inhabit a thallus.

The fossil lichens from the Lower Devonian (415 million years ago) are composed of algae or cyanobacteria and fungi layers.

Lichens, along with few other organisms, introduced nitrogen fixation and photosynthesis into terrestrial environments approximately 450 million years ago.

[9] They played a significant role in making the harsh terrestrial environment suitable for the colonization of other organisms such as land plants, animals, and insects.

[9] Even today lichens make many unsuitable extreme habitats more suitable for species to colonize and survive.

[9] Furthermore, lichens are a critical bioindicators of habitat disturbances, freshwater quality, air pollution, carbon dioxide measures, and long term ecological continuity of undisturbed forests.

[11] Moreover, lichens are critical in climate change and global warming studies to understand the effects of increasing greenhouse gasses such as carbon dioxide in natural environments.

[10] The position of Trebouxia at the base of the food chain as a primary producer is critical for the maintenance of freshwater, marine, and terrestrial ecosystems.