Pictures of Trebouxiophyceae:                      

The class Trebouxiophyceae, named after the type genus Trebouxia with the suffix -phyceae, meaning "alga," is a group of algae with worldwide distribution that inhabits freshwater, marine environments, terrestrial habitats, or exists in symbiosis within other organisms.

They are generally microscopic algae, though some species can form thalli visible to the naked eye. The vast majority of species are unicellular or form colonies of a few cells, often with the cells being more or less rounded and lacking noticeable ornamentations or structures. Reproduction is mostly asexual, occurring through simple division, by the formation of new cells within the parent cell, by spores, or producing swimming cells. In the stages of the life cycle where they possess flagella, they appear in apical pairs, with basal bodies displaced in a clockwise direction. Cell division occurs through the formation of a phycoplast, and when centrioles are present, they appear at the equator of the cell.

Some species in this group live freely, but others can only survive in symbiosis or by parasitizing other organisms. They often live in symbiosis with fungi, forming lichens. Other species parasitize plants, forming spots on leaves, though they remain photosynthetic. The most aggressive species, which lack photosynthetic chloroplasts, parasitize insects or vertebrates, including humans, where they can cause serious diseases.

The class Trebouxiophyceae is an ancient group, closely related to typical green algae (classes Chlorophyceae and Ulvophyceae).