Compositrema is a genus of lichen-forming fungi in the family Graphidaceae. It has four species. The genus was circumscribed in 2012 by lichenologists Eimy Rivas Plata, Robert Lücking, and Helge Thorsten Lumbsch, with C. cerebriforme assigned as the type species. The genus is distinguished by its unique, composite ascomata (i.e., fruiting bodies with a made of both thallus tissue and bits of host tissue), which sets it apart from the otherwise similar genus Stegobolus.
The genus Compositrema was circumscribed in 2012 by lichenologists Eimy Rivas Plata, Robert Lücking, and H. Thorsten Lumbsch. The type species, Compositrema cerebriforme, was jointly described by Jesús Ernesto Hernández Maldonado and Lücking. The name of the genus makes reference to its "composite" ascomata, a feature that starkly distinguishes it from the genus Stegobolus. The ending -trema is derived from the Latinised Greek neuter noun meaning "perforation; aperture; opening; orifice". Phylogenetically, Compositrema is closely akin to Stegobolus but is genetically quite distant, forming a strongly supported clade that is a distant relative of the Myriotrema album group. Compositrema is in the tribe Ocellularieae of the subfamily Graphidoideae in the Graphidaceae. Two species were initially included in this genus, and two others added in 2014.
Compositrema lichens have a pale green-grey to olive-green thallus that is smooth to uneven, with a dense that has a tissue structure. Sometimes, isidia are present on these lichens. The contains scattered clusters of calcium oxalate crystals among its cells. The, or the reproductive parts of the lichen, are grouped into distinct pseudostromata, which are rounded to angular and can be either or sessile. The -like part of these pseudostromata is pale brown and dusted with a thin white . On a more microscopic level, Compositrema lichens have unbranched, and each ascus contains eight that are ellipsoid in shape with thick septa and lens-shaped . These ascospores range from colourless to pale brown and react with iodine to give a violet-blue colour. Psoromic acid is a secondary metabolite (lichen product) that occurs in Compositrema.