Trentepohlia jolithus

Trentepohlia jolithus (basionym: Byssus jolithus) is an alga species in the genus Trentepohlia. Despite being a member of the group of green algae (Chlorophyta), it is usually colored bright orange or red due to the presence of carotenoid pigments.

Trentepohlia jolithus occurs in cool, humid regions. It mostly grows on rocks, boulders and concrete walls, but it has also been reported growing on wood. It often forms large orange mats, particularly in forests in northern Europe. In Germany, it is called "Veilchenstein" or "Veilchenmoos" due to the smell of violet which emanates from the alga when wet. It is widespread in Europe, and may have a cosmopolitan distribution, but records from tropical regions need reassessment. In Mount Gongga, China, a variety of this species (T. jolithus var. yajiagengensis) forms reddish-orange mats on rocks, and has become a tourist attraction.