Trentepohlia aurea

Trentepohlia aurea, called orange rock hair, is a filamentous green alga that lives on land, belonging to the family Trentepohliaceae. It grows on rocks, old walls, concrete, tree bark, rotting wood, mosses, and damp soil.

Its orange color comes from carotenoid pigments inside the cells. This species serves as the type for the genus Trentepohlia. In the British Isles, it is the most common and widespread member of the genus, although the name has often been incorrectly applied to other species, such as Trentepohlia abietina, so many past records need reexamination.

The alga forms small, bushy tufts colored golden yellow to orange-brown; dried specimens appear duller. Its thallus is made of erect filaments rising from a reduced network of prostrate filaments. The erect filaments reach up to 1.5 mm tall and can be unbranched or lightly to heavily branched. They consist of cylindrical cells 10–20 μm wide (rarely 30 μm), with lengths 1.5 to 4 times their width.

Prostrate filaments are swollen and up to 18 μm wide. Gametangia are globular, ovoid, or flask-shaped, measuring 18–40 μm wide. Zoosporangia are 25–30 μm wide and 27–40 μm long.