Trentepohlia aurea (L.) Mart. s.l.

    Terrestrial filamentous green alga. The carotenoids pigments that conceal the green chlorophylla are responsible of the conspicuous yellow-orange colour. This green alga is very often a symbiont of lichenized fungi and in that case, acts as the photobiont, but also may have its own independence as described here. The free stage is characterized by small velvet cushions 0.3-2 mm tall, made of ascending filaments, sometimes in very high number, and capable of covering large surfaces (and even a whole wall!), golden-yellow, yellow-orange, orange and as much orange as its grows in shadowed places. Prefers alkaline substrata, humid, shadowed, common on walls, cement in areas with constant humidity notably under bidges where it may develop large colonies visible a long way off. Less frequent on humid and shaded barks (Quercus, Prunus, ...). Doubtless, this species is the most common of its group among which several other species exist but are difficult to distinguish. Worth to report, Trentepohlia abietina (Flotow) Hansgirg that grows on bark of coniferous trunks. In the same habitat we can also find another "free" photobiont: Trentepohlia umbrina which is redder.