British distribution (genus): Throughout Britain, free-living in terrestrial habitats, also as lichen photobionts.
The genus Trentepohlia would not, at first glance, be taken as a green alga. Free-living species are mostly yellow to bright orange or red-brown in colour, due to the orange pigment, haematochrome (β-carotene), which usually hides the green of the chlorophyll. The genus is terrestrial, commonly on rocks, walls, tree bark and, especially in the tropics, on leaf surfaces. The haematochrome no doubt protects the alga's chlorophyll in these subaerial habitats. Trentepohlia is also a widespread photosynthetic symbiont (photobiont) in lichens.
John, in John et al., (2002), based on recent work in Ireland by Rindi & Guiry (2002), recognises five British and Irish species and states that there are at least twelve known species in the genus. Van den Hoek et al. (1995) consider that there are about forty species, mostly tropical and subtropical. There is general agreement that the genus requires taxonomic revision. In Britain, the best known is T. aurea (L.) Martius, which forms conspicuous, bright orange, small cushions on rock faces and sometimes on tree trunks (especially Quercus (oak)). It is most common in wetter, western districts and in upland areas, and prefers lime-rich substrates.
As with other Trentepohlia species, T. aurea is filamentous, with both creeping and erect filaments. Creeping filaments produce gametangia; aerial filaments produce sporangia (neither illustrated here). T. aurea is recognised by its aerial filaments being 10-20 µm in diameter and in the filaments not being constricted at the cell junctions. The chloroplasts are small, discoid (in this species, ribbon-like in certain others) and lack pyrenoids. As seen below, the haematochrome makes the chloroplasts seem orange or brown under the microscope and they are hidden amongst the droplets of the haematochrome itself. Aerial filaments end with a capping cell. The cell walls are lamellate (layered).
Although the best known, T. aurea is probably not the most common species in Britain. Trentepohlia is not uncommon on damp, sheltered walls, at least in west Scotland, and material photographed and collected at a Renfrewshire site (March 2002), on an old, mortared, sandstone wall on the edge of a wooded ravine, was found to differ from T. aurea in its significantly narrower, more tapering filaments.
Such material may well have been named as T. calamicola in the past, see the key in Pentecost (1984), but John's (op. cit.) revision of the genus provided more species to choose from. The Renfrewshire material keys out instead to T. abietina (Flotow) Hansgirg, and it matches very well the photographs on the accompanying CD-ROM and the photomicrographs and notes in Rindi & Guiry (op. cit.). Although T. abietina is said to be known in the British Isles only from scattered localities in Ireland, mostly on trees but also on limestone rocks within forested areas, it is also predicted to be widespread in the "wetter westernmost areas of the British Isles", i.e. including west Scotland. Further experience suggests that T. abietina is, indeed, frequent on tree trunks (sometimes as small and inconspicuous colonies) and on shaded rock faces in this area.
T. calamicola, as now understood, is a greenish species, yellow only when dry, and seemingly rare, known for certain only in Ireland and Cornwall, though considered "probably more widely distributed".
Trentepohlia is also a widespread photobiont in lichens, particularly in tropical crustose species. Friedl & Büdel (1996) list its presence in lichens of the orders Arthroniales, Gyalectales, 'Sphaeriales' and Ostropales. The large, crustose lichen genus Opegrapha ('scribble lichens') (Opegraphales) is one such genus in Britain with Trentepohlia symbionts. It is not clear which or how many Trentepohlia species occur as lichen symbionts - a "Trentepohlia lichenicola" has been described, but John (op. cit.) notes that at least two of the free-living species are known to occur as lichen photobionts. As with other lichen photobionts, algal morphology changes markedly within the lichen thallus, the typical filamentous structure being lost.
Since fungal hyphae are often associated with free-living Trentepohlia colonies, perhaps the symbiosis can be viewed as varying levels of parasitism, though Opegrapha can occupy exposed habitats that would not be suitable for the alga alone.
The photomicrograph is of Trentepohlia within the lichen Opegrapha niveoatra (Borrer) Laundon, a species of nutrient-rich tree bark. It is intended to add a photograph of this lichen at a later date; another Trentepohlia-containing Opegrapha species is shown below.
The class Trentepohliophyceae differs from other green algae in the structure of the flagella (in the biflagellate gametes and in the quadriflagellate zoospores). Presence of polyhydroxyalcohols is also a distinctive character and the mode of cell division is distinctive. Van den Hoek et al. (1995) give a good outline account of these features.
The class contains the single order, the Trentepohliales, and four genera, including Trentepohlia. The related genera are Phycopeltis, Printzina and Cephaleuros, all of which grow mainly on leaves in the warmer and more humid parts of the world, though there are records of each from the British Isles. Cephaleuros species can be parasitic and they cause substantial economic damage to tropical crops (cocoa, tea, etc.).