Roccellaceae

Lichens to dye for
Published 28 November 2022

Most lichen species, it is probably fair to say, are little recognized for their economic impact on humans. Among the most notable exceptions is Roccella tinctoria, a fruticose lichen found on rocks and cliffs on the coast of Portugal and northern Africa, as well as on the nearby Macaronesian islands. This species has long been used as the source of orchil, a red or purple dye. Orchil may be used to dye wool or silk without the need for a mordant and was particularly popular in Renaissance-era Europe. The Ruccellai family of Florence, one of that city’s wealthiest dynasties, derived their name from orchil (Tehler & Irestedt 2007). Prepared another way, the dye extracted from R. tinctoria forms litmus which many of you may recall being used as a pH indicator in high school chemistry.

Orchil Roccella tinctoria, copyright Norbert Nagel.

Roccella tinctoria is a representative of the Roccellaceae, a family of saxicolous and corticolous lichens found most abundantly in subtropical coastal habitats, particularly in arid and Mediterranean-climate regions. As currently circumscribed, Roccellaceae are typically characterised by a corticate thallus containing trentepohlioid photobionts, rounded ascomata with a thalline margin, a dark brown hypothecium, cylindrical-clavate asci, and ascospores lacking a gelatinous sheath (Ertz et al. 2015). Roccellaceae are representatives of the Arthoniomycetes, a distinct clade of fungi from the Lecanoromycetes to which belong the greater number of familiar lichens.

Dirina massiliensis, copyright Anita Gould.

The growth habit of Roccellaceae is varied, including both crustose and fruticose forms. Phylogenetic analyses indicate that changes in growth habit have occurred multiple times over the family’s evolution, from crustose to fruticose and back again (Tehler & Irestedt 2007). Species may reproduce sexually via the production of ascospores or asexually via the production of soredia, small ‘parcels’ containing fungal hyphae and associated algal photobionts. Production of both reproductive structures on a single thallus is rare, and often reproductive mode is used to distinguish species: for instance, Roccella tinctoria represents the asexual counterpart to the sexual R. canariensis.

Whereas lichen species are generally widespread, Roccellaceae are unusual in that species are often highly endemic. This may extend to higher taxa: for instance, the diverse genus Roccellina is almost entirely restricted to the Southern Hemisphere (Ertz et al. 2015). For Roccellaceae, at least, the classic proposal for micro-organisms that “everything is everywhere” simply does not apply.

References

Ertz, D., A. Tehler, M. Irestedt, A. Frisch, G. Thor & P. van den Boom. 2015. A large-scale phylogenetic revision of Roccellaceae (Arthoniales) reveals eight new genera. Fungal Diversity 70: 31–53.

Tehler, A., & M. Irestedt. 2007. Parallel evolution of lichen growth forms in the family Roccellaceae (Arthoniales, Ascomycota). Cladistics 23: 432–454.

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