From Plant Press, Vol. 28, No. 1, January 2025.
By Ian Medeiros.
A recent study in the American Journal of Botany (2024; https://doi.org/10.1002/ajb2.16441) provides a first view of the southern African biodiversity of the green algal genus Trebouxia (Chlorophyta), the most common photosynthetic partner in lichens.
Trebouxia in a squash preparation of lichen thallus. Hale 78605 (US).
Southern Africa, particularly the Greater Cape Floristic Region, is a biodiversity hotspot for vascular plants and hosts a unique biota of lichen-forming fungi (mycobionts). However, the region’s lichen-forming algae (photobionts) had hardly been studied with molecular data or put into a global phylogenetic context. In the unicellular green alga Trebouxia, as with other lichen photobiont lineages, genomic variation far exceeds the limited morphological variation, and thus molecular data are essential for recognizing species.
To begin the process of applying a modern classification scheme to Trebouxia in South Africa and Namibia, an international team of scientists from the United States, Belgium, Chile, Hungary, Malaysia, Namibia, Panama, Poland, and South Africa sequenced the internal transcribed spacer (ITS) and rbcL from ca. 150 lichen thalli representing diverse mycobionts and biomes, from desert and fynbos to Afrotemperate forest and montane grassland. These sequences were incorporated into curated, global datasets; putative species were delimited with both phylogeny and operational taxonomic unit (OTU) clustering approaches as a way of ensuring that conclusions were robust to uncertain species boundaries.
We found that the Trebouxia biota of this region contains an estimated 43 species, representing 20–30% of global Trebouxia biodiversity. This is comparable to other well-sampled regions of the Southern Hemisphere (i.e., Bolivia and Kenya). On one hand, the results exemplify an “everything is everywhere” approach to microbial biogeography, with a majority of the southern African Trebouxia species shared with Bolivia, Kenya, or the Mediterranean basin. Conversely, seven species were only found in southern Africa, although further sampling will be required to assess whether they are truly southern African endemics.
Sampling for lichen photobionts in the northern mistbelt forest of the Buffelskloof Nature Reserve, South Africa. (photo by József Geml)
Patterns in species distribution and the mycobiont–photobiont association echoed what has been found elsewhere in the world. For example, the foggy Namib desert coast was dominated by species of Trebouxia clade A, something also seen in the Atacama Desert of Chile. The mycobiont genus Parmotrema was strictly associated with species from Trebouxia clade C, as has been observed in the Neotropics and Northern Hemisphere. Mean annual precipitation, but not mean annual temperature or precipitation seasonality, was found to have a significant effect on the composition of Trebouxia communities.
Only five of the species-level lineages recognized in this study have been formally described, highlighting the substantial Linnean shortfall in Trebouxia, but also providing a blueprint for future efforts to recollect, culture, and formally describe these species.