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Symbionts and changing environment: Lichen diversity and photobiont associations in tropical mountain ecosystems

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Tropical lichens reveal extent of biodiversity

A new study of lichens found on an African mountainside reveal a treasure trove of genetic diversity.

Climate Change and Environment

A lichen is not a single organism, but instead comprises a stable symbiotic association between a fungus and algae and/or cyanobacteria. While some temperate lichens have been intensively studied, almost nothing is currently known about most tropical taxa, and knowledge of African lichens is particularly sparse. As part of the Marie Skłodowska-Curie Individual Fellowships grant, the EU-funded Horizon 2020 Tropical lichens project produced the first modern account of symbiont diversity in lichens of tropical mountains. “We studied both fungal and cyanobacterial symbionts with DNA methods and focused the sampling on cyanobacterial lichens which live in the moist montane forests of Taita Hills, Kenya,” says project coordinator Prof. Jouko Rikkinen. These forests represent a global biodiversity hot spot, but little information existed on their lichen biota. Researchers therefore surveyed remnant forest patches along a steep natural climatic gradient on the slope of a high tropical volcano to determine the effects of environmental change on lichen symbiotic organisms. Unprecedented diversity Scientists found high fungal diversity in all the cyanolichen groups analysed. In some genera, over half of the identified species appear to be previously unknown and now await formal description. “The results highlight that the species concepts and nomenclature of many lichen lineages in tropical Africa need to be revised,” Prof. Rikkinen explains. Some of the new species may be local endemics that have a very limited range and only exist in special microhabitats. Such organisms are under serious risk of extinction due to continuing human disturbance. Prof. Rikkinen comments: “The genetic diversity of cyanobacterial symbionts was unprecedented and hardly any genotypes had been previously reported from lichens sampled from other parts of the world.” Researchers focused on cyanolichens because “in addition to their contribution to primary production through photosynthesis, they can have an even more significant impact on the nitrogen cycle due to their nitrogen-fixing cyanobacterial symbionts,” states Prof. Rikkinen. Epiphytic lichens and bryophytes growing on tree trunks and branches also play an important role in water capture in cloud forests, acting as ‘water towers’ for surrounding dry lowlands. Dramatic scientific advances The project demonstrated how the application of modern DNA techniques has revolutionised lichen systematics and ecology. “The type of information gained through the Tropical lichens project was totally out of reach only some years ago and is rapidly leading to profound changes in our understanding of the extent and ecological significance of genetic diversity in lichen-symbiotic organisms,” states Prof. Rikkinen. Tropical lichens advanced scientific understanding of the diversity of biological associations that are generally labelled under the term lichen symbiosis. It also produced the fungal universal barcode marker (the internal transcribed spacer) and several cyanobacterial marker sequences from hundreds of lichen specimens, which together with the voucher specimens provide a stable and accessible basis for future research. The work conducted by the project thus represents an important step forward in the study of lichen symbiont diversity in the tropics, and especially in Africa. Prof. Rikkinen concludes: “The data collected has direct implications on environmental conservation and strongly underlines the crucial importance of preserving the remaining moist montane forests as a refuge for unique and in many cases still largely unknown biological diversity.”

Keywords

Tropical lichens, lichens, symbiont, Africa, montane, biodiversity, cyanolichen, fungal universal barcode marker, internal transcribed spacer, genotype

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