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Aug 07, 2023

Bright orange lichens can use their pigments as a ‘sunscreen’

Fungi in orange lichens can avoid the toxic effects of bright pigments, allowing

Fungi in orange lichens can avoid the toxic effects of bright pigments, allowing them to handle high UV loads.

Lichens are curious plant-like organisms that consist of one fungus species and at least one alga or cyanobacteria living in a symbiotic relationship. A family of lichens, known as Teloschistaceae, are often brightly coloured orange - a rare trait compared to closely related species. The pigments responsible for the intense hue often have toxic effects on the organism itself, causing scientists to wonder how it protected itself while synthesising its orange colouration.

Research has shown that fungi in orange lichens are able to avoid the toxic effects of bright pigments by transporting them out of their cells, creating a ‘sunscreen effect’. A new meta genomics study published by researchers from Imperial College London and the Royal Botanic Gardens, Kew now reveals how lichens can use their orange hue to reflect sunlight, while avoiding their toxic effects.

The study, published in Genome Biology and Evolution, reveals that the fungi in this class of lichen has a gene responsible for transporting the orange pigments out of the cell shortly after they are created.

Lead author Theo Llewellyn, a PhD candidate from the Department of Life Sciences at Imperial, says that the unexpected discovery came from a gene survey that isolated sequences from the algae and fungi that make up lichens. The fungi in orange lichens had evolved to handle toxic pigments, he says: "What we were really excited and surprised about was that we saw that right next to the gene that is responsible for making these pigments, there's a second gene, which is specialised to be able to transport that pigment and get it out of the cell."

These transporter genes allowed the fungi to rid themselves of the pigment before it could accumulate and become toxic, researchers explain. The transporter gene was not found in non-orange lineages/lichen groups.

The study was conceived, led and partly funded by an RBG Kew project. Dr Ester Gaya, Senior Research Leader in Mycology at RBG Kew and a Teloschistaceae expert, says that she is pleased with the group's findings. She says: "We had worked for years in this group of lichens, and after unveiling that an adaptive radiation into arid habitats had been mediated by these orange pigments, I always wondered why. Why did they evolve to produce such toxic pigments that could kill themselves?"

Pigments which are responsible for colouring lichens from a golden yellow to a crimson red are known as ‘anthraquinones,’ which also have UV-protectant properties.

RBG Kew natural product researcher Dr Tom Prescott, who is also a co-author on the study, concurred, adding "anthraquinones have long been recognised as being quite toxic, including to fungi, so it's always been a mystery as to how they make these compounds without poisoning themselves." This study helps explain this.

"These pigments that they produce are insoluble in water," Llewellyn says, "Once the lichen produces them, they start to crystallise and they form a layer on top of the lichen."

This enables the lichen to reflect UV and visible blue light. The scientists say that, while it is still not clear which organisms in the lichen benefit the most from this sunscreen effect, they speculate that both the fungi and the algae are protected. The algae use sunlight to photosynthesise and produce sugars for the lichen system, but too much of it can be harmful since it can cause DNA damage.

The thick layer of crystals can reflect harmful radiation while still allowing some solar radiation to pass through for photosynthesis.

Co-author Professor Timothy Barraclough, professor at the Department of Biology, University of Oxford, and a visiting professor in the Department of Life Sciences at Imperial, says that understanding these mechanisms can help shed light on how these orange lichens are able to fare in different habitats.

"This particular group has been especially successful and has spread out into challenging habitats with high UV loads," Professor Barraclough says – pointing to how the family contains more than 1000 species, and can be found in countries like South Africa, Namibia and Australia.

Professor Barraclough points to the unique structure of lichens that make them difficult to study: "They’re widespread but slightly unusual forms of life that are made up of a collaboration between fungi and algae, and possibly many more partners. In the presence of all of these partners, it makes it very complicated to pull out information about genes."

The metagenomic study that the group produced sequenced small fragments of DNA from the lichen and mapped it to databases of known organisms. By analysing which DNA sequences most closely resembled those found in fungi and algae, the researchers were able to identify which sequences belonged to each partner.

Llewellyn says that the next stage of research will try to investigate the other qualities of anthraquinones, such as their potential antimicrobial properties that allow lichens to outcompete other fungi and microbes.

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Llewellyn, T., Nowell, R.W., Aptroot, A., Temina, M., Prescott, T.A.K., Barraclough, T.G., Gaya, E., 2023. Metagenomics shines light on the evolution of "sunscreen" pigment metabolism in the Teloschistales (lichen-forming ascomycota). Genome Biology and Evolution, 15(2), DOI: 10.1093/gbe/evad002