by Gauri Gharpure – Lichen are one of the best known examples of mutualism (or symbiosis in scientific terms). They are organisms formed of a photosynthesizing partner (usually cyanobacteria or algae) and a “housing” and/or structural fungal partner (usually Ascomycota). Together they make intricate structures called thalli (sing. thallus) that can exist in many different forms: crustose, which adhere very strongly with their substrate like rocks; foliose, which has flat, leaf-like thallus; fruticose, which has branch-like projections; etc. These structures are together involved in photosynthesis, the exchange of nutrients, forming the structure and so on.
The following photos show the diversity of lichen.
Recent evidence has shown that, unlike what scientists believed for the last nearly 150 years, lichen have more than one fungal partner; they can have up to three! These partners can be ascomyetes or basidiomycetes. Work by groups referenced below has shown the details of the microbial diversity in the lichen thalli using molecular tools to identify them on the basis of genetic sequences and markers, construct their phylogenetic tree (indicating their evolutionary history), microscopy to visualize their position and sometimes role in the thalli.
One such study by Tuovinen et al published in January 2020 showed the structure of these organisms in detail. One of the key points was visualization of the structure of these thalli, revealing much more complexity than previously thought. They were able to visualize the cell-cell contact between these partners and show close contact between the haustoria of the fungi and the algal cells. This suggests that this is a mutualistic relationship, and not parasitic as previously thought as the haustoria do not penetrate the algal cells. They also speculate that both the number and identity of fungal species taking part in this symbiotic relationship is specific to the lichen itself.
Another study by Spribille et al even shows that variations in the phenotype of closely related lichen, which couldn’t be explained earlier, were caused by the variation in abundance of the third basidiomycete (fungal) partner. They also showed the ultrastructure of these thalli in detail.
Such studies answer previous questions, but they raise many more interesting questions as well. How is the number and identity of the fungal species determined? What is the exact nature of the relationship with the algal component: is it really mutualistic or parasitic or commensalistic, or something entirely different? How do nutrients etc. get exchanged in such systems? The list and possibilities are endless and something definitely worth watching out for.
- Tuovinen, V., Ekman, S., Thor, G., Vanderpool, D., Spribille, T., & Johannesson, H. (2019). Two basidiomycete fungi in the cortex of wolf lichens. Current Biology, 29(3), 476-483.
- Spribille, T., Tuovinen, V., Resl, P., Vanderpool, D., Wolinski, H., Aime, M. C., … & Mayrhofer, H. (2016). Basidiomycete yeasts in the cortex of ascomycete macrolichens. Science, 353(6298), 488-492.
- What’s in a Lichen? How Scientists Got It Wrong for 150 Years (https://www.youtube.com/watch?v=Fkw_VF5zDT0