A Partnership That Builds Worlds
Across the tundra of Finnmark, vast, colorful mats of Cladonia lichens blanket the landscape. These complex structures are not single plants but composite organisms, the result of a symbiotic partnership between a fungus (the mycobiont) and a photosynthetic partner (the photobiont), usually a green alga or cyanobacterium. The fungus, typically an ascomycete, makes up the bulk of the lichen's body, or thallus. It provides a physical structure that protects the photobiont from harsh conditions like extreme temperatures, desiccation, and high ultraviolet radiation. In return, the microscopic algae, embedded within the fungal tissues, perform photosynthesis, supplying the fungus with carbohydrates.
This self-contained life-support system allows lichens to colonize environments where neither partner could survive alone. They are pioneers, often the first organisms to grow on bare rock. Over immense timescales, they physically and chemically break down stone, trapping dust and organic matter to create the first foundations of soil, allowing for mosses and vascular plants.
The Reindeer Lichen of Finnmark
A dominant group in the Arctic is Cladonia, a genus of fruticose, or shrubby, lichens. The most famous species carpeting the ground in Finnmark is Cladonia rangiferina, commonly known as reindeer lichen. This pale, grey-green lichen grows in complex, three-dimensional tufts that can be up to 8 cm high. It is an important food source for reindeer, especially during the harsh winters when other vegetation is scarce.
Reindeer lichen grows extremely slowly, with average growth rates estimated at just 3 to 5 millimeters per year in Arctic lands. A patch of lichen can take decades to recover after being grazed or trampled. This slow growth shows the challenging conditions and the lichen's strategy of endurance over rapid expansion. The photobiont in C. rangiferina is the green alga Trebouxia, which gives the lichen its characteristic color.
Masters of Chemistry and Time
Lichens are sophisticated chemists, producing secondary metabolites not found elsewhere in nature. Cladonia species produce compounds like usnic acid, a dibenzofuran that is particularly effective at absorbing UV radiation. This pale yellow pigment acts as a natural sunscreen for the delicate photosynthetic cells within. These chemical compounds can also deter herbivores.
The lichens are resilient. They can enter a state of suspended animation called cryptobiosis when dehydrated, shutting down their metabolism almost completely to survive long periods of drought or extreme cold. Some species have even survived direct exposure to the vacuum and radiation of outer space. Experiments on the International Space Station showed that lichens could endure the harsh environment for 18 months and resume normal metabolic activity, including photosynthesis, upon their return to Earth. This durability, combined with their slow growth, makes them some of the longest-living organisms. The field of lichenometry uses the predictable growth rates of certain species to date geological surfaces, revealing histories of retreating glaciers and rockfalls that span thousands of years.