Seventeen per cent of the 110,000 described fungal species sustain mutualistic relationships with mainly green algal species in the lichen class Trebouxiophyceae or the order Trentepohliales. In their self-sustaining symbiotic relationship, the alga, or photobiont, supplies the fungal partner, or mycobiont, with carbon through photosynthesis, while receiving protection from ultraviolet (UV) radiation through chemical metabolites produced by the mycobiont.
In this study covering more than 10,000 lichenized fungal species, researchers explored the global distributions of trebouxiophycean and trentepohlialean photobionts, while assessing the role of lichen substances and their properties driving these patterns.
By combining GBIF-mediated occurrences of fungal species and their associated photobionts with data on lichen substances and derived properties, the authors modelled a network of relationships between the environment, occurrence probabilities and chemical properties of metabolites.
The model revealed a clear dominance of lichenized Trebouxiophyceae at higher latitudes, while trentepohlialean lichens showed a higher occurrence probability in the tropics. Both the potential for environmental dissolution and UV-B radiation showed maximum values at lower latitudes.
The occurrence probability of Trebouxiophyceae photobionts was positively linked with the occurrence of lichen substances, which decreased with probability for leaching and thus dissolution. Further, Trebouxiophyceae substances had higher UV-B absorbance and lower solvability than those of Trentepohliales.
Together, the study's findings suggest that the overall environmental drivers of lichen fungi and algae distributions, such as water availability and solar radiation, are mediated by chemical and UV-protective properties of lichen substances.