Diversity, ecology and specificity in Antarctic lichens

The extreme climate conditions make Antarctica an environment where only the hardiest organisms can survive. The terrestrial vegetation communities of the ice-free areas of this continent (c. 2%) are mainly comprised of mosses, fungi and lichens, i.e. fungal species (mycobionts) which form a symbiotic association with algae or cyanobacteria (photobionts). Although Antarcticas ecosystems represent some of the simplest in the world, it is increasingly becoming clear that their biodiversity is far greater than previously thought and that spatial variations in species diversity are complex, possibly reflecting regional to local variations in climate. However, the mechanisms that connect climate and lifes diversity and evolution in Antarctica are still poorly understood owing to limited taxon and climate data sampling in many areas of the continent. On this background, the current research project investigates the role of environmental (climatic) factors driving lichen diversity, distribution and evolution in Antarctica and adjacent South American regions, using saxicolous lecideoid lichens as a model system. This aim will be accomplished by integrating analytical approaches from molecular phylogenetics, ecological niche modelling (ENM), and morphology/anatomy on the basis of a worldwide unique collection of about 700 specimens with a focus on circum-Antarctic coverage. There are two major and specific issues addressed: (1) the identity, geographic distribution and phylogenetic relationships of the photobiont and mycobiont lineages in the target regions; and (2) the extent of specificity between these symbiotic partners and how this relates to their degree of environmental (climate-related) niche overlap. The research envisaged here has the potential to provide an unprecedented broad-scale view of the contribution of ecological (climate-related) processes to the origin and maintenance of species and lineage boundaries in lecideoid lichens and to the selectivity of mycobionts towards their photobionts throughout the southern Polar region. Whilst the focus is on lichens, resolving this latter and still controversial issue may have also wider implications for symbiosis research in general, as it explicitly tests ecological (climate- related) explanations for patterns of symbiotic distribution, diversity, and evolution in climatically heterogeneous environments.

Organisms inhabiting climatically extreme regions are sensitive to changes in environmental conditions. Current climate warming, for instance, forces cold-adapted lichens to shift their natural geographic distributions, which may cause changes in the interactions between fungal (mycobiont) and algal partner (photobiont) that form the lichen symbiosis. Especially, in lichens the independently distributed symbiotic partners show varying degrees of specialisation to each other. They are requiring different climatic conditions which can be defined by the size of their ecological niche. Accordingly, lichens are ideal model systems to study the effects of climate warming on species interactions and diversity. This project aimed at gaining more insights in the aspects of myco-/photobiont interactions of saxicolous lecideoid lichens in climatically extreme environments of Continental, Maritime Antarctica and Subantarctic areas in southern South America. The terrestrial vegetation communities in these regions are dominated by lichens, which act as vegetation forming pioneer-organisms. The goals of this project were (1) to reveal the identity and geographic distribution of the mycobiont and photobiont species; (2) to quantify the extent of specificity between these symbiotic partners and how this relates to their environmental niche breadth; (3) to generate area-covering climate (BIOCLIM) variables for the Antarctic continent that were not available before; and (4) to construct genealogical trees of Antarctic and South-American myco- and photobionts in global context. Altogether 700 lichen specimen were investigated, 500 for Antarctica and 200 for the subantarctic areas in southern South America. The results showed a surprisingly high diversity of these lichens and their independently distributed symbiotic partner, especially for southern South America, but also with a smaller amount for Antarctica. Several endemic species and locally differentiated subgroups of globally distributed species point to different separating events such as geographic isolation, limited distribution by the Antarctic Circumpolar Current system and regional glacier refugia. The generalist myco- and photobionts are by far the most widespread group and occur frequently. However, species specialised to specific climatic conditions are more likely to be present in smaller numbers. This supports the assumption that highly specific species with a small ecological niche are more endangered than generalists. The diverging patterns of dispersal and their causes of this cosmopolitan group of lichens are still under-researched. They are very sensitive to environmental changes, and consequently can be used as a powerful indicator. Therefore, it is important to create larger data sets along the assumed distribution routes such as high mountain ranges worldwide. With these data it will be possible to better understand colonization and dispersal events and the impact of climate change on the symbiotic relationship of this so far quite overlooked group of crustose lichens. A follow-up project has already been submitted to ensure the necessary further work on this topic.