9 Lichen–Bacterial Interactions

Abstract

Lichens, generally known as a fungal lifestyle with tight integration of algal partners in a joint structure, are classic examples of self-sustained symbioses. More recently, the associations of bacterial communities with lichens have been characterized in more detail by culture-independent approaches. The colonization of bacteria on the lichen thallus can be displayed directly by fluorescence in situ hybridization and confocal laser scanning microscopy, while the structure of the community can be precisely determined by amplicon sequencing. The prevalence of Alphaproteobacteria has been observed, while other bacterial lineages are usually present in lower amounts. Despite the ubiquity and a better understanding of bacterial communities on lichens, their functions remained largely elusive so far. We applied a multi-omics approach to explore the functional potential of the microbiome, with the lung lichen Lobaria pulmonaria as the model. The results suggested multiple functions in the symbiotic system, supporting growth and fitness of their hosts: nutrient supply, especially nitrogen, phosphorus, and sulfur, resistance against biotic stress factors (pathogen defense), resistance against abiotic factors, support by production of vitamins, detoxification of metabolites, and degradation of older parts of the lichen thallus. Thus lichen thalli are well designed as smart harvesters of bacterial contributions. We outline a comprehensive model of the symbiosis depicting the functional multiplayer network including the bacterial partners.