Schizoxylon as an experimental model for studying interkingdom symbiosis

Experiments to re-synthesise lichens so far focused on co-cultures of fungal and algal partners. However, recent studies have revealed that bacterial communities colonise lichens in a stable and host-specific manner. We were therefore interested in testing how lichenised fungi and algae interact with selected bacteria in an experimental setup. We selected the symbiotic system of Schizoxylon albescens and the algal genera Coccomyxa and Trebouxia as a suitable model. We isolated bacterial strains from the naturally occurring bacterial fraction of freshly collected specimens and established tripartite associations under mixed culture experiments. The bacteria belong to Actinobacteria, Firmicutes and Proteobacteria and corresponded to groups already found associated with fungi including lichens. In mixed cultures with Coccomyxa, the fungus formed a characteristic filamentous matrix and tightly contacted the algae; the bacteria distributed in small patches between the algal cells and attached to the cell walls. In mixed cultures with Trebouxia, the fungus did not develop the filamentous matrix, but bacterial cells were observed to be tightly adhering to the fungal hyphae. Our experiments show that this tripartite fungal-algal-bacterial model system can be maintained in culture and can offer multiple opportunities for functional studies based on experiments under controlled conditions in the laboratory.