Enforced fungal-algal symbioses in alginate spheres.

The thallus structure of the lichen symbiosis provides a fungal shelter for the growth of algal partners. The long-living thallus also provides a habitat for other fungi, but experimental studies, which could inform us about the details of their interactions have hardly been conducted. We present a new approach by embedding axenically cultured strains of fungi together with algae in alginate spheres, which allows easy transfer of co-cultures on solid media. As the growth rates of the organisms are differentially triggered by the underlying medium, alginate embedding can help to adjust optimal parameters for stable culture of the combined symbionts. In our experiments, direct contacts between hyphae and algae and the formation of layered structures were observed in a fungus that is living as a commensal in the host lichen without visible symbiotic structures. The growth of primary lichen symbionts cannot be accelerated by alginate embedding so far, but our approach could artificially enforce symbiotic interactions that are not normally observed in nature.