Many topics of lichen biology deal with questions about how the different symbiotic partners (mycobionts, photobionts, cyanobionts) interact within the lichen thallus. The separation, isolation, and culture of the lichen symbionts or components offer researchers insights into functional aspects of the lichen symbiosis, such as identifying parameters essential for their growth in the aposymbiotic state or triggers for producing secondary metabolites (polyketides, shikimic acid derivatives, etc.) in culture. Furthermore, culturing provides a means for investigating how lichen symbionts respond to each other, how they recognize each other through chemical signals, and how a functional symbiosis is established. Many of these fundamental problems in lichenology have been investigated (Chapter 5), but not fully resolved. Apart from the questions that arise from investigating only the typical lichen bionts, one can utilize more advanced molecular methods to study other associated partners of the lichen symbiosis, including molds, yeasts, lichenicolous fungi, lichenicolous lichens, and parasitic bacteria located on the surface of or within the thalli.
Over the past 20 years many culture experiments have been undertaken to improve culture methods for lichen symbionts, in general, and also to re-establish lichen symbioses (di- or tripartite partnerships of ascomycetous and basidiomycetous lichens) under artificial conditions (Fig. App. 1). Such experiments help to answer basic questions, like how the lichen fungus transforms from a relatively unstructured mycelium into a highly organized thallus. Such resynthesis experiments can significantly extend our knowledge about symbiont coordination and steps in thallus ontogeny.