Experimental Lichenology and Microbiology of Lichens: Culture Experiments, Secondary Chemistry of Cultured Mycobionts, Resynthesis, and Thallus Morphogenesis

Abstract

Lichens are unique and fascinating symbiotic organisms. They have been studied for more than two centuries as morphological entities, although their fungal-algal composition (symbiosis) was only recognized in 1869 by Schwendener. The discovery of the symbiotic double nature of lichens led to a controversy among scientists of the late 19th century. To some extent the discussion of whether lichens should be considered as whole or composite plants initiated trials to separate the symbiotic partners, fungus and alga, in order to obtain scientific proof of the composed morphologies of diverse lichens. These early experiments to separate and recombine lichen symbionts can be considered as the birth of As most of the early manipulations with lichen bionts and whole lichens did not have the expected success to reconstitute a fully developed thallus from its components (resynthesis) or to grow whole lichen thalli in culture, experimental lichenology remained a nearly unexplored scientific field for many decades. However, successful culture and resynthesis experiments that were performed during the pioneer period (e.g., Bonnier 1887, 1889; Stahl 1877) were ignored and overlooked. Finally, the composite and symbiotic nature of lichens was accepted by most lichenologists; mainly by successful isolations of the algal partners, which were identified as typical representatives of green algae and/or blue-green algae, recently named cyanobacteria. Additionally, an exceptional work of culturing lichens was presented by Thomas (1939), who cultured different mycobionts under various physiological conditions and temperature regimes. Moreover, Thomas reported the first successful resynthesis of Cladonia pyxidata. The early work on the culture of Endocarpon pusillum by Stahl (1877) and Geitler (1938) and Cladonia by Thomas (1939) was partly reproduced and completed by Ahmadjian and Heikkili (1970) and Bertsch and Butin (1967), Stocker-W6rgt6tter and Tuirk (1988), and Wagner and Letrouit-Galinou (1988). Since the 1970's, the improvement and optimization of culture conditions of lichen-forming fungi have been a major goal of experimental lichenology. A modern approach to improvement of culture conditions, adapting microbiological methods for lichens, was presented by Ahmadjian (1973). In his contribution Methods for isolating lichen mycoand photobionts, Ahmadjian introduced a manual on how tools and nutrient media can be prepared for the isolation and culture of lichen symbionts. It was postulated by Ahmadjian that the best units for mycobiont isolations are spores, because they represent the true lichen fungus and should show relatively low contamination compared with hyphae obtained from the thallus pieces that may contain a variety of micro-organisms like bacteria, epiphytic algae, and fungi. Further methods for the culture of lichen symbionts were also presented by Bubrick (1988). Spore isolations became very popular among lichenologists and had strong influence on experimental lichenology during the following 20 years Highly time consuming efforts were initiated to grow lichens and mycobionts from single spores (e.g., Ahmadjian & Jacobs 1981; Culberson et al. 1992; Honegger & Kutasi 1990; Stocker-Wirg6tter & TUrk 1989a). These single spore isolations provided mycobiont cultures with a decreased rate of contamination than polyspore isolates, but only low quantities of lichen material were harvested after long incubation periods of up to 2-3 yr. Several of those efforts were done by lichen chemists (e.g., Culberson et al. 1992; Leuckert et al. 1990) and focussed on the production of lichen substances by spore-derived cultures. The question of whether the production of lichen secondary compounds is a feature of the lichen fungus alone or a characteristic of the mycobiont living in symbiosis with a specific