Usnic acid and Triacylglycerides Production by the Cultured Lichen Mycobiont of Ramalina celastri

A strain of the lichen mycobiont of Ramalina celastri, isolated from ascospores, was cultured axenically on two solid media containing high amounts of the carbon source: sucrose in MY10 and mannitol in BMRM. Usnic acid, the major cortical lichen metabolite, was produced by the colonies grown on MY10, with a very high yield (7.9%) in comparison with that in the lichen thallus. Mycelia grown on BMRM did not produce the lichen secondary metabolite and rendered triacylglycerides (8.5%) instead. Analysis by GC-MS of the fatty acid methyl esters revealed the presence of oleic, palmitic and stearic acids as the main triacylglyceride constituents. The present results highlight the impact of the culture conditions on the lichen mycobiont secondary metabolism and confirm that MY10 is a useful medium to obtain usnic acid from mycobionts in the laboratory.