Spatial Mapping of Lichen Metabolites Using Mass Spectrometry: Application to the Crustose Lichen Ophioparma Ventosa

Lichens are self-sustaining symbiotic partnerships (fungus and green algae and/or cyanobacteria) producing a wide array of unique compounds that exert varied bioactivities [1]. The successful harnessing of lichens unique chemodiversity relies on refined dereplicative techniques to reduce time and costs in downstream analyses. Given the extracellular localization of their metabolites, the ambient mass spectrometry technique DART-MS (Direct Analysis in Real Time) appears most suitable to provide complete chemical profiles from unprocessed pieces of lichens [2,3]. Likewise, owing to UV-absorbing properties of the main structural classes of lichen metabolites, we also demonstrated the advantages offered by LDI-MS for the accelerated dereplication of lichen extracts [4]. Allocation of lichen substances in the lichen thallus is also a challenge to discuss their role. Through the specific example of the Alpine crustose lichen Ophioparma ventosa, we herein illustrate the adequacy of in situ DART-MS for the chemical profiling of unprocessed lichen samples, bypassing costly and tedious steps of solvent extractions and possible artefacts coming along with it. Compared analyses of both lower and upper faces of the thallus could then provide a first estimation of metabolites distribution with ventosic and divaricatic acids being ascribed to the lower parts of the lichen and haemoventosin, thamnolic and usnic acids rather confined to the upper slices of the lichen fruiting bodies. The sharp mapping of all secondary metabolites (except ventosic acid) is subsequently established with a 50 µm high resolution record using LDI-MSI, and discussed with regard to their ecological significance. Altogether, these innovative mass spectrometric approaches appear to be of outstanding interest to (i) alleviate the dereplication holdup of compounds from lichen sources and (ii) advance the understanding of lichen biology through their chemical mapping.