Lichens at the limits of life: past perspectives and modern technology

Lichens are found at the limits of polar life in the Antarctic Dry Valleys under extreme conditions of UVB radiation, desiccation and low temperature. They may even have evolved in the final phase of microbial community development in analogous niches on Mars. The endolithic lichen habitat is an avoidance strategy, but epilithic lichens can tolerate environmental extremes by producing protective pigments (e.g. scytonemin, B-carotene and parietin), water-replacement molecules (e.g. trehalose) and compatible solutes (e.g. the polyol erythritol). Enhanced UVB radiation during the Antarctic "Ozone Hole" makes pigment micro-distributionof great importance in the stratified symbioses of crustose and endolithic lichens. Non-invasive laser-based Fourier Transform Raman spectroscopy (FTRS) is an eminently suitable tool for not only identifying key pigments in mixed assemblages of biomolecules, but for locating them spatially in situ without disrupting the community. A variety of lichen pigments and other functional biomolecules, whose unique Raman spectral "fingerprints" have been determined, are described. FTRS of the Antarctic Xanthoria spp. Acarospora spp. and endolithic lichens has shown the distribution of pigments relative to the light gradient. The non-contact approach of FTRS makes it a suitable system for exobiological research, to detect microbial biomolecules in inaccessible strata. These include the profiles of drill-holes in sedimentary deposits and endolithic strata to be made by future landers on Mars.