Hymenelia prevostii and H. coerulea are common endolithic lichen species on limestone outcrops in the eastern alpine mountains. We investigated their photosynthetic properties under controlled laboratory conditions and analyzed them with respect to relevant meso- and macroclimatic parameters to determine specific adaptations to the extreme habitat.

For the first time, it can be shown that endolithically living lichens are able to utilize water vapor alone to gain positive net photosynthesis (NP) rates. With respect to liquid water, both species reached optimum NP rates at low water content (H. prevostii: 0.26 mm, H. coerulea: 0.07 mm rainfall equivalent), and NP was increasingly depressed at water suprasaturation. In their preferred microhabitat on steep limestone outcrops in alpine regions, endolithic lichens avoid inundation over long time spans, but could perfectly utilize the high air humidity of monthly means between 60 and 80%. Due to light reduction by endolithic growth, both species revealed relatively high photosynthetic light compensation and saturation values, thus avoiding the high light intensities of the alpine habitat. Both lichen species were able to perform 90% of their optimal NP over a wide temperature range and still reached about one-third of their maximum NP rates at 2 °C, demonstrating their capability to cope with the large temperature changes occurring in their habitat.