The terrestrial vegetation in Antarctica is restricted to small, very isolated, ice-free areas on the continent and on the islands adjacent to the Peninsula region. Lichens, a symbiosis between a fungus and an alga or cyanobacterium, form the most prominent component in the vegetation. They have the greatest number of species compared with other cryptogamic and unicellular organisms. They are able to colonize all types of solid substrata and have a low mineral nutrient demand. Their physiological vigour is derived from high freezing tolerance and the ability to be photosynthetically activated by water vapour uptake from snow at temperatures as low as c. -20°C. Long-term monitoring in their natural habitats demonstrates that lichens are photosynthetically active at suboptimal temperatures. Locally, however, they can benefit from meltwater on insolated rocks, but only for short periods in early summer. Although at this time they may be exposed to substrate temperatures of > 20°C and strong light they do not suffer from photostress. If, in winter, lichens are covered and kept dark by snow with temperatures close to 0°C, lichens enter a negative carbon balance. This effect may be relevant to lichens under conditions of global or regional warming.