The lichen Turgidosculum complicatulum (formerly Mastodia tesselata) occurs in the shore-zone of Marion Island (sub-Antarctic: 47°S,38°E). Net CO2 exchange in the lichen is dominated by a strong temperature-dependence of respiration rate. The light/temperature response of photosynthesis is such that under the prevailing climatic regime on the island the lichen, if sufficiently hydrated, would exhibit near-maximal photosynthesis rates for 75% of the photoperiod over the year. A photosynthetic response model predicts that the lichen's net annual carbon acquisition is 3.1 g C g-1 year-1 under the current solar radiation and temperature regime at the island. The model predicts that changes in temperature and radiation by the amounts known to have occurred in the past few decades, and even greater changes (temperature increase by up to 2°C, radiation by up to 10%), would negligibly affect the annual amount of carbon acquired provided the thalli remain hydrated. Incorporating hydration/desiccation cycles into the model resulted in a substantial lowering of annual net C exchange. However, attempts to include the increase in aridity known to have occurred at the island since 1971 gave conflicting scenarios for the effect on annual C acquisition, depending on whether atmospheric drying or thallus drying was considered.