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Photosynthetic photon flux density (PPFD), air relative humidity and thallus temperature (TT) were measured in Usnea antarctica over a period of 12 months at a coastal rock on King George Island, South Shetland Islands. TT had an annual amplitude of c. 54 K with the maximum of +27.4 degrees C recorded in December and minimum of -27.3 degrees C in July. Daily maximum TT exceeded 0 degrees C every month except in June and July when they were below -0.4 degrees C and -1.6 degrees C, respectively. Daily minimum temperatures were always below zero. Diel courses of PPFD showed pronounced seasonal differences between summer (more than 2000 mu mol, m(-2) s(-1)) and winter (less than 50 mu mol m(-2) s(-1) in July). Daily sum of PPFD was highest (more than 30 mol m(-2) d(-1)) in December and lowest (0.1 mol m(-2) d(-1)) in July. A photosynthesis model was used to estimate the potential annual primary production from habitat PPFD and TT. The estimated potential annual carbon balance of U. antarctica was 323 mg CO2 g(-1) dry weight y(-1) assuming that the lichen was always at optimal moisture. The potential carbon balance indicates that primary production is severely limited by low PPFD and subzero TT in the austral winter. Nevertheless, the PPFD and Tr would allow metabolic activity during winter but with a negative carbon balance during May-August. Spring and autumnal months are revealed as probably the most important periods for lichen primary production because PPFD exceeds photosynthetic light compensation point during the daytime and ambient moisture conditions frequently favour a positive carbon balance.