We quantified the metabolic cost to the Antarctic leafy liverwort Cephaloziella varians of responding to an abrupt increase in ultraviolet B (UVB) radiation exposure in the natural environment at Rothera Point on the western Antarctic Peninsula (67 °34′S, 68 °07′W). The liverwort was protected from exposure to UVB radiation for 44 days with screens containing Mylar polyester, after which time its thalli, which are normally black in colour, had become green owing to reduced concentrations of an anthocyanidin, identified here as riccionidin A, in thallus tips. Thalli were then exposed to an abrupt increase in UVB radiation by removing the screens. The thalli became visibly darker within 48 h of the screens being removed, resynthesizing riccionidin A to the same concentration as that present in thalli outside screens during this period. Chlorophyll fluorescence measurements indicated that nonphotochemical quenching was higher in the thalli formerly under the screens than in those not previously covered with screens, but that Fv/Fm and photochemical quenching were the same in the two groups of thalli. We used data from aqueous phase oxygen electrode measurements to calculate an estimate for carbon fixation by C. varians during the 48 h after the screens were removed. Assuming a photosynthetic quotient for Antarctic bryophytes of 1, these analyses indicated that the minimum weight of carbon used to synthesize riccionidin A was equivalent to 1.85% of the carbon fixed by thalli during the 48 h after the abrupt increase in UVB radiation exposure.