Effects of UV-B radiation on inorganic carbon acquisition by the marine microalga Dunaliella tertiolecta (Chlorophyceae)

The chlorophyte alga Dunaliella tertiolecta was exposed to UV-B radiation (UVBR) and the characteristics of dissolved inorganic carbon (DIC)-dependent oxygen evolution, carbon fixation and CO-concentrating mechanism (CCM) activity were examined. Exposure response measurements were carried out under 2.8 W m(-2) UVBR (unweighted), and the maximum quantum yield of photosystem II (PSII). Phi(PSII-max), was monitored until cell Suspensions were similar to50% inhibited. Under these conditions, although photosystem II (PSII) activity was severely inhibited. photosystem I capacity decreased by only < 15%. UVBR-treated cells exposed to (HCo3-)-Co-14 took up the same amount of label as did untreated cells, though the proportion fixed into organic matter decreased under UVBR and the internal DIC pool increased. The increase in internal DIC pools was paralleled by an increase in conductance for DIC, measured as the increase in the initial slope of oxygen evolution vs [DIC] plots. Thus, although carbon assimilation rates are inhibited by UVBR, CCM activity is unaffected. The ecological significance of these findings is discussed.