Potential effects of rising tropospheric concentrations of CO2 and O-3 on green-algal lichens

Parmelia sulcata Taylor was used as a model to examine the effects of elevated CO2 and/or O-3 on green algal lichens. Thalli were exposed for 30 d in duplicate controlled-environment chambers to two atmospheric concentrations of CO2 ('ambient' [350 mu mol mol(-1)] and 'elevated' [700 mu mol mol(-1)] 24 h d(-1)) and two O-3 regimes ('non-polluted' air [CF, < 5 nmol mol(-1)] and 'polluted' air [15 nmol mol(-1) overnight rising to a midday maximum of 75 nmol mol(-1)]), in a factorial design. Elevated CO2 or elevated O-3 depressed the light saturated rate of CO2 assimilation (A(sat)) measured at ambient CO2 by 30%, and 18%, respectively. However, despite this effect ultrastructural studies revealed increased lipid storage in cells of the photobiont in response to CO2-enrichment. Simultaneous exposure to elevated O-3 reduced CO2-induced lipid accumulation and reduced A(sat) in an additive manner. Gold-antibody labelling revealed that the decline in photosynthetic capacity induced by elevated CO2 and/or O-3 was accompanied by a parallel decrease in the concentration of Rubisco in the algal pyrenoid (r = 0.93). Interestingly, differences in the amount of Rubisco protein were not correlated with changes in pyrenoid volume. Measurements of in vivo chlorophyll-fluorescence induction kinetics showed that the decline in A(sat) induced by elevated CO2 and/or O-3 was not associated with significant changes in the photochemical efficiency of photosystem (PS)II. Although the experimental conditions inevitably imposed some stress on the thalli, revealed a significant decline in the efficiency of PS II photochemistry, and enhanced starch accumulation in the photobiont over the fumigation period, the study shows that the green-algal lichen symbiosis might be influenced by future changes in atmospheric composition. Photosynthetic capacity, measured at ambient CO2, was found to be reduced after a controlled 30 d exposure to elevated CO2 and/or O-3 and this effect was associated with a parallel decline in the amount of Rubisco in the pyrenoid of algal chloroplasts.