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 fo r 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' a ir [CF, < 5 nmol mol(-1)] and 'polluted' air [15 nmol mol(-1) overnigh t rising to a midday maximum of 75 nmol mol(-1)]), in a factorial desi gn. Elevated CO2 or elevated O-3 depressed the light saturated rate of CO2 assimilation (A(sat)) measured at ambient CO2 by 30%, and 18%, re spectively. However, despite this effect ultrastructural studies revea led increased lipid storage in cells of the photobiont in response to CO2-enrichment. Simultaneous exposure to elevated O-3 reduced CO2-indu ced lipid accumulation and reduced A(sat) in an additive manner. Gold- antibody labelling revealed that the decline in photosynthetic capacit y induced by elevated CO2 and/or O-3 was accompanied by a parallel dec rease 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 c hlorophyll-fluorescence induction kinetics showed that the decline in A(sat) induced by elevated CO2 and/or O-3 was not associated with sign ificant changes in the photochemical efficiency of photosystem (PS)II. Although the experimental conditions inevitably imposed some stress o n the thalli, revealed a significant decline in the efficiency of PS I I photochemistry, and enhanced starch accumulation in the photobiont o ver the fumigation period, the study shows that the green-algal lichen symbiosis might be influenced by future changes in atmospheric compos ition. 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.