Potential effects of rising tropospheric concentrations of CO2 and O3 on green‐algal lichens

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Abstract
Pormelia sulcata Taylor was used as a model to examine the effects of elevated CO2 and/or O3 on green algal lichens. Thalli were exposed for 30 d in duplicate controlled-environment chambers to two atmospheric concentrations of CO2 ('ambient' [350μmol mol-1 ] and 'elevated' [700μmol mol-1 ] 24 h d-1 ) and two O3 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 O3 depressed the light saturated rate of CO2 , assimilation Asat ) measured at ambient CO2 , by 30% and 18%, respectively. However, despite this effect ultrastructure) studies revealed increased lipid storage in cells of the photobiont in response to CO2 -enrichment. Simultaneous exposure to elevated O3 reduced CO2 -induced lipid accumulation and reduced Asat in an additive manner. Gold-antibody labelling revealed that the decline in photosynthetic capacity induced by elevated CO2 and/or O3 was accompanied by a parallel decrease in the concentration of Rubiscoa 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 Asat induced by elevated CO2 , and/or O2 , 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 as a significant decline in the efficiency of PS II photochemistry, and enhanced starch accumulation in the photobiont over the fornication 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 O3 and this effect was associated with a parallel decline in the amount of Rubisco in the pyrenoid of algal chloroplasts.