Potential effects of rising tropospheric concentrations of CO and O on green-algal lichens.

Pormelia sulcata Taylor was used as a model to examine the effects of elevated CO and/or O on green algal lichens. Thalli were exposed for 30 d in duplicate controlled-environment chambers to two atmospheric concentrations of CO ('ambient' [350μmol mol ] and 'elevated' [700μmol mol ] 24 h d ) and two O regimes ('non-polluted' air [CF, < 5 nmol mol ] and 'polluted' air [15 nmol mol overnight rising to a midday maximum of 75 nmol mol ]), in a factorial design. Elevated CO , or elevated O depressed the light saturated rate of CO , assimilation A ) measured at ambient CO , by 30% and 18%, respectively. However, despite this effect ultrastructure) studies revealed increased lipid storage in cells of the photobiont in response to CO -enrichment. Simultaneous exposure to elevated O reduced CO -induced lipid accumulation and reduced A in an additive manner. Gold-antibody labelling revealed that the decline in photosynthetic capacity induced by elevated CO and/or O 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 A induced by elevated CO , and/or O , 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 CO , was found to be reduced after a controlled 30 d exposure to elevated CO , and/or O and this effect was associated with a parallel decline in the amount of Rubisco in the pyrenoid of algal chloroplasts.