Long-term responses of the green-algal lichen Parmelia caperata to natural CO enrichment.

Acclimation to elevated CO was investigated in Parmelia caperata originating from the vicinity of a natural CO spring, where the average daytime CO concentration was 729 ± 39 μmol mol dry air. Thalli showed no evidence of a down-regulation in photosynthetic capacity following long-term exposure to CO enrichment in the field; carboxylation efficiency, total Ribulose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO assimilation, and the light-saturated rate of CO assimilation (measured under ambient and saturating CO concentrations) were similar in thalli from the naturally CO enriched site and an adjacent control site where the average long-term CO concentration was about 355 μmol mol. Thalli from both CO environments exhibited low CO compensation points and early saturation of CO uptake kinetics in response to increasing external CO concentrations, suggesting the presence of an active carbon-concentrating mechanism. Consistent with the lack of significant effects on photosynthetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO. Detailed intrathalline analysis revealed a decreased investment of nitrogen in Rubisco in the pyrenoid of algae located in the elongation zone of thalli originating from elevated CO, an effect associated with a reduction in the percentage of the cell volume occupied by lipid bodies and starch grains. Although these differences did not affect the photosynthetic capacity of thalli, there was evidence of enhanced limitations to CO assimilation in lichens originating from the CO-enriched site. The light-saturated rate of CO assimilation measured at the average growth CO concentration was found to be significantly lower in thalli originating from a CO-enriched atmosphere compared with that of thalli originating and measured at ambient CO. At lower photosynthetic photon flux densities, the light compensation point of net CO assimilation was significantly higher in thalli originating from elevated CO, and this effect was associated with higher usnic acid content.