Long-term responses of the green-algal lichen Parmelia caperata to natural CO2 enrichment

8 pages, figures, and tables statistcs.
Acclimation to elevated CO2 was investigated in Parmelia caperata originating from the vicinity of a natural CO2 spring, where the average daytime CO2 concentration was 729 39 lmol mol)1 dry air. Thalli showed no evidence of a down-regulation in photosyn- thetic capacity following long-term exposure to CO2 enrichment in the ®eld; carboxylation e ciency, total Ribulose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO2 assimilation, and the light-saturated rate of CO2 assimilation (measured under ambient and saturating CO2 concen- trations) were similar in thalli from the naturally CO2 enriched site and an adjacent control site where the average long-term CO2 concentration was about 355 lmol mol)1. Thalli from both CO2 environments exhibited low CO2 compensation points and early sat- uration of CO2 uptake kinetics in response to increasing external CO2 concentrations, suggesting the presence of an active carbon-concentrating mechanism. Consistent with the lack of signi®cant e ects on photosynthetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO2. 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 CO2, an e ect associated with a reduction in the percentage of the cell volume occupied by lipid bodies and starch grains. Although these dif- ferences did not a ect the photosynthetic capacity of thalli, there was evidence of enhanced limitations to CO2 assimilation in lichens originating from the CO2-en- riched site. The light-saturated rate of CO2 assimilation measured at the average growth CO2 concentration was found to be signi®cantly lower in thalli originating from a CO2-enriched atmosphere compared with that of thalli originating and measured at ambient CO2. At lower photosynthetic photon ¯ux densities, the light com- pensation point of net CO2 assimilation was signi®- cantly higher in thalli originating from elevated CO2, and this e ect was associated with higher usnic acid content.