Long-term responses of the green-algal lichen Parmelia caperata to naturalCO2 enrichment

Acclimation to elevated CO2 was investigated in Parmelia caperata originati ng from the vicinity of a natural CO2 spring, where the average daytime CO2 concentration was 729 +/- 39 mu mol mol(-1) dry air. Thalli showed no evid ence of a down-regulation in photosynthetic capacity following long-term ex posure to CO2 enrichment in the field; carboxylation efficiency, total Ribu lose bisphosphate carboxylase/oxygenase (Rubisco) content, apparent quantum yield of CO2 assimilation, and the light-saturated rate of CO2 assimilatio n (measured under ambient and saturating CO2 concentrations) were similar i n thalli from the naturally CO2 enriched site and an adjacent control site where the average long-term CO2 concentration was about 355 mu mol mol(-1). Thalli from both CO2 environments exhibited low CO2 compensation points an d early saturation of CO2 uptake kinetics in response to increasing externa l CO2 concentrations, suggesting the presence of an active carbon-concentra ting mechanism. Consistent with the lack of significant effects on photosyn thetic metabolism, no changes were found in the nitrogen content of thalli following prolonged exposure to elevated CO2. Detailed intrathalline analys is revealed a decreased investment of nitrogen in Rubisco in the pyrenoid o f algae located in the elongation zone of thalli originating from elevated CO2, an effect associated with a reduction in the percentage of the cell vo lume occupied by lipid bodies and starch grains. Although these differences did not affect the photosynthetic capacity of thalli, there was evidence o f enhanced limitations to CO2 assimilation in lichens originating from the CO2-enriched site. The light-saturated rate of CO2 assimilation measured at the average growth CO2 concentration was found to be significantly lower i n thalli originating from a CO2-enriched atmosphere compared with that of t halli originating and measured at ambient CO2, At lower photosynthetic phot on flux densities, the light compensation point of net CO2 assimilation was significantly higher in thalli originating from elevated CO2 and this effe ct was associated with higher usnic acid content.