Gas exchange and carbon isotope discrimination in lichens: evidence for interactions between CO2-concentrating mechanisms and diffusion limitation

The characteristics of gas exchange and carbon isotope discrimination were determined for a number of lichen species, representing contrasting associations between fungal (mycobiont) and photosynthetic (photobiont) organism. These parameters were evaluated with regard to the occurrence of any CO2-concentrating mechanism (CCM) expressed specifically by the green algal (phycobiont) or cyanobacterial (cyanobiont) partner. Carbon isotope discrimination (delta) fell into three categories. The highest delta, found in lichens comprising a phycobiont plus cyanobacteria limited to pockets in the thallus (known as cephalodia), ranged from 24 to 28 per thousand, equivalent to a carbon isotope ratio (delta 13C) of around-32 to -36 per thousand vs. Pee Dee Belemnite (PDB) standard. Further evidence was consistent with CO2 supply to the carboxylating system entirely mediated by diffusion rather than a CCM, in that thallus CO2 compensation point and online instantaneous delta were also high, in the range normally associated with C3 higher plants. For lichens consisting of phycobiont or cyanobiont alone, organic material delta formed two distinct ranges around 15 per thousand (equivalent to a delta 13C of - 23 per thousand). Thallus compensation point and instantaneous delta were lower in the cyanobiont group, which also showed higher maximum rates of net photosynthesis, whether expressed on the basis of thallus dry weight chlorophyll content or area. These data provide additional evidence for the activity of a CCM in cyanobiont lichens, which only show photosynthetic activity when reactivated with liquid water. Rates of net CO2 uptake were lower in both phycobiont associations, but were relatively constant across a wide working range of thallus water contents, usually in parallel with on-line delta. The phycobiont response was consistent whether photosynthesis had been reactivated with liquid water or water vapour. The effect of diffusion limitation could generally be seen with a 3-4 per thousand decrease in instantaneous delta at the highest water contents. The expression of a CCM in phycobiont algae, although reduced compared with that in cyanobacteria, has already been related to the occurrence of pyrenoids in chloroplasts. In view of the inherent requirement of cyanobacteria for some form of CCM, and the smaller pools of dissolved inorganic carbon (DIC=CO2+HCO3 +CO3(2-) associated with phycobiont lichens, it appears that delta characteristics provide a good measure of the magnitude of any CCM, albeit tempered by diffusion limitation at the highest thallus water contents.