UPTAKE AND FIXATION OF CO2 IN LICHEN PHOTOBIONTS

Lichens are the symbiotic phenotype of nutritionally specialised fungi that live in symbiosis with photosynthesising algal and/or cyanobacte rial photobionts. Although the initiation and maintenance of the metab olic activity of lichens require that water is taken up and stored, ex cess water may potentially limit the photosynthetic activity of the li chen if this causes swelling of the fungal hyphae, which may impede th e diffusion of CO2 to the photobiont. In free-living algae and cyanoba cteria this potential limitation of photosynthesis has partly been com pensated for by the evolution of a CO2 concentrating mechanism (CCM). This mechanism operates under conditions of low CO2 availability in th eir environment, such as when the diffusion of CO2 is slow or when HCO 3- is the dominating inorganic carbon source. This paper gives a brief presentation of the function of the CCM in free-living cyanobacteria and microalgae and summarises recently obtained evidence for the prese nce of this mechanism in lichens with cyanobacterial Nostoc and green algal Trebouxia photobionts. However, the CCM is absent in some photob iont genera; evidence for this is also presented and possible reasons for this absence are discussed.