PHOTOSYNTHETIC CARBON ACQUISITION IN THE LICHEN PHOTOBIONTS CACCOMYXAAND TREBOUXIA (CHLOROPHYTA)

Processes involved in photosynthetic CO2 acquisition were characterise d for the isolated lichen photobiont Trebouxia erici (Chlorophyta, Tre bouxiophyceae) and compared with Coccomyxa (Chlorophyta), a lichen pho tobiont without a photosynthetic CO2-concentrating mechanism. Comparis ons of ultrastructure and immune-gold labelling of ribulose-1,5-bispho sphate carboxylase-oxygenase (Rubisco; EC 4.1.1.39) showed that the ch loroplast was larger in T. erici and that the majority of Rubisco was located in its centrally located pyrenoid. Coccomyxa had no pyrenoid a nd Rubisco was evenly distributed in its chloroplast. Both species pre ferred CO2 rather than HCO3- as an external substrate for photosynthes is, but T. erici was able to use CO2 concentrations below 10-12 mu M m ore efficiently than Coccomyxa. In T. erici, the lipid-insoluble carbo nic anhydrase (CA; EC 4.2.1.1) inhibitor acetazolamide (AZA) inhibited photosynthesis at CO2 concentrations below 1 mu M, while the lipid-so luble CA inhibitor ethoxyzolamide (EZA) inhibited CO2-dependent O-2 ev olution over the whole CO2 range. EZA inhibited photosynthesis also in Coccomyxa, but to a much lesser extent below 10-12 mu M CO2. The inte rnal CA activity of Trebouxia, per unit chlorophyll (Chi), was ca 10% of that of Coccomyxa. Internal CA activity was also detected in homoge nates from T. erici and two Trebouxia-lichens (Lasallia hispanica and Cladina rangiferina). In all three, the predominating CA had a-type ch aracteristics and was significantly inhibited by low concentrations of AZA, having an I-50 below 10-20 ruM. In Coccomyxa a beta-type CA pred ominates, which is much less sensitive to AZA. Thus, the two photobion ts differed in three major characteristics with respect to CO2 acquisi tion, the subcellular location of Rubisco, the relative requirement of CA and the biochemical characteristics of their predominating interna l CA. These differences may be linked to the ability of Trebouxia to a ccumulate dissolved inorganic carbon internally, enhancing their CO2 u se efficiency at and below air-equilibrium concentrations (10-12 mu M CO2) in comparison with Coccomyxa.