THE ROLE OF THE CHLOROPLAST IN INORGANIC CARBON UPTAKE BY EUKARYOTIC ALGAE

The role of the chloroplast in the adaptation to low CO2 by eukaryotic algae is reviewed. Eukaryotic algae can grow on very low CO2 levels b ecause of the presence of a CO2 concentrating mechanism (CCM). This re view is focused on the localization of key photosynthetic enzymes such as ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and carb onic anhydrase as well as the location of presumptive components of th e CCM and photorespiratory cycle within the chloroplast. Previous immu nolocalization studies place as much as 99% or as little as 5% of the cell's Rubisco in the chloroplast pyrenoid. These different results ar e summarized, and an alternative explanation is provided. The differen t results appear to be due to the growth regime of the algae as well a s differences in quantitation. Evidence suggests that a large majority of Rubisco is located within the pyrenoid. We have also summarized th e recent discovery of a thylakoid-bound carbonic anhydrase that is ess ential to growth on low CO2. A model depicting a possible role for thi s carbonic anhydrase in photosynthesis is presented.