The role of the chloroplast in inorganic carbon uptake by eukaryotic algae

The role of the chloroplast in the adaptation to low CO 2 by eukaryotic algae is reviewed. Eukaryotic algae can grow on very low CO 2 levels because of the presence of a CO 2 concentrating mechanism (CCM). This review is focused on the localization of key photosynthetic enzymes such as ribulose-1,5-bisphosphate carboxylase-oxygenase (Rubisco) and carbonic anhydrase as well as the location of presumptive components of the CCM and photorespiratory cycle within the chloroplast. Previous immunolocalization studies place as much as 99% or as little as 5% of the cell's Rubisco in the chloroplast pyrenoid. These different results are summarized, and an alternative explanation is provided. The different results appear to be due to the growth regime of the algae as well as differences in quantitation. Evidence suggests that a large majority of Rubisco is located within the pyrenoid. We have also summarized the recent discovery of a thylakoid-bound carbonic anhydrase that is essential to growth on low CO 2 . A model depicting a possible role for this carbonic anhydrase in photosynthesis is presented.Key words: chloroplast, algae, pyrenoid, carbonic anhydrase, photosynthesis.