Model of the Carbon Concentrating Mechanism in Chloroplasts of Eukaryotic Algae

A generic chloroplast-based model for the carbon concentrating mechanism (CCM) in eukaryotic algae is presented. The fine structure of chloroplasts is represented by separate compartments: marginal and bulk stroma, pyrenoid, girdle lamella, bulk thylakoids, and central lamella traversing the pyrenoid. The roles of the individual structural elements of the chloroplast with respect to the CCM and the effect of carbonic anhydrase activity in various compartments are analysed. Hypothetical HCO-3transport into the acidic thylakoid lumen is adjusted by imposing an optimization principle: a given [CO2] at the site of RuBisCO is achieved with minimum energy costs for the CCM. Our model is highly efficient in terms of saturation of RuBisCO carboxylase activity and the affinity of the chloroplast for CO2, if either a girdle lamella or a pyrenoid is present. The highest efficiency is achieved with a pyrenoid. A eukaryotic CCM is not necessarily associated with accumulation of dissolved inorganic carbon (DIC) as in cyanobacteria. Chloroplasts are categorized into four types corresponding to morphological characteristics of all major algal classes with regard to the presence of pyrenoids, girdle lamellae, and the distribution of CA activity.