Models of CO 2 concentrating mechanisms in microalgae taking into account cell and chloroplast structure

Abstract

Detailed mathematical models have been developed for the functioning of CO 2 concentration mechanisms in microalgae. The models treat a microalgal cell as several compartments: pyrenoid, chloroplast stroma, cytoplasm and periplasmic space. Cases for both the active bicarbonate transport through the plasmalemma and the passive CO 2 diffusion through it with the subsequent concentrating of CO 2 inside the chloroplast are analyzed. CO 2 evolution from bicarbonate inside the pyrenoid is modeled. The great diffusion resistance for CO 2 flux from the pyrenoid is caused by a starch envelope and the concentric thylakoid membranes surrounding it. The role of carbonic anhydrase in the periplasmic space, cytoplasm and inside the chloroplast is evaluated numerically. The models also offer an explanation for the absence of `short-circuited' inorganic carbon fluxes between the external medium and the cytoplasm under active bicarbonate transport through the plasmalemma and in the presence of carbonic anhydrase in the cytoplasm. If the cytoplasm is driven from the space between a chloroplast envelope and plasmalemma upon the microalgae adaptation to low concentration of the dissolved inorganic carbon, the inorganic carbon leak might be avoided. The models reproduce accurately the majority of known experimental data. The high efficiency of CO 2 concentrating mechanisms in microalgae can be explained by a considerable diffusion resistance for CO 2 flux from the pyrenoid and by the effective scavenging of CO 2 leaking outward from the chloroplast to cytoplasm and from cell to periplasmic space.