CO2 Acquisition, Concentration and Fixation in Cyanobacteria and Algae

Abstract

Aquatic photosynthetic organisms face a number of unique problems with regard to the supply of CO2 for photosynthesis. These stem largely from the physical chemistry of the water phase in which they live, where the diffusion of C1 species is slow and can exists as both CO2 and HCO3 − depending on the pH of the medium. Given the constraints, a number of solutions have evolved to optimize photosynthetic CO2 fixation in algae and cyanobacteria. The two chief strategies that are apparent are the development of CO2-concentrating mechanisms based on the active uptake of both CO2 and HCO3 − and the evolution of more efficient forms of Rubisco which are able to fix CO2 at limiting levels of CO2. This chapter examines aspects of co-evolution of Rubisco and CO2-concentrating mechanisms in both algae and cyanobacteria. Particular emphasis is placed on what is known about the mechanism of the operation of carbon concentrating mechanisms (CCMs) in cyanobacteria and green microalgae. In cyanobacteria, multiple active C1 transporters drive the CCM on the plasma membrane. These are energized by photosynthetic ATP and NADPH production, with the NAD(P)H dehydrogenase complexes playing a critical role. A pool of HCO3 − is accumulated within the cell and this is used by the Rubisco-containing carboxysome to generate CO2 within this localized micro-environment. Carboxysomal carbonic anhydrase is crucial to this CO2 generation process. For green microalgae, active C1 transport occurs at both the plasma membrane and the chloroplast envelope. A HCO3 pool is accumulated in the chloroplast stroma, with the aid of photosynthetic energy, and this pool is used to elevate CO2 around Rubisco. Rubisco is primarily localized to the pyrenoid. A crucial part of this conversion appears to be a thylakoid lumen carbonic anhydrase, which may use the lumenal protons to drive this process. The CCM is inducible in nature in both cyanobacteria and algae, increasing its affinity forexternal C1 when cells are grown at limiting C1 conditions. In both green and non-green algae, the presence of significant CCM activity is correlated with the presence of pyrenoids and single chloroplasts within cells. This chapter examines the possible diversity of CCM operation among the green and non-green algae, highlighting possible variation in CCM operations compared to the models developed for green microalgae. Considerable work remains to be done to identify specific variation of CCM mechanismsin non-green algae.