The Mechanism of Inorganic Carbon Uptake by Cyanobacteria: Energization and Activation by Light

Abstract

The photosynthesis and growth of cyanobacteria depend on their ability to accumulate inorganic carbon (C_i) within the cells, since the apparent K_m (CO₂) of their Rubisco is about thirty times higher than the available CO₂ concentration in the environment (1). This ability to actively accumulate Cj internally depends on the concentration of CO₂ experienced by the cells during growth (2,3). Low CO₂ (L) adapted cells are capable of concentrating C_i to as much as x 1000 its extracellular level (2). The apparent pho-tosynthetic affinity of L cells is therefore much higher than that observed in high CO₂ (H) grown cells (2). While the signal for adaptation has not yet been identified, it is clear that it depends on the concentration ratio of CO₂/O₂ rather than on [CO₂] as such (4). In green algae it has been shown that adaptation from L to H occurs at a certain phase of the cell cycle regardless of the [CO₂] present (5). The adaptation of cells from one CO₂ level to another involves a syndrome of changes at different levels of cell organization and metabolic steps (6).