Photosynthesis and Inorganic Carbon Usage by the Marine Cyanobacterium, Synechococcus sp

The marine cyanobacterium, Synechococcus sp. Nägeli (strain RRIMP N1) changes its affinity for external inorganic carbon used in photosynthesis, depending on the concentration of CO₂ provided during growth. The high affinity for CO₂ + HCO₃⁻ of air-grown cells (K_½ < 80 nanomoles pH 8.2]) would seem to be the result of the presence of an inducible mechanism which concentrates inorganic carbon (and thus CO₂) within the cells. Silicone-oil centrifugation experiments indicate that the inorganic carbon concentration inside suitably induced cells may be in excess of 1,000-fold greater than that in the surrounding medium, and that this accumulation is dependent upon light energy. The quantum requirements for O₂ evolution appear to be some 2-fold greater for low CO₂-grown cells, compared with high CO₂-grown cells. This presumably is due to the diversion of greater amounts of light energy into inorganic carbon transport in these cells.

A number of experimental approaches to the question of whether CO₂ or HCO₃⁻ is primarily utilized by the inorganic carbon transport system in these cells show that in fact both species are capable of acting as substrate. CO₂, however, is more readily taken up when provided at an equivalent concentration to HCO₃⁻. This discovery suggests that the mechanistic basis for the inorganic carbon concentrating system may not be a simple HCO₃⁻ pump as has been suggested. It is clear, however, that during steady-state photosynthesis in seawater equilibrated with air, HCO₃⁻ uptake into the cell is the primary source of internal inorganic carbon.