The kinetic properties of ribulose-1,5-bisphosphate carboxylase/oxygenase may explain the high apparent photosynthetic affinity of Nannochloropsis sp. to ambient inorganic carbon

The marine unicellular alga Nannochloropsis sp. (Eustigmatophyceae) exhibits high apparent affinity for extracellular inorganic carbon (Ci) despite the fact that its ability to accumulate Ci within the cells is relatively low. Kinetic investigation of carboxylation enzyme, ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO), isolated from Nannochloropsis suggests that the latter discrepancy can be accounted for by the high affinity of RubisCO for CO₂. A K_m(CO₂) of 7-10 μM was obtained both by a standard radiolabeling approach and by novel methodology using membrane inlet mass spectrometry. The latter allows precise determination of the changes in the concentrations of dissolved CO₂ and O₂ as the reaction proceeds. The kinetic parameters of the oxygenase reaction, deduced from measurements of oxygen level, indicated a high K_m(O₂) (about 1 mM) and high V_max (3.9 μmol O₂ min ^-1 mg^-1 protein) values, compared to those observed in green algae. Thus, despite Nannochloropsis RubisCO's low K_m(CO ₂), an unusually low specificity factor of 27 was calculated, lower than observed in cyanobacteria and close to values found in anaerobic organisms. We proposed that the elevated CO₂ level within the cells, indicated by massive net efflux of CO₂ during steady state photosynthesis, is essential for its growth under the high O₂ concentrations prevailing in the environment.