Growth and Photosynthesis of the Cyanobacterium Synechococcus leopoliensis in HCO 3 − -Limited Chemostats

Abstract

Synechococcus leopoliensis was grown in HCO3−-limited chemostats. Growth at 50% the maximum rate occurred when the inorganic carbon concentration was 10 to 15 micromolar (or 5.6 to 8.4 nanomolar CO2). The O2 to CO2 ratios during growth were as high as 192,000 to 1. At growth rates below 80% the maximum rate, essentially all the supplied inorganic carbon was converted to organic carbon, and the cells were carbon limited. Carbon-limited cells used HCO3− rather than CO2 for growth. They also exhibited a very high photosynthetic affinity for inorganic carbon in short-term experiments. Cells growing at greater than 80% maximum growth rate, in the presence of high dissolved inorganic carbon, were termed carbon sufficient. These cells had photosynthetic affinities that were about 1000-fold lower than HCO3−-limited cells and also had a reduced capacity for HCO3− transport. HCO3−-limited cells are reminiscent of the air-grown cells of batch culture studies while the carbon sufficient cells are reminiscent of high-CO2 grown cells. However, the low affinity cells of the present study were growing at CO2 concentrations less than air saturation. This suggests that supranormal levels of CO2 not required to induce the physiological changes usually ascribed to high CO2 cells.