Phosphate limited cultures of the cyanobacterium Synechococcus are capable of very rapid, opportunistic uptake of phosphate

. Phosphate uptake rates were measured in Synechococcus R-2 incubated in artificial secondary- and tertiary-treated sewage. . Phosphate uptake was measured using chemical assay and (32)p incorporation. Intracellular pH was measured using accumulation of C-14-labelled weak acids and bases and membrane potentials using Rb-86(+)/valinomycin. . Synechococcus cells are capable of very rapid, opportunistic uptake of phosphate (10-30 nmol m(-2) s(-1)) even though net uptake by growing cultures was < 0.5 nmol m(-2) s-1. Kin and Vmax in the light were not significantly different at pH(o) 7.5 and 10. The mean Kin values were 1.91 +/- 0.41 mmol m(-3) and 0.304 +/- 0.055 mmol m(-3) for P-sufficient (secondary-treated sewage) and P-deficient (tertiary-treated sewage) cells, respectively. The transport systems probably recognize both H2PO4- and HPO42-. Intracellular inorganic phosphate is +28 to +56 kJ mol(-1) from electrochemical equilibrium. In P-sufficient cells uptake is very slow in the dark (c. 0.1 nmol m(-2) s(-1)) but phosphate-starved cells can opportunistically take up P about 100 times faster . . Two separate ATP-driven phosphate uptake mechanisms (1 PO4 in per ATP) appear to be responsible for phosphate uptake by the cells. They have different Kin values, different light/dark responses and electrical behaviour.