Phosphate limited cultures of the cyanobacterium Synechoccus 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 32P incorporation. Intracellular pH was measured using accumulation of 14C-labelled weak acids and bases and membrane potentials using 86Rb+/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. Km and Vmax in the light were not significantly different at pHo 7.5 and 10. The mean Km 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 Km values, different light/dark responses and electrical behaviour.