Phosphate limited cultures of the cyanobacterium <i>Synechococcus</i> are capable of very rapid, opportunistic uptake of phosphate

Abstract

Summary 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 14 C‐labelled weak acids and bases and membrane potentials using 86 Rb + /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 &lt; 0.5 nmol m −2 s −1 . Km and Vmax in the light were not significantly different at pH o 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 H 2 PO 4 − and HPO 4 2− . 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 PO 4 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.