Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

Abstract

Phosphate uptake rates in Synechococcus R-2 in BG-11 media (a nitrate-based medium, not phosphate limited) were measured using cells grown semi-continuously and in continuous culture. Net uptake of phosphate is proportional to external concentration. Growing cells at pHo 10 have a net uptake rate of about 600 pmol m−2 s−1 phosphate, but the isotopic flux for 32P phosphate was about 4 nmol m−2 s−1. There appears to be a constitutive over-capacity for phosphate uptake. The Km and Vmax, of the saturable component were not significantly different at pHo 7.5 and 10, hence the transport system probably recognizes both H2PO−4and HPO2−4. The intracellular inorganic phosphate concentration is about 3 to 10 mol m−3, but there is an intracellular polyphosphate store of about 400 mol m−3. Intracellular inorganic phosphate is 25 to 50 kJ mol−1 from electrochemical equilibrium in both the light and dark and at pHo 7.5 and 10. Phosphate uptake is very slow in the dark (≈ 100 pmol m−2 s−1) and is light-activated (pHo 7.5≈1.3 nmol m−2 s−1, pHo 10≈600 pmol m−2 s−1). Uptake has an irreversible requirement for Mg2+ in the medium. Uptake in the light is strongly Na+-dependent. Phosphate uptake was negatively electrogenic (net negative charge taken up when transporting phosphate) at pHo 7.5, but positively electrogenic at pHo 10. This seems to exclude a sodium motive force driven mechanism. An ATP-driven phosphate uptake mechanism needs to have a stoichiometry of one phosphate taken up per ATP (1 PO4 in/ATP) to be thermodynamically possible under all the conditions tested in the present study.