Phosphate Uptake in the Cyanobacterium Synechococcus R-2 PCC 7942

Abstract

Phosphate uptake rates in Synechococcus R-2 in BG11 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 pH010 have a net uptake rate of about 600 pmol m~s phosphate, but the isotopk flux for P phosphate was about 4 nraol m~s~'. There appears to be a constitutive over-capacity for phosphate uptake. The Km and Vmtx of the saturable component were not significantly different at pH0 7.5 and 10, hence the transport system probably recognizes both H2PO4 and HPO ,. The intracellular inorganic phosphate concentration is about 3 to 10 mol m~, but there is an intracellular polyphosphate store of about 400 mol m~. Intracellular inorganic phosphate is 25 to 50 kJ mol' 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~s~) and is light-activated (pHo 7.5«1.3nmolms~, pHo 10«600 pmol m^ ). Uptake has an irreversible requirement for Mg 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 PO4,0/ATP) to be thermodynamically possible under all the conditions tested in the present study.