SODIUM-TRANSPORT AND THE ORIGIN OF THE MEMBRANE-POTENTIAL IN THE CYANOBACTERIUM SYNECHOCOCCUS R-2 (ANACYSTIS, NIDULANS) PCC-7942

Synechococcus R-2 (PCC 7942) actively extruded Na+ in the light and dark, and over a very wide range of pH(o) (5 to 10), [Na+]o and [K+]o (0.1 to 300 mol m-3). The Na+ motive force or Na+ electrochemical potential (DELTA-mu-Na+i,o) was -93 +/- 3 mV in 0.1 mol m-3 Na+o, rose to a maximum of -150 +/- 6 mV in 50 mol m-3 Na+o then fell off to -112 +/- 3 mV in 300 mol m-3 Na+o. [Na+]i was virtually independent of pHo (5 to 10), (E)Na+i,o ranged from 24 +/- 11 mV (pH(o) 7) to 8 +/- 3 mV (pH(o) 10). DELTA-mu-Na+i,o decreased in parallel to DELTA-psi(i,o) in increasing [K+]o. Under control conditions ([Na+]o = 18 mol m-3), [Na+]i was 10.87 +/- 0.98 mol m-3, DELTA-mu-Na+i,o = -139 +/- 3 mV (light); [Na+]i = 28.9 +/- 2.3 mol m-3, DELTA-mu-Na+i,o = -115 +/- 3 mV (dark). Na+ fluxes and permeabilities were much higher in the light: phi-Na+ = 275 +/- 38 nmol m-2 s-1, (P)Na+in = 3 +/- 0.4 nm s-1 (light); phi-Na+ = 5.97 +/- 0.60 nmol m-2 s-1, (P)Na+in = 0.073 +/- 0.007 nm s-1 (dark). Phi-Na+ in the light was insensitive to dichlorophenyldimethylurea (DCMU). Dicyclohexylcarbodiimide (DCCD) inhibited phi-Na+ in the light by about 75% and [Na+]i doubled to 21.2 +/- 2.4 (4) mol m-3 but in the dark phi-Na+ increased to 16.5 +/- 3.8 nmol m-2 s-1 (20) and [Na+]i increased to 136 +/- 34 mol m-3 (20). Phi-Na+out In the light was directly proportional to [Na+]o. In alkaline pH(o) the proton motive force (pmf(i,o)) was near zero but DELTA-mu-Na+i,o exceeded -100 mV. The active extrusion of Na+ could not be driven by pmf(i,o). In the light, Na+ was probably actively extruded by an electrogenic 3 Na+/ATP pump which in turn drives H+ efflux via a H+out/2 Na+in antiport at pH(o) less than or equal to 9. The Na+ conductance, under control conditions, was (G)Na+in = 2.44 (+/- 0.49) S m-2, which is comparable to the conductance of the proton circuits found in giant algal cells, but (G)Na+in fell off rapidly in lower [Na+]o. DELTA-psi(i,o) is apparently generated by electrogenic pumping of Na+ in the light and dark and is not a K+/Na+ Goldman diffusion potential.