Membrane Potential and pH Control in the Cyanobacterium Synechococcus R-2 (Anacystis nidulans) PCC 7942

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Summary The membrane potential (Δψi,o) and intracellular pH (pHi) in the cyanobacterium Synechococcus R-2 (Anacystis nidulans) (PCC 7942) were measured. Δψi,o was measured using valinomycin-mediated uptake of 86Rb+. Δψi,o under control conditions (BG-11 medium, pHo 7.5, [K+]o 0.35 mol m-3, [Na+]o 18.04 mol m-3) was -126 ± 1.2 mV (light) and -115 ± 3.1 mV (dark). Anaerobic conditions in the dark did not depolarise Δψi,o any further. DCMU did not depolarise Δψi,o to the value observed in the dark. The following parameters all refer to cells in the light. Nigericin (5 mmol m-3) depolarised Δψi,o to -98 mV and carbonyl cyanide p-trifluoromethoxy phenylhydrazone (FCCP) (5 mmol m-3) to -78 mV; nigericin + FCCP depolarised by more than the two separately (-62 mV). Monensin (100 mmol m-3) depolarised Δψi,o to -90 mV. Synechococcus actively took up K+ at [K+]o below 1 mol m-3 but K+ was in electrochemical equilibrium at higher [K+]o where increasing [K+]o depolarised Δψi,o by 56 ± 3 mV/log [K+]o. Δψi,o was insensitive to [Na+]o below 50 mol m-3. EK+i,o and ΔµK+i,o were independent of [Na+]o at concentrations below 30 mol m-3. No evidence was found for an interaction between pHo and [K+]i at 0.35 mol m-3 K+o. In neutral and alkaline pHo the Δψi,o was -120 to -130 mV; in acid pHo Δψi,o depolarised to -80 to -90 mV. pHi was measured using [14C]-5,5 dimethyl oxazolidine 2,4 dione (DMO) and [14C]-ethylamine. pHi was very stable; ranging from 7.16 ± 0.03 to 7.55 ± 0.02 over a pHo range from 5 to 10 in the light. The pmf1,o (proton motive force) progressively fell from -220 ± 3 mV at the pHo 5 to + 32 ± 6 mV at pHo 10 by 48.9 ± 11 mV/pHo. Synechococcus actively extruded H+ at all pHo below 9 but H+ was actively taken up at pHo 10.