Plasma membrane electron transport coupled to Na+ extrusion in the halotolerant alga Dunaliella

The halotolerant alga Dunaliella adapts to exceptionally high salinity and maintains low [Na+](in) at hypersaline solutions, suggesting that it possesses efficient mechanisms for regulating intracellular Na+. In this work we examined the possibility that Na+ export in Dunaliella is linked to a plasma membrane electron transport (redox) system. Na+ extrusion was induced in Dunaliella cells by elevation of intracellular Na+ with Na+-specific ionophores. Elevation of intracellular Na+ was found to enhance the reduction of an extracellular electron acceptor ferricyanide (FeCN). The quinone analogs NQNO and dicumarol inhibited FeCN reduction and led to accumulation of Na+ by inhibition of Na+ extrusion. These inhibitors also diminished the plasma membrane potential in Dunaliella. Anaerobic conditions elevated, whereas FeCN partially decreased intracellular Na+ content. Cellular NAD(P)H level decreased upon enhancement of plasma membrane electron transport. These results are consistent with the operation of an electrogenic NAD(P)H-driven redox system coupled to Na+ extrusion in Dunaliella plasma membrane. We propose that redox-driven Na+ extrusion and recycling in Dunaliella evolved as means of adaptation to hypersaline environments. (C) 2001 Elsevier Science B.V. All rights reserved.