Membrane electrogenesis and sodium transport in filamentous nitrogen-fixing cyanobacteria

Transport of Na+ and its relationship with membrane potential (delta psi m) was examined in Anabaena L-31 (a fresh water cyanobacterium) and Anabaena torulosa (a brackish water cyanobacterium) which require Na+ for diazotrophic growth. The data on the effect of N,N'-dicyclohexylcarbodiimide indicated that delta psi m was generated by electrogenic proton extrusion predominantly mediated by ATPase(s). In addition, operation of a plasmalemmabound, non-ATP-requiring, H+-pumping terminal oxidase was suggested by the sensitivity of delta psi m to anaerobiosis, cyanide and azide, all of which inhibit aerobic respiration. The response of delta psi m to external pH and external Na+ or K+ concentrations indicated that a diffusion potential of Na+ or K+ may not contribute significantly to delta psi m. Kinetic studies showed that Na+ influx was unlikely to be a result of Na+/NA+ exchange but was a carrier-mediated secondary active transport insensitive to low concentrations (less than 10 mM) of external K+. There was a close correspondence between changes in delta psi m and Na+ influx; all the treatments which caused depolarisation (such as low temperature, dark, cyanide, azide, anaerobiosis, ATPase inhibitors) lowered Na+ influx whereas treatments which caused hyperpolarisation (such as 2,4-dinitrophenol, nigericin) enhanced Na+ influx. Remarkably low intracellular Na+ concentrations were maintained by these cyanobacteria by means of active efflux of the cation. The basic mechanism of Na+ transport in the fresh water and the brackish water cyanobacterium was similar but the latter demonstrated less influx, more efficient efflux, more affinity of carriers for Na+ and less accumulation of Na+, all attributes favouring salt tolerance.