Membrane electrogenesis and sodium transport in filamentous nitrogen-fixing cyanobacteria

Abstract

Transport of Na⁺ and its relationship with membrane potential (Δψ_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 Δψ_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 Δψ_m to anaerobiosis, cyanide and azide, all of which inhibit aerobic respiration. The response of Δψ_m to external pH and external Na⁺ or K⁺ concentrations indicated that a diffusion potential of Na⁺ or K⁺ may not contribute significantly to Δψ_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 (< 10 mM) of external K⁺. There was a close correspondence between changes in Δψ_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.