Sodium-dependent silicate transport in the apochlorotic marine diatom Nitzschia alba

Abstract

Silicate uptake by Nitzschia alba cells is higher in medium containing Na(+) than in media lacking Na(+) but containing K(+), Rb(+), NH(4) (+), Li(+), or choline(+). The initial rate is inhibited by monensin and gramicidin but not by valinomycin or nigericin and is less sensitive to inhibition by carbonyl cyanide m-chlorophenylhydrazone (CCCP). In isolated membrane vesicles, silicate is taken up when a Na(+) gradient is imposed across the membrane or is generated by cytoplasmic Na(+),K(+)-ATPase. H(+) or K(+) gradients in either direction do not stimulate uptake. Na(+)-gradient-dependent uptake is inhibited by monensin but not by CCCP, valinomycin, or vanadate, which inhibits the cytoplasmic Na(+),K(+)-ATPase. Uptake increases if an internally negative potential is imposed across the membrane. The vesicular uptake shows saturation kinetics with a K(m) of 62 muM and a V(max) of 4.1 nmol/mg of protein per min. In intact cells, the initial rate of silicate uptake increases with pH up to 9.5. Thus, in N. alba, silicate is symported with Na(+), and the transport system is driven by the Na(+) gradient that is generated and maintained across the membrane by the activity of Na(+),K(+)-ATPase.