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

Silicate uptake by Nitzschia alba cells is higher in medium containing Na+than in media lacking Na+but containing K+, Rb+, NH4 +, 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 Kmof 62 μ M and a Vmaxof 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.