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

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Abstract
Silicate uptake by N. alba cells was higher in medium containing Na+ than in media lacking Na+ but containing K+, Rb+, NH4+, Li+, or choline+. The initial rate was inhibited by monensin and gramicidin, but not by valinomycin or nigericin and was less sensitive to inhibition by carbonyl cyanide m-chlorophenylhydrazone (CCCP). In isolated membrane vesicles, silicate was taken up when Na+ gradient was imposed across the membrane, or was generated by cytoplasmic Na+, K+-ATPase. H+ or K+ gradients in either direction did not stimulate uptake. Na+-gradient-dependent uptake was inhibited by monensin but not by CCCP, valinomycin or vanadate, which inhibited the cytoplasmic Na+, K+-ATPase. Uptake increased if an internally negative potential was imposed across the membrane. Vesicular uptake showed saturation kinetics with a Km of 62 .mu.M, and a Vmax 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. In N. alba, silicate is transported with Na+, and the transport system is driven by the Na+ gradient that apparently is generated and maintained across the membrane by the activity of Na+, K+-ATPase.