DIATOM MINERALIZATION OF SILICIC-ACID .3. SI(OH)4 BINDING AND LIGHT DEPENDENT TRANSPORT IN NITZSCHIA-ANGULARIS

Silicic acid binding, possibly at a membrane site is suggested as the first step in Si(OH)4 metabolism in the marine diatom Nitzschia angularis. We estimate 1738±133 Si(OH)4 binding sites·μm-2 cell surface and a turnover of 4-12 Si(OH)4 molecules·s-1 at maximum transport velocity. All studies were carried out using 68Ge(OH)4 as a tracer for Si(OH)4. After Si(OH)4 uptake, rapid (<1 min) transformation or intracellular binding was demonstrated; yet the extractable pool size after 1 min was indicative of transport against a concentration gradient. Initial uptake kinetics were linear for 150 s and saturation kinetics were demonstrated with kinetic parameters of 560 pmol Si(OH)4·106 cells-1·min-1 (Vmax) 4.2 μmol Si(OH)4·L-1(Ks). A metabolic energy requirement for transport was suggested by inhibition of uptake by agents that uncouple or inhibit phosphorylation; transport also was sensitive to agents that block sulfhydryl groups. Such characteristics are consistent with Si(OH)4 transport being an active carrier mediated process. Rates of Si(OH)4 transport were regulated during various growth stages and during the synchronized cell cycle. In Si(OH)4 starved cells, blocked at the initiation of silica frustule formation, cycloheximide treatment caused a rapid decline of transport rate. Blocked cells, placed in the dark, maintained high transport rates for 6 h after which there was a loss of activity during the following 18 h. Light dependent recovery of transport ability in 12 h predarkened cells was dependent on de novo protein synthesis. © 1979 Springer-Verlag.