Μm Si Research Articles

DIATOM MINERALIZATION OK SILICIC ACID. I Si(OH)4 TRANSPORT CHARACTERISTICS IN NAVICULA PELLICULOSA

SUMMARYSilicic acid transport was studied in the photosynthetic diatom Navicula pelliculosa (Bréb.) Hilse using [68Ge] germanic acid (68Ge(OH)4) as a tracer of silicic acid (Si(OH)4). The initial uptake rate of Si(OH)4 was dependent on cell number, pH, temperature, light and was promoted by certain monovalent cations in the medium. Na+ was more effective than K+, whereas Li+ and NH+4 were ineffective at promoting uptake.Uncouplers and inhibitors of oxidative phosphorylation and of photophosphorylation reduced uptake by 40–99% of control values. Uptake was also especially sensitive to the sulfhydryl blocking agents at 10−5 M and to the ionophorous compound valinomycin (10−7 M) which inhibited uptake by 82%.The Si(OH)4 transport system displayed Michaelis‐Menten‐type saturation kinetics with kinetic parameters of KS= 4.4 p. mol Si(OH)4· 1−1, Vmax= 334 pmol Si(OH)4· 106 cells−1· min−1. Calculations of the acid soluble silicic acid pool size based on 60 s uptake at 20 μM Si(OH)4 suggested that intracellular levels of Si could reach 20 mM and as much as 5 mM could exist as free silicic acid, representing maintenance of a 250‐fold concentration gradient compared with the medium.Efflux from preloaded cells was dependent on temperature and the Si(OH)4 concentration of the external medium. In the presence of 100 μMM “cold” Si(OH)4, approximately 30% of the Si(OH)4 in preloaded cells was exchanged in 20 min.The initial uptake rate of Si(OH)4 in logarithmic phase cells was constant, but the uptake rate increased in a linear fashion for 6 h in stationary phase cells.These results suggest that the first step in silica mineralization by diatoms is the active transmembrane transport of Si(OH)4 by an energy dependent, saturable, membrane‐carrier mechanism which requires the monovalent cations Na+ and K+ and is sensitive to sulfhydryl blocking agents. Silicic acid transport activity also appears to be regulated during different growth stages of the diatom.

KINETICS OF SILICON‐LIMITED GROWTH IN THE FRESHWATER DIATOM ASTERIONELLA FORMOSA1,2

SUMMARYGrowth rates of two clones of the freshwater planktonic diatom Asterionella formosa Hass. were measured under conditions in which external silicon concentrations controlled growth. Clone AfOH2 from Lake Ohrid, Yugoslavia, had a higher maximum growth rate (μmax= 1.11 doublings/day) and apparent half‐saturation constant (Ksi] + Sio= 1.93 μM Si) than clone L262 from Lake Windermere, England. (μmax= 0.61 doublings/day; Ksi+ Sio= 1.09 μM Si). Klim, the silicon concentration at μ= 0.9 μmax, is 13.8 μM Si for clone AfOH2 and 6.5 μM Si for clone L262. These values agree well with published field observations showing A. formosa populations decreasing below 0.5 mg/l SiO2 (= 8.4 μM Si). Calculations of yield gave a range of 0.5–1.5 μM Si/106 cells for clone AfOH2 and 0.6–1.9 μM Si/106 cells for clone L262.

Fabrication of a miniature 8K-bit memory chip using electron-beam exposure

A fully-ion-implanted miniature 8192-bit random-access memory chip has been fabricated using electron-beam lithography with minimum linewidth between 1 and 1.5 μm and advanced Si FET technology. Device structure, processing steps, mask transfer, and reactive ion etching processes capable of fabricating device structures in the micrometer and submicrometer dimensions are described. With a minimum linewidth of 1.25 μm, the memory chip occupies an area of 1.1×1.6 mm with an array density of 5 million bits/in.2 (0.8 million bits/cm2). A typical readout access time of 90 ns was measured on a functional chip.

Silica in the sea — its forms and dissolution rate

Reactive, particulate, and total silica were investigated in seawater samples collected in the Oyashio and Kuroshio areas of the western North Pacific and Funka Bay near Hokkaido Island. The concentrations of particulate silica and suspended inorganic matter in the Japan Sea were also determined. A vertical diffusion model was first applied to the profiles of particulate silica to estimate the rate of dissolution of particulate silica in the intermediate water of the Japan Sea. The dissolution rate, which corresponds to the rate of increase of reactive silica, was estimated to be 0·04–0·4 μg-atoms Si 1 −1 γr −1, providing the vertical eddy diffusivity is 1–10 cm 2 s −1 and the settling velocity of particulate matter is (1·16–11·6) × 10 −4 cm s −1. Reactive silica is the dominant species of silica in seawater. No colloidal silica is detectable in samples taken either near shore or in open seas, in any of the whole water column from the surface to near the deep-sea bed. Particulate silica concentrations varied from 0·1 to 2·0 μM Si in the open seas and from 0·5 to 11 μM Si in Funka Bay. Higher values were observed in the surface and subsurface waters in the Oyashio area and the bottom waters in the coastal region.

KINETICS OF SILICON‐LIMITED GROWTH IN THE MARINE DIATOM THALASSIOSIRA PSEUDONANA HASLE AND HEIMDAL (=CYCLOTELLA NANA HUSTEDT)1,2

SUMMARYThe half‐saturation constant (K8) for growth and the maximum growth rate (μmax) were determined for 2 clones of Thalassiosira pseudonana (=Cyclotella nana) under conditions in which external silicon concentrations controlled growth. The estuarine clone (3H) had a higher half‐saturation constant and maximum growth rate (K8= 0.98 μM Si; μmax= 3.6 divisions/day) than the clone from the Sargasso Sea (K8= 0.19 μM Si; μmax= 2.1 divisions/day). The K8 values for each clone are such that the silicate levels found at certain times in both the Sargasso Sea and the coastal regions are rate limiting to growth, hence can be of significance to plant production and to species succession. The yield data are consistent with the concept that growth rate and cellular silicon content vary together in silicon‐limited cultures.

High-frequency fall-off of Impatt diode efficiency

Large-signal operation of Si and GaAs abrupt junction Impatt diodes is studied with a Read type analysis. Modifications are made to the existing Read type analysis to take into account a finite width of the avalanche region. Read's original assumption of a constant total particle current in the avalanche region is kept. A series of diodes with varying space charge layer widths are analyzed at typical operating conditions respectively. It is found, under many simplifying assumptions, that oscillation efficiency of the Si p + n abrupt junction diode remains essentially the same up to about 100 GHz and starts to degrade appreciably beyond that. The cause is the saturation of ionization rates at high electric field. The GaAs diode is found to perform better up to 50 GHz but become comparable beyond 100 GHz. Experimental data in the literature indicate that the tunnel current has practically no effect even in the 0·1 μm Si diode, which is the narrowest studied. This seems to be the case with the GaAs diode too.