Cycl opal

Klug et al. (1999) selected RNA sequences that bind the Sel B protein of E. coli, an elongation factor which recognizes a stem-loop structure and drives the incorpor-ation of selenocysteine in response to an opal (UGA) codon immediately upstream of the structure. The wild-type hairpin recognized by Sel B in the dehydrogenase G mRNA was used as a competitor for the selection allowing the RNA pool to evolve very quickly. In four cycles high affinity aptamers (Kd 1 nM) were selected. Several aptamers bound 50-fold... 

The isotopic composition of silicon in diatom opal has been investigated as a proxy for the degree of silicate utilization by diatoms, based on the fact that diatoms fractionate the silicon isotopes ( °Si and Si) during uptake (De La Rocha et al., 1997, 1998). This application is analogous to the use of nitrogen isotopes to study nitrate utilization, but with important differences. On the one hand, the upper ocean cycle is arguably simpler for silica than bio-available nitrogen, which bodes well for the silicon isotope system. 

Recent studies have very clearly delineated the importance of aluminum in sediments for both the opal dissolution rate and the observed asymptotic H4Si04 concentration. The incorporation of aluminum into opal and the hypothesized reprecipitation of H4Si04 from dissolving opal suggest that the silica cycle may exert a significant influence on the cycling of major constiments... 

The Phanerozoic organisms that maintain the concentration of silica in seawater at such low levels also control the distribution of siliceous sediments. Since diatoms became major participants in the silica cycle, these sediments have dominantly been deep-sea accumulations of relatively soluble opal-A. This phase is... 

Silica precipitation is controlled by the concentration of silica in solution, itself related to silica availability and, to a lesser extent, the duration of wetting/drying cycles (Knauth, 1994). The most soluble silica species will be precipitated first from any supersaturated solution (Millot, 1960, 1970). This is usually amorphous silica, which is both poorly ordered and has a higher solubility under neutral conditions (60-130ppm) compared with opal-C (20-30ppm) and quartz (6-10ppm) (Williams et al., 1985). Silica monomers polymerise and aggregate to form colloids from solutions, and these colloids may precipitate to form opal-A (Iler, 1979 Williams et al., 1985). If solutions are supersaturated with respect to quartz then... 

De La Rocha, C. L. 2006. Opal-based proxies of paleoenvironmental conditions. Global Biogeochem-ical Cycles, 20, GB4S09, doi 10.1029/2005... 

W.H., Jickells, T.D. and King, P., 1996. The benthic cycle of biogenic opal at the Bermuda Atlantic Time Series site.- Deep-Sea Research I, 43, 383-409. 

Silicate is a very important nutrient in the ocean. Unlike other major nutrients such as phosphate and nitrate or ammonium, which are needed by almost all marine plankton, silicate is an essential chemical only for certain biota such as diatoms, radiolarian, sihcoflageUates, and siliceous sponges. However, this biology is one of the most important producers in marine. The estimation shows that diatoms contribute more than 40% of the entire primary production. Therefore, silicate cycling has received significant scientific attention in recent years and many scientists have studied silicate behavior in marine environments. Biogenic silicate is the amorphous content extracted by chemical methods, which is named as biogenic opal or opal in brief. The concentration of dissolved silicate in the world ocean is about 70.6 pmol/L and the net input of dissolved silicate from land to ocean is (6.1 2.0)x 10 mol (calculated by Si) every year, and the primary contribution (about 80%) comes from river. 

The ceramic monolithic stmctures made from inverted opal silicon carbonitride (see Section 6.2), which had been developed by Mitchell et al. [460], were applied for propane steam reforming. A 5 wt.% mthenium catalyst was coated onto the monoliths. Full conversion was achieved at fairly high reactor temperatures of 900 °C. Despite the low S/C ratio of 1.33, stable operation of the catalyst and reactor at 800 °C and 60% conversion was achieved through several hours of operation and more than 15 thermal cycles without apparent coke formation. 

As a comparison, electrodes made of amorphous macroporous Si in an "inverse opal" structure had initial capacities of 2500 mAh/g using the C/10 rate, which faded to about 1500 mAh/g for C/5,500 mAh/g for C/2, and no capacities for rates 1C and higher. The authors attributed the poor cycling performance at higher currents to the poor electronic conductivity of a-Si. This illustrates the superior electronic transport properties of the SiNW electrode, since each NW is still connected to the current collector even after becoming amorphous. 

He contributed to discoveries of the photosynthetic carbon reduction cycle. In addition, the opal glass method for the spectral characterization of chloroplasts was firmly established during this stay at Berkeley. Invited by Stacy French, he spent 1955-56 at the Carnegie Foundation Research Laboratories, Stanford, California studying the greening of etiolated... 

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