Ovid Technologies, Inc. Email Service ------------------------------ Search for: from 20 [1 and 19] keep 2-3,11,13,23,29,31 Citations: 1-7 Citation <1> Accession Number PREV200100080645 Author/Editor/Inventor Kroeger Nils. Deutzmann Rainer. Bergsdorf Christian. Sumper Manfred [a]. Institution [a] Lehrstuhl Biochemie I, Universitaet Regensburg, 93053, Regensburg: manfred.sumper@vkl.uni-regensburg.de Germany. Title Species-specific polyamines from diatoms control silica morphology. Source Proceedings of the National Academy of Sciences of the United States of America. [ print] 97(26). December 19, 2000. 14133-14138. Abstract Biomineralizing organisms use organic molecules to generate species-specific mineral patterns. Here, we describe the chemical structure of long-chain polyamines (up to 20 repeated units), which represent the main organic constituent of diatom biosilica. These substances are the longest polyamine chains found in nature and induce rapid silica precipitation from a silicic acid solution. Each diatom is equipped with a species-specific set of polyamines and silica-precipitating proteins, which are termed silaffins. Different morphologies of precipitating silica can be generated by polyamines of different chain lengths as well as by a synergistic action of long-chain polyamines and silaffins. Citation <2> Accession Number PREV200100054726 Author/Editor/Inventor Martin-Jezequel Veronique. Hildebrand Mark [a]. Brzezinski Mark A. Institution [a] Marine Biology Research Division, Scripps Institution of Oceanography, University of California, San Diego, 9500 Gilman Dr., La Jolla, CA, 92093-0202: mhildebrand@ucsd.edu USA. Title Silicon metabolism in diatoms: Implications for growth. Source Journal of Phycology. [ print] 36(5). October, 2000. 821-840. Abstract Diatoms are the world's largest contributors to biosilicification and are one of the predominant contributors to global carbon fixation. Silicon is a major limiting nutrient for diatom growth and hence is a controlling factor in primary productivity. Because our understanding of the cellular metabolism of silicon is limited, we are not fully knowledgeable about intracellular factors that may affect diatom productivity in the oceans. The goal of this review is to present an overview of silicon metabolism in diatoms and to identify areas for future research. Numerous studies have characterized parameters of silicic acid uptake by diatoms, and molecular characterization of transport has begun with the isolation of genes encoding the transporter proteins. Multiple types of silicic acid transporter gene have been identified in a single diatom species, and multiple types appear to be present in all diatom species. The controlled expression and perhaps localization of the transport! ers in the cell may be factors in the overall regulation of silicic acid uptake. Transport can also be regulated by the rate of silica incorporation into the cell wall, suggesting that an intracellular sensing and control mechanism couples transport with incorporation. Sizable intracellular pools of soluble silicon have been identified in diatoms, at levels well above saturation for silica solubility, yet the mechanism for maintenance of supersaturated levels has not been determined. The mechanism of intracellular transport of silicon is also unknown, but this must be an important part of the silicification process because of the close coupling between silica incorporation and uptake. Although detailed ultrastructural analyses of silica deposition have been reported, we know little about the molecular details of this process. However, proteins occluded within silica that promote silicification in vitro have recently been characterized, and the application of molecular techniqu! es holds the promise of great advances in this area. Cellular energy for silicification and transport comes from aerobic respiration without any direct involvement of photosynthetic energy. As such, diatom silicon metabolism differs from that of other major limiting nutrients such as nitrogen and phosphorous, which are closely linked to photosynthetic metabolism. Cell wall silicification and silicic acid transport are tightly coupled to the cell cycle, which results in a dependency in the extent of silicification on growth rate. Silica dissolution is an important part of diatom cellular silicon metabolism, because dissolution must be prevented in the living cell, and because much of the raw material for mineralization in natural assemblages is supplied by dissolution of dead cells. Perhaps part of the reason for the ecological success of diatoms is due to their use of a silicified cell wall, which has been calculated to impart a substantial energy savings to organisms that hav! e them. However, the growth of diatoms and other siliceous organisms has depleted the oceans of silicon, such that silicon availability is now a major factor in the control of primary productivity. Much new progress in understanding silicon metabolism in diatoms is expected because of the application of molecular approaches and sophisticated analytical techniques. Such insight is likely to lead to a greater understanding of the role of silicon in controlling diatom growth, and hence primary productivity, and of the mechanisms involved in the formation of the intricate silicified structures of the diatom cell wall. Citation <3> Accession Number PREV200000121585 Author/Editor/Inventor Del Amo Yolanda [a]. Brzezinski Mark A. Institution [a] Department of Ecology, Evolution and Marine Biology and Marine Science Institute, University of California at Santa Barbara, Santa Barbara, CA, 93106 USA. Title The chemical form of dissolved Si taken up by marine diatoms. Source Journal of Phycology. 35(6). dEC., 1999. 1162-1170. Abstract Results of past studies of the pH-dependent Si uptake kinetics of Phaeodactylum tricornutum Bohlin suggested that the anion SiO(OH)3- is the chemical form of dissolved Si taken up by marine diatoms. We determined the chemical form of Si taken up by three other marine diatom species and P. tricornutum by examining the kinetics of Si use under two dramatically different SiO(OH)3-:Si(OH)4 ratios in seawater by varying pH from apprxeq8 to apprxeq9.6. Uptake rates were determined using a precise and sensitive 32Si tracer methodology. The pH-dependent uptake kinetics obtained for all species except P. tricornutum suggest that marine diatoms transport Si(OH)4. The half-saturation constant (Km) varies strongly as a function of pH for all species when the substrate of transport is assumed to be SiO(OH)3-. Kinetic curves for Thalassiosira pseudonana (Hustedt) Hasle et Heimdal, Thalassiosira weissflogii (Grunow) G. Fryxell et Hasle, and Cylindrotheca fusiformis Reimann et Lewin have st! atistically identical values of Km at each pH when the substrate for transport is assumed to be Si(OH)4 (T. pseudonana and T. weissflogii) or total dissolved silicon (C. fusiformis). In contrast, P. tricornutum exhibits unusual biphasic uptake kinetics: uptake conforms to Michaelis-Menten kinetics up to 15 to 25 muM, above which uptake increases linearly. This enigmatic response may have biased conclusions drawn from past experiments using this species. However, based on the consistency of the results for the three other species, a new model of Si transport in marine diatoms is proposed on the basis of the direct formation of a complex between the Si-transport protein and Si(OH)4. Citation <4> Accession Number PREV200000015076 Author/Editor/Inventor Kroeger Nils [a]. Deutzmann Rainer. Sumper Manfred. Institution [a] Lehrstuhl Biochemie I, Universitaet Regensburg, 93053, Regensburg Germany. Title Polycationic peptides from diatom biosilica that direct silica nanosphere formation. Source Science (Washington D C). 286(5442). Nov. 5, 1999. 1129-1132. Abstract Diatom cell walls are regarded as a paradigm for controlled production of nanostructured silica, but the mechanisms allowing biosilicification to proceed at ambient temperature at high rates have remained enigmatic. A set of polycationic peptides (called silaffins) isolated from diatom cell walls were shown to generate networks of silica nanospheres within seconds when added to a solution of silicic acid. Silaffins contain covalently modified lysine-lysine elements. The first lysine bears a polyamine consisting of 6 to 11 repeats of the N-methyl-propylamine unit. The second lysine was identified as epsilon-N,N-dimethyl-lysine. These modifications drastically influence the silica-precipitating activity of silaffins. Citation <5> Accession Number PREV199699242453 Author/Editor/Inventor Lobel K D. West J K. Hench L L [a]. Institution [a] Dep. Materials Sci. Eng., Univ. Florida, PO Box 116400, Gainesville, FL 32611-6400 USA. Title Computational model for protein-mediated biomineralization of the diatom frustule. Source Marine Biology (Berlin). 126(3). 1996. 353-360. Abstract The mechanism of silicification from which the intricate cell walls of diatoms emerge, as well as the stereochemical relationship between the wall's organic casing and the siliceous materials within, continue to elude current technology. The present study further develops Hecky et al.'s standing model of the organic-inorganic interface with semi-empirical computer simulations of a biosilicification pathway. Polycondensation reactions between silicic acid molecules and a hydroxyl-rich beta-sheet protein template results in a stereochemically-compatible chemisorbed tetrasiloxane ring. The 24-stage reaction pathway has an activation barrier of +15.4 kcal mol-1 and results in a net stabilization of -28.0 kcal mol-1. Spatial matching and favorable thermodynamics support the theory of protein-mediated biomineralization of the diatom. Citation <6> Accession Number PREV199497197229 Author/Editor/Inventor Brzezinski Mark A [a]. Conley Daniel J. Institution [a] Dep. Biol. Sci., Univ. California, Santa Barbara, CA 93106 USA. Title Silicon deposition during the cell cycle of Thalassiosira weissflogii (Bacillariophyceae) determined using dual rhodamine 123 and propidium iodide staining. Source Journal of Phycology. 30(1). 1994. 45-55. Abstract The relatively non-toxic dye, rhodamine 123 (R123), was incorporated into the frustule of Thalassiosira weissflogii Grun. clone ACTIN in direct proportion to biogenic silica (BSi). R123 was used together with the DNA stain propidium iodide to track and quantify Si deposition during the cell cycle of T. weissflogii using flow cytometry. Silicon deposition was not continuous through the cell cycle. Deposition of the valves occurred during M phase. The hypocingulum was largely deposited during G1 with some suggestion of minor girdle band deposition during G2. Silicon deposition did not occur during S phase. Assuming that a complete frustule consists of an epivalve, epicingulum, hypocingulum, and hypovalve, then 40% of cellular BSi was contained within the cingulum of T. weissflogii with 60% present in the valves. These percentages correspond to 0.38 pmol Si in the two cingula and 0.57 pmol Si in the valves. Temporal differences in the timing of silicic acid uptake and depositio! n during the cell cycle of T. weissflogii suggested that deposition of both the new valves and the cingulum is supported by an internal pool of dissolved Si acquired during G2. Citation <7> Accession Number PREV199395055296 Author/Editor/Inventor Brzezinski Mark A. Institution Univ. California, Santa Barbara, Calif. 93106, USA. Title Cell-cycle effects on the kinetics of silicic acid uptake and resource competition among diatoms. Source Journal of Plankton Research. 14(11). 1992. 1511-1539. Abstract Current models of nutrient utilization by planktonic algae generally assume that cells transport nutrients continuously. This assumption is violated in the case of Si utilization by diatoms. Silic acid transport is confined to specific portions of the cell cycle such that only a fraction of the cells in a population are likely to be transporting silic acid at any given time. A theoretical framework for describing the growth and nutrient-uptake kinetics of populations of cells exhibiting cell-cycle-dependent nutrient uptake is presented. Thalassiosira weissflogii was used as a model organism. Kinetic parameters determined at steady state, assuming cells take up Si continuously, underestimated both the V-m and K-s of individual cells by nearly an order of magnitude. Likewise, kinetic constants determined from short-term experiments assuming continuing uptake were shown to be dependent on the initial physiological state of the population and to dramatically underestimate the ki! netic parameters of individual cells. The response of steady-sate populations to large pulses of dissolved Si was also examined. The ratio of the average maximum specific uptake rate of the culture to the average maximum observable specific growth rate (V'm-avg/ cxa mu-m) increased from unity at a relative growth rate of 1 to over 24 at low relative growth rates. The increase in V'm-avg/ cxa mu-m with decreasing relative growth rate under nitrogen limitation has been hypothesized to be an adaptation by cells to exploit micropatches of NH-3. In contrast, the predicted increase in this ratio for Si-limited cells was due entirely to changes in the distribution of cells within the cell cycle caused by Si stress. These findings, if empirically verified, will require a revision of our views of how diatoms are adapted to low-nutrient environments.