ALBERT

Description: Arabidopsis genes encoding enzymes for each of the eight steps in l-arginine (Arg) synthesis were identified, based upon sequence homologies with orthologs from other organisms. Except for N-acetylglutamate synthase (NAGS; EC 2.3.1.1), which is encoded by two genes, all remaining enzymes are encoded by single genes. Targeting predictions for these enzymes, based upon their deduced sequences, and subcellular fractionation studies, suggest that most enzymes of Arg synthesis reside within the plastid. Synthesis of the l-ornthine (Orn) intermediate in this pathway from l-glutamate occurs as a series of acetylated intermediates, as in most other organisms. An N-acetylornithine:glutamate acetyltransferase (NAOGAcT; EC 2.3.1.35) facilitates recycling of the acetyl moiety during Orn formation (cyclic pathway). A putative N-acetylornithine deacetylase (NAOD; EC 3.5.1.16), which participates in the “linear” pathway for Orn synthesis in some organisms, was also identified. Previous biochemical studies have indicated that allosteric regulation of the first and, especially, the second steps in Orn synthesis (NAGS; N-acetylglutamate kinase (NAGK), EC 2.7.2.8) by the Arg end-product are the major sites of metabolic control of the pathway in organisms using the cyclic pathway. Gene expression profiling for pathway enzymes further suggests that NAGS, NAGK, NAOGAcT and NAOD are coordinately regulated in response to changes in Arg demand during plant growth and development. Synthesis of Arg from Orn is further coordinated with pyrimidine nucleotide synthesis, at the level of allocation of the common carbamoyl-P intermediate.

Description: This paper presents seasonal variations of the wind characteristics and wind turbine characteristics in the regions around Elazig, namely Maden, Agin and Keban. Mean wind speed data in measured hourly time series format is statistically analyzed for the six year period 1998–2003. The probability density distributions are derived from the time series data and their distributional parameters are identified. Two probability density functions are fitted to the measured probability distributions on a seasonal basis. The wind energy characteristics of all the regions is studied based on the Weibull and Rayleigh distributions. Energy calculations and capacity factors for the wind turbine characteristics were determined for wind machines of different sizes between 300 and 2300 kW. It was found that Maden is the best region, among the regions analyzed, for wind characteristics and wind turbine characteristics.

Description: Measuring and modeling the surface charge of clays, and more especially smectites, has become an important issue in the use of bentonites as a waste confinement material aimed at retarding migration of water and solutes. Therefore, many studies of the acid–base properties of montmorillonite have appeared recently in the literature, following older studies principally devoted to cation exchange. It is striking that beyond the consensus about the complex nature of the surface charge of clays, there are many discrepancies, especially concerning the dissociable charge, that prevents intercomparison among the published data. However, a general trend is observed regarding the absence of common intersection point on raw titration curves at different ionic strengths. Analysis of the literature shows that these discrepancies originate from the experimental procedures for the preparation of the clays and for the quantification of their surface charge. The present work is an attempt to understand how these procedures can impact the final results. Three critical operations can be identified as having significant effects on the surface properties of the studied clays. The first one is the preparation of purified clay from the raw material: the use of acid or chelation treatments, and the repeated washings in deionized water result in partial dissolution of the clays. Then storage of the purified clay in dry or wet conditions strongly influences the equilibria in the subsequent experiments respectively by precipitation or enhanced dissolution. The third critical operation is the quantification of the surface charge by potentiometric titration, which requires the use of strong acids and bases. As a consequence, besides dissociation of surface sites, many secondary titrant consuming reactions were described in the literature, such as cation exchange, dissolution, hydrolysis, or precipitation. The cumulated effects make it difficult to derive proper dissociation constants, and to build adequate models. The inadequation of the classical surface complexation models to describe the acid–base behavior of clays is illustrated by the electrokinetic behavior of smectites, which is independent from the pH and the ionic strength. Therefore, there is still a need on one hand for accurate data recorded in controlled conditions, and on the other hand for new models taking into account the complex nature of the charge of clays.

Description: Dinoflagellates are an important component of the extant eukaryotic plankton. Their organic-walled, hypnozygotic cysts (dinocysts) provide a rich, albeit incomplete, history of the group in ancient sediments. Building on pioneering studies of the late 1970s and 1980s, recent drilling in the Southern Ocean has provided a wealth of new dinocyst data spanning the entire Paleogene. Such multidisciplinary studies have been instrumental in refining existing and furnishing new concepts of Paleogene paleoenvironmental and paleoclimatic reconstructions by means of dinocysts. Because dinocysts notably exhibit high abundances in neritic settings, dinocyst-based environmental and paleoclimatic information is important and complementary to the data derived from typically more offshore groups as planktonic foraminifera, coccolithophorids, diatoms and radiolaria. By presenting case-studies from around the globe, this contribution provides a concise review of our present understanding of the paleoenvironmental significance of dinocysts in the Paleogene (65–25 Ma). Representing Earth's greenhouse–icehouse transition, this episode holds the key to the understanding of extreme transient climatic change. We discuss the potential of dinocysts for the reconstruction of Paleogene sea-surface productivity, temperature, salinity, stratification and paleo-oxygenation along with their application in sequence stratigraphy, oceanic circulation and general watermass reconstructions.

Description: Geochemical and petrographical studies of lavas and ignimbrites from the Quaternary Nisyros–Yali volcanic system in the easternmost part of the Hellenic arc (Greece) reveal insight into magma generating processes. A compositional gap between 61 and 68 wt.% SiO2 is recognized that coincides with the stratigraphic distinction between pre-caldera and post-caldera volcanic units. Trace element systematics support the subdivision of Nisyros and Yali volcanic units into two distinct suites of rocks. The variation of Nd and Hf present day isotope data and the fact that they are distinct from the isotope compositions of MORB rule out an origin by pure differentiation and require assimilation of a crustal component. Lead isotope ratios of Nisyros and Yali volcanic rocks support mixing of mantle material with a lower crust equivalent. However, 87Sr/86Sr ratios of 0.7036–0.7048 are incompatible with a simple binary mixing scenario and give low depleted mantle extraction ages (〈0.1 Ga), in contrast with Pb model ages of 0.3 Ga and Hf and Nd model ages of ca. 0.8 Ga. The budget of fluid-mobile elements Sr and Pb is likely to be dominated by abundant hydrous fluids characterised by mantle-like Sr isotope ratios. Late stage fluids probably were enriched in CO2, needed to explain the high Th concentrations. The occurrence of hydrated minerals (e.g., amphibole) in the first post-caldera unit with the lowermost 87Sr/86Sr ratio of 0.7036±2 can be interpreted as the result of the increased water activity in the source. The presence of two different plagioclase phenocryst generations in the first lava subsequent to the caldera-causing event is indicative for a longer storage time of this magma at a shallower level. A model capable of explaining these observations involves three evolutionary stages. First stage, assimilation of lower crustal material by a primitive magma of mantle origin (as modelled by Nd–Hf isotope systematics). This stage ended by an interruption in replenishment that led to an increase of crystallization and, hence, an increase in viscosity, suppressing eruption. During this time gap, differentiation by fractional crystallization led to enrichment of incompatible species, especially aqueous fluids, to silica depolymerisation and to a decrease in viscosity, finally enabling eruption again in the third stage.

Description: Diatoms are a major component of phytoplankton community. They tend to dominate under natural high-nutrient concentrations, as well as during artificial Fe fertilisation experiments. They are main players in the biogeochemical cycle of carbon (C), as they can account for 40% of the total primary production in the Ocean and dominate export production, as well as in the biogeochemical cycles of the other macro-nutrients, nitrogen (N), phosphorus (P), and silicon (Si). Another important nutrient is Fe, which was shown to have a direct or indirect effect on nearly all the biogeochemical parameters of diatoms. In the present paper, an inventory is made of the growth, physiology and fate of many diatom species, including maximum growth rate, photosynthetic parameters (maximum specific rate of photosynthesis, photosynthetic efficiency and light adaptation parameter), nutrient limitation (half-saturation constant for growth/uptake), cellular elemental ratios, and loss terms (sinking rates, autolysis rates and grazing rates). This is a first step for improvement of the parameterisation of physiologically based phytoplankton growth and global 3D carbon models. This review is a synthesis of a large number of published laboratory experiments using monospecific cultures as well as field data. Our compilation confirms that size is an important factor explaining variations of biogeochemical parameters of diatoms (e.g. maximum growth rate, photosynthesis parameters, half-saturation constants, sinking rate, and grazing). Some variations of elemental ratios can be explained by adaptation of intracellular requirements or storage of Fe, and P, for instance. The important loss processes of diatoms pointed out by this synthesis are (i) sinking, as single cells as well as through aggregation which generally greatly increases sinking rate, (ii) cell autolysis, which can significantly reduce net growth rates, especially under nutrient limitation when gross growth rates are low, and (iii) grazing by both meso- and micro-zooplankton. This review also defines gaps concerning our knowledge on some important points. For example, we need to better know which iron species is available for phytoplankton, as well as the impact of Fe on the variation of the elemental ratios, especially in terms of assimilation and regeneration of C and N. A better quantification of prey selection by microzooplankton and mesozooplankton in natural environments is also needed, including preference for the various phytoplankton and zooplankton species as well as for aggregates and faecal pellets.

Description: The Tehuantepec Ridge is one of the most prominent lithospheric structures of the Cocos Plate, yet its tectonic evolution hasremained poorly constrained until now. Calculated ocean floor ages and spreading rates, morphostructural analysis of the ridgeand the surrounding ocean floor are used to infer the tectonic evolution and pattern of the Tehuantepec Ridge and associatedstructures. The ocean floor age estimates south of the Tehuantepec Ridge suggest an age of ~26 Ma at the Middle Americatrench. A mean age difference across the Tehuantepec Ridge of ~7 Ma suggests that the Tehuantepec Ridge would be formed asa long transform fault on the Guadalupe plate, prior to 15 Ma.The full-spreading rate estimates show that there might be a differential full-spreading rate across the transform faultbetween 15 and 5 Ma, when the oceanic plate just south of Clipperton Fracture Zone was decelerating between 15 and 10 Ma,then accelerating for the next 5 Ma. We propose a model in which between 13 and 8 Ma temporarily transpressionalintratransform-spreading centers would have been existing as a consequence of the East Pacific Rise onset with an angle of~108clockwise with respect to the ceased Mathematician ridge. Our model estimates indicate that between 21 and 15 Ma aprominent pseudofault was formed south of the Tehuantepec Ridge due to an unstable offset on the Mathematician ridge thatmigrated northward. The Tehuantepec Ridge and the pseudofault trace bound a deeper oceanic basin than the surrounding area,with no corresponding anomalous depth on the western side of the East Pacific Rise. The complete absence of a pseudofaulttrace and a deeper oceanic basin on the western side of the East Pacific Rise, as well as the spreading rate changes, suggest theexistence of a microplate embedded into the Cocos Plate just south of the Tehuantepec Ridge. Also, the asymmetric and sharpmorphology of the Tehuantepec Ridge suggests that it may be the expression of a major transpressional structure along theformer transform fault on the Guadalupe plate 15–20 Ma ago.