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1. Primary consumers play important roles in the cycling of nutrients in headwater streams, storing assimilated nutrients in growing tissue and recycling them through excretion. Although environmental conditions in most headwater streams and their surrounding terrestrial ecosystems vary considerably over the course of a year, relatively little is known about the effects of seasonality on consumer nutrient recycling these streams. In this study, we measured nitrogen accumulated through growth and excreted by the grazing snail Elimia clavaeformis (Pleuroceridae) over the course of 12 months in Walker Branch, identifying close connections between in-stream nitrogen processing and seasonal changes in the surrounding forest. 2. Nitrogen processing rates were positively correlated with ecosystem respiration, which was driven by leaf phenology on streamside trees. Snail nitrogen assimilation, growth and excretion were relatively high in spring before leaf emergence, low in summer when canopy shade was extensive and high again in autumn after leaf-fall. During the time that snails grazed primarily on epilithon (winter, spring and summer), growth and excretion rates followed changes in light and epilithon biomass. In autumn, when snails primarily grazed fallen leaves, leaf-associated microbes provided large subsidies of nitrogen for the snails. Nitrogen accumulation in snail biomass was greater in the 2 months following leaf-fall than at any other time of the year. 3. Snails were less important as nitrogen sinks than as sources of recycled nitrogen in Walker Branch. Over the course of the year, snails excreted approximately 12 times more nitrogen than they accumulated in biomass. Nitrogen accrued during growth in spring was subsequently lost in summer when primary production declined and snails underwent tissue loss. Catabolic losses represented >40% of the nitrogen excreted by the snails in summer. Net nitrogen growth efficiency (growth/assimilation), which varied with food availability, was only 8% for the entire year. Neither growth nor excretion was positively correlated with nitrogen concentrations in grazing substrata. 4. Snails achieved high standing crops and were significant contributors to nitrogen spiralling in Walker Branch. On an areal basis, nitrogen in snail biomass (mgN m-2) was two to five times greater than that in epilithon biomass, depending on the season. Snails assimilated and excreted up to 50% of the nitrogen initially taken up by autotrophs and leaf microbes, and they were likely to have additional effects on nitrogen spiraling through egestion and the cropping of assimilative biomass. Primary consumers like Elimia are important catalysts of nutrient movement through headwater streams, decreasing residence times and facilitating fluxes to downstream waters [ABSTRACT FROM AUTHOR]

Animal excretion can be a significant nutrient flux within ecosystems, where it supports primary production and facilitates microbial decomposition of organic matter. The effects of excretory products on nutrient cycling have been documented for various species and ecosystems, but temporal variation in these processes is poorly understood. We examined variation in excretion rates of a dominant grazing snail, Elimia clavaeformis, and its contribution to nutrient cycling, over the course of 14 months in a well-studied, low-nutrient stream (Walker Branch, east Tennessee, USA). Biomass-specific excretion rates of ammonium varied over twofold during the study, coinciding with seasonal changes in food availability (measured as gross primary production) and water temperature (multiple linear regression, R = 0.57, P = 0.053). The contribution of ammonium excretion to nutrient cycling varied with seasonal changes in both biological (that is, nutrient uptake rate) and physical (that is, stream flow) variables. On average, ammonium excretion accounted for 58% of stream water ammonium concentrations, 26% of whole-stream nitrogen demand, and 66% of autotrophic nitrogen uptake. Phosphorus excretion by Elimia was contrastingly low throughout the year, supplying only 1% of total dissolved phosphorus concentrations. The high average N:P ratio (89:1) of snail excretion likely exacerbated phosphorus limitation in Walker Branch. To fully characterize animal excretion rates and effects on ecosystem processes, multiple measurements through time are necessary, especially in ecosystems that experience strong seasonality. [ABSTRACT FROM AUTHOR]

The article discusses the nonpartisan foreign policy of Afghanistan. It states that assumptions were anchored in theories of international relations such as the classical realist theory developed by Hans Morgenthau, whose work was published in a tense era shortly after World War II and at the onset of the Cold War. The policy is reportedly consistent with the theoretical framework that can be viewed as the end of a U.S. doctrine of separation of foreign policy from domestic considerations.

Summary 1. While the balance of light and nutrients is known to influence the food quality of herbivores by altering algal phosphorus and nitrogen content, the combined effects of light and nutrients on fatty acid synthesis in freshwater periphyton are relatively unknown. In this study, we manipulated light and phosphorus concentration in large, flow-through experimental streams to examine their effects on both elemental stoichiometry and fatty acid content in periphyton. 2. Two levels of phosphorus (4 and 80 μg L−1) and three of light (17, 40, 110 μmol photons m−2 s−1) were applied in a factorial design in two separate experiments. Diatoms dominated periphyton communities in both experiments, comprising >95% of algal biovolume. Periphyton growth in the streams was simultaneously affected by both resources, even at low rates of supply. 3. Periphyton C/P and C/N ratios increased with light augmentation and decreased with phosphorus enrichment, and consistent with the light : nutrient hypothesis (LNH). Light effects were strongest in streams with low phosphorus concentrations. 4. Periphyton fatty acids reflected the dominance of diatoms : palmitic (16 : 0), palmitoleic (16 : 1ω7) and eicosapentanoic (20 : 5ω3) were the principal saturated (SAFA), monounsaturated (MUFA) and polyunsaturated fatty acids (PUFA), respectively. Linoleic (18 : 2ω6) and linolenic (18 : 3ω3) acids, characteristic of chlorophytes and cyanophytes, were rare, comprising <2% of total fatty acids. 5. Periphyton fatty acid profiles were highly sensitive to light and phosphorus. The proportion of fatty acids comprised by SAFA and MUFA increased with light augmentation and decreased with phosphorus enrichment, whereas PUFA decreased with light and increased with phosphorus. Light effects on fatty acid composition were strongest in phosphorus-poor streams. PUFA declined with increasing light/phosphorus ratios in the streams, whereas 'energy' fatty acids (16 : 0 and 16 : 1) increased. The ratio of SAFA/PUFA was strongly and positively correlated with C/P and C/N ratios. SAFA and MUFA, normalised to dry mass, increased two- to threefold with increasing light, while PUFA normalised to dry mass was not significantly affected by light. 6. Similarities in the responses of fatty acids and elemental stoichiometry to light and phosphorus treatments suggested that they were influenced by a common mechanism. Both components of food quality appeared to be sensitive to light-regulated rates of carbon fixation which, when coupled with insufficient supplies of phosphorus, caused diatom cells to store surplus carbon in SAFA, MUFA and other carbon-rich compounds that diluted both essential fatty acids and mineral nutrients. [ABSTRACT FROM AUTHOR]

Light and nutrients play pivotal roles in determining the growth of autotrophs, yet the potential for synergistic interactions between the two resources in algal communities is poorly understood, especially in stream ecosystems. In this study, light and phosphorus were manipulated in large experimental streams to examine resource colimitation and synergy in stream periphyton. Whole-stream metabolism was simultaneously limited by light and phosphorus. Increasing the supply of either light or phosphorus resulted in significant increases in primary production and the transformation of the streams from heterotrophy to autotrophy. Resource-driven changes in periphyton community structure occurred in concert with changes in production. Algal assemblages in highly shaded streams were composed primarily of small diatoms such as Achnanthidium minutissima, whereas larger diatoms such as Melosira varians predominated at higher irradiances. Phosphorus enrichment had relatively little effect on assemblage structure, but it did substantially diminish the abundance of Meridion circulare, a diatom whose mucilaginous colonies were conspicuously abundant in phosphorus-poor, high-light streams. Bacterial biomass declined relative to algal biomass with increases in primary productivity, regardless of whether the increases were caused by light or phosphorus. Synergistic effects on primary production appeared to occur because the availability of one resource facilitated the utilization of the other. Light increased the abundance of large diatoms, which are known to convert high concentrations of nutrients into primary production more effectively than smaller taxa. Phosphorus enrichment led to the replacement of Meridion circulare by non-mucilaginous taxa in phosphorus-enriched streams, and we hypothesize that this change enabled more efficient use of light in photosynthesis. Higher ratios of chlorophyll a : biomass in phosphorus-enriched streams may have also led to more efficient photon capture and higher photosynthetic rates. Synthesis. Our results underscore the potential for resource colimitation, even in habitats where a single resource is as strongly limiting as is light in shaded streams. The capacity of autotrophic communities to respond to more than one limiting resource suggests that prevailing single-resource models of ecosystem productivity are overly simplistic. [ABSTRACT FROM AUTHOR]

1. There is much concern that filter-feeding Asian carp will invade the Laurentian Great Lakes and deplete crucial plankton resources. We developed bioenergetic models, using parameters from Asian carp and other fish species, to explore the possibility that planktonic food resources are insufficient to support the growth of silver carp ( Hypophthalmichthys molitrix) and bighead carp ( H. nobilis) in the Great Lakes. 2. The models estimated basic metabolic requirements of silver and bighead carp under various body sizes, swimming speeds and reproductive stages. These requirements were then related to planktonic food resources and environmental temperature to predict when and where silver and bighead carp may survive in the Great Lakes, and how far they may travel. 3. Parameter values for respiration functions were derived experimentally in a coordinated study of silver and bighead carp, while consumption parameters were obtained from the literature on silver carp. Other model parameters lacking for Asian carp, such as those for egestion and excretion, were obtained from the literature on other fish species. 4. We found that full-sized bighead carp required 61.0 kJ d−1 just to maintain their body mass at 20 °C, approximately equivalent to feeding in a region with 255 μg L−1 macrozooplankton (dry) or 10.43 μg L−1 chlorophyll a. Silver carp energy requirements were slightly higher. 5. When applied to various habitats in the Great Lakes, our results suggest that silver and bighead carp will be unable to colonise most open-water regions because of limited plankton availability. However, in some circumstances, carp metabolism at lower temperatures may be low enough to permit positive growth even at very low rations. Positive growth is even more likely in productive embayments and wetlands, and the modelled swimming costs in some of these habitats suggest that carp could travel >1 km d−1 without losing biomass. 6. The simulation (and firmly hypothetical) results from this modelling study suggest when and where Asian carp could become established in the Great Lakes. Given the potential consequences to Great Lakes ecosystems if these filter feeders do prove capable of establishing reproducing populations, efforts to keep Asian carp out of the Great Lakes must not be lessened. However, we do encourage the use of bioenergetic modelling in a holistic approach to assessing the risk of Asian carp invasion in the Great Lakes region. [ABSTRACT FROM AUTHOR]

Abstract: Ribosomal protein S20 is a primary binding protein that bridges the 5′ domain and the 3′ minor domain of the 16S ribosomal RNA (rRNA) in the 30S ribosomal subunit. Using time-dependent dimethyl sulfate modification, we have determined that as it is bound to 16S rRNA, protein S20 causes rapid protection of bases A246, A274, A279, and A282 in the stem region of helix 11 in the 5′ domain and moderately fast modifications of helix 44 bases A1433 and A1434 in the 3′ minor domain. At a later time, enhancements occur with bases A181and A190 in helix 9, bases A325 and A327 in helix 13, and base C264 at the distal end of helix 11 in the 5′ domain of 16S rRNA. The modifications that occur in the stem region of helix 11 are distant from the binding site of protein S20, as determined from the crystal structure. Simultaneous addition of protein S17 with S20 to the complex significantly alters the modifications caused by protein S20 in the stem region of helix 11 but does not alter the remaining modifications. Our results indicate that protein S20 is binding to at least two alternate 16S rRNA sites during the early assembly process. [ABSTRACT FROM AUTHOR]

1. The light : nutrient hypothesis (LNH) states that algal nutrient content is determined by the balance of light and dissolved nutrients available to algae during growth. Light and phosphorus gradients in both laboratory and natural streams were used to examine the relevance of the LNH to stream periphyton. Controlled gradients of light (12–426 μmol photons m−2 s−1) and dissolved reactive phosphorus (DRP, 3–344 μg L−1) were applied experimentally to large flow-through laboratory streams, and natural variability in canopy cover and discharge from a wastewater treatment facility created gradients of light (0.4–35 mol photons m−2 day−1) and DRP (10–1766 μg L−1) in a natural stream. 2. Periphyton phosphorus content was strongly influenced by the light and DRP gradients, ranging from 1.8 to 10.7 μg mg AFDM−1 in the laboratory streams and from 2.3 to 36.9 μg mg AFDM−1 in the natural stream. Phosphorus content decreased with increasing light and increased with increasing water column phosphorus. The simultaneous effects of light and phosphorus were consistent with the LNH that the balance between light and nutrients determines algal nutrient content. 3. In experiments in the laboratory streams, periphyton phosphorus increased hyperbolically with increasing DRP. Uptake then began levelling off around 50 μg L−1. 4. The relationship between periphyton phosphorus and the light : phosphorus ratio was highly nonlinear in both the laboratory and natural streams, with phosphorus content declining sharply with initial increases in the light : phosphorus ratio, then leveling off at higher values of the ratio. 5. Although light and DRP both affected periphyton phosphorus content, the effects of DRP were much stronger than those of light in both the laboratory and natural streams. DRP explained substantially more of the overall variability in periphyton phosphorus than did light, and light effects were evident only at lower phosphorus concentrations (≤25 μg L−1) in the laboratory streams. These results suggest that light has a significant negative effect on the food quality of grazers in streams only under a limited set of conditions. [ABSTRACT FROM AUTHOR]

The light nutrient hypothesis posits that herbivore growth is increasingly constrained by low food quality as the ratio of light to nutrients increases in aquatic ecosystems. We tested predictions of this hypothesis by examining the effects of large seasonal cycles in light and nutrients on the mineral content of periphyton and the growth rate of a dominant herbivore (the snail Elimia clavaeformis) in two oligotrophic streams. Streambed irradiances in White Oak Creek and Walker Branch (eastern Tennessee, USA) varied dramatically on a seasonal basis due to leaf phenology in the surrounding deciduous forests and seasonal changes in sun angle. Concentrations of dissolved nutrients varied inversely with light, causing light: nitrate and light: phosphate to range almost 100-fold over the course of any individual year. Periphyton nitrogen and phosphorus concentrations were much lower than the concentrations of these elements in snails, and they bottomed out in early spring when streambed irradiances were highest. Snail growth, however, peaked in early spring when light: nutrient ratios were highest and periphyton nutrient concentrations were lowest. Growth was linearly related to primary production (accounting for up to 85% of growth variance in individual years), which in turn was driven by seasonal variation in light. Conceptual models of herbivore growth indicate that growth should initially increase as increasing light levels stimulate primary production, but then level off, and then decrease as the negative effects of decreasing algal nutrient content override the positive effects of increased food production. Our results showed no, evidence of an inflection point where increasing ratios of light to nutrients negatively affected growth. Snail growth in these intensively grazed streams is probably unaffected by periphyton nutrient content because exploitative competition for food reduces growth rates to levels where the demand for nitrogen and phosphorus is small enough to be satisfied by even low levels of these nutrients in periphyton. Competition for limited food resources in habitats where herbivore densities are uncontrolled by predation or other mortality factors should strongly influence the potential for herbivores to be limited by mineral deficits in their food. [ABSTRACT FROM AUTHOR]

Abstract: Ribosomal subunit assembly is initiated by the binding of several primary binding proteins. Results from chemical modification studies show that 16S ribosomal RNA undergoes striking structural rearrangements when protein S17 is bound. For the first time, we are able to distinguish and order these structural rearrangements by using time-dependent chemical probing. Initially, protein S17 binds to a portion of helix 11, inducing a kink-turn in that helix that bends helix 7 toward the S17–helix 11 complex in a hairpin-like manner, allowing helix 7 to bind to protein S17. This structural change is rapidly stabilized by interactions at the distal and proximal ends of both RNA helices. Identifying the dynamic nature of interactions between RNA and proteins is not only essential in unraveling ribosome assembly, but also has more general application to all protein–RNA interactions. [Copyright &y& Elsevier]

Invasive Asian carps Hypophthalmichthys spp. are an ecological threat to non-native aquatic ecosystems throughout the world, and are poised to enter the Laurentian Great Lakes. Little is known about how these filter-feeding planktivores grow and impact zooplankton communities in mesotrophic to oligotrophic systems like the Great Lakes. Our purpose was to determine how different plankton densities affect bighead carp H. nobilis biomass and how bighead carp affect zooplankton species composition. We conducted a 37-day indoor mesocosm experiment (volume = 678 l) with high and low plankton treatments (zooplankton dry mass ≈ 1,900 and 700 μg l−1; chlorophyll a = 25 and 14 μg l−1, respectively) in the presence and absence of juvenile bighead carp (mean = 5.0 g, 8.5 cm). Carp lost weight in the low plankton treatment and gained weight in the high plankton treatment, suggesting that food availability may be a limiting factor to bighead carp growth in regions of low plankton densities. In the presence of carp, zooplankton shifted from Daphnia to copepod dominance, while in the absence of carp, Daphnia remained dominant. Chydorids and ostracods increased in the presence of carp, but only in the low plankton treatment, suggesting that the impact of bighead carp on zooplankton species composition may vary with zooplankton density. Chlorophyll was higher in the absence of carp than in the presence. Chlorophyll and zooplankton densities in many Great Lakes ecosystems are substantially lower than our low treatment conditions, and thus our results suggest that Asian carp establishment in these regions may be unlikely. [ABSTRACT FROM AUTHOR]

1. This study investigated the combined effects of light and phosphorus on the growth and phosphorus content of periphyton. To investigate the potential for colimitation of algal growth by these two resources, diatom-dominated periphyton communities in large flow-through laboratory streams were exposed under controlled conditions to simultaneous gradients of light and phosphorus. 2. Periphyton growth rate was predictably light-limited by the subsaturating irradiances (12–88 μmol photons m−2 s−1) used in this experiment. However, phosphorus concentration also limited growth rate: growth increased hyperbolically with increasing soluble reactive phosphorus (SRP), reaching a threshold of growth saturation between 22 and 82 μg L−1. 3. Periphyton phosphorus content was strongly and nonlinearly related with SRP, reaching a maximum at 82 μg L−1 SRP. Contrary to the Light : Nutrient Hypothesis, periphyton phosphorus content did not decrease with increasing light, even at the lowest concentrations of SRP. Periphyton phosphorus was highly correlated with periphyton growth rate (Spearman's ρ = 0.63, P < 0.005). 4. Multiple regression analysis reinforced evidence of simultaneous light and phosphorus limitation. Both light and periphyton phosphorus content were significant variables in multiple regressions with growth parameters as dependent variables. Light alone accounted for 67% of the variance in periphyton biomass, and the addition of periphyton phosphorus as an additional independent variable increased the total amount of variance explained to 81%. 5. Our results did not support the hypothesis that extra phosphorus is required for photoacclimation to low light levels. Rather, the effect of additional phosphorus may have been to accommodate increased requirements for P-rich ribosomal RNA when growth was stimulated by increased light. The potential colimitation of periphyton growth by phosphorus and light at subsaturating irradiances has important implications in both theoretical and applied aquatic ecology. [ABSTRACT FROM AUTHOR]

Abstract: The Ribosome Builder is a software project that provides tools and techniques to create dynamic models of macromolecular systems from the rapidly growing numbers of atomic structural models. It includes a computer program that allows the user to assemble the multiple molecular components within a 3D space and to define the hypothetical interactions of these components with the initial goal of understanding protein translation at an atomic level of detail. The program employs a simplified molecular dynamics forcefield that can simulate the long time-scale events, such as docking of translation factors and mRNA translocation. An embedded scripting language and Application Programming Interface (API) enable the creation of Steered Molecular Dynamics (SMD) simulations through the programmable application of external forces and torques on atoms and bonds. A graphical interface is provided for displaying and interacting with models, recording movies of molecular dynamics movements, and creating annotated 3D simulations of complex macromolecular events. Initial applications of the project include simulation of tetraloop folding, docking of an mRNA on the 30S subunit and a schematic simulation of the translation elongation cycle. The program is an open source project released under the GNU public license. [Copyright &y& Elsevier]

Despite the key role microalgae play in introducing toxicants into aquatic food webs, little is known about the effects of environmental factors on metal accumulation by these primary producers. Environmental factors such as light and nutrients alter growth rates and may consequently influence metal concentrations in microalgae through growth dilution. Laboratory experiments suggested that metal uptake and elimination by microalgal biofilms were gradual enough to enable dilution of metals within the biofilms by photosynthetically accrued carbon, and a simple kinetic model of metal accumulation predicted significant variation in metal content due to growth dilution over the natural range of microalgal growth rates. The ratio of metal uptake to carbon uptake by microalgal biofilms decreased exponentially with increasing light in short-term laboratory experiments because photosynthesis was much more sensitive to a light gradient than was metal uptake. The effect of light on biofilm metal concentrations was confirmed in situ with a long-term experiment in which experimental shading of biofilms in a metal-contaminated stream decreased biofilm growth rates and caused a 3&yimes; increase in biofilm concentrations of twelve metals, including methylmercury. Slow growth at the primary producer level is a likely contributor to higher biotic metal concentrations in shaded, oligotrophic, or cold ecosystems. [ABSTRACT FROM AUTHOR]

Microalgae can absorb contaminants from the aqueous environment, and harvesting microalgae has been proposed as a method to purify water. However, rapid growth of microalgae (stimulated by increased light, for example) results in lowered tissue concentration of contaminant. This reduction has been observed to propagate to herbivores. Here we investigate (with simulation and supporting analytical argument) the propagation of growth dilution in all trophic levels of a food chain. We are concerned with concentration as well as overall mass of contaminant in each level, for different functional relationships between levels. We find that transient (i.e., prompt) growth dilution occurs for all levels. However, the new steady state concentrations can increase or decrease, depending on functional relationships (e.g., ratio versus prey dependence). These results, which have implications for pollution control, call for experimental testing. [Copyright &y& Elsevier]

1. Streambed light regimes change dramatically when riparian trees gain leaves in spring and lose them in autumn. This study examined the effect of these changes on periphyton photosynthetic characteristics, primary production, and light utilisation efficiency in two eastern Tennessee streams. 2. Photosynthesis–irradiance responses were measured at intervals covering leaf emergence and abscission in spring and autumn. Photosynthetic efficiency (αchl ) increased with declining streambed irradiances during spring leaf emergence, but returned to pre-emergence values after autumn leaf fall. The onset of photosaturation (I k ) displayed the opposite pattern, decreasing during leaf emergence and increasing after leaf fall. Both αchl and I k were closely associated (P < 0.01) with daily integrated streambed irradiance, as were periphyton carotenoids. Internal shading by photoprotective carotenoids is hypothesised to account for lower αchl when streambed irradiances are high. 3. An in situ shading experiment confirmed that the temporal changes observed in periphyton photosynthetic characteristics and carotenoids were primarily the result of changing light levels and not other environmental factors (e.g. nutrients, temperature). 4. Daily chlorophyll-specific primary production (PP chl ) was calculated with P –I models and recorded streambed irradiances. In both streams, PP chl was the highest in early spring when trees were leafless, and then declined markedly as leaves emerged, reaching a minimum in summer. PP chl increased after leaf abscission, but was still lower than it was in early spring, when the sun was higher and daylength was longer. A hyperbolic tangent equation fit to PP chl and daily integrated irradiance (r 2 =0. 85) suggested that primary production was light saturated at 4–8 mol m–2... [ABSTRACT FROM AUTHOR]

Examines the effects of leaf emergence at multiple levels in two headwater streams in eastern Tennessee. Light; Temperature and discharge; Nutrients; Periphyton biomass and pigments; Primary producers; Herbivores.

Summary 1 This laboratory study examined the effect of a gradient of UV-B radiation (280–320 nm) on photosynthesis and food quality of periphyton, the trophic base of many freshwater benthic communities. Four irradiances of UV-B (0, 0.6, 1.2, and 2.3 W m-2) were delivered by UV-B lamps (313 nm peak irradiance) over a 13-day period in the first experiment and over a 4-h period in the second experiment. These irradiances were roughly equivalent to 0, 1, 2, and 4 times the ambient biologically effective (DNA) midsummer, midday UV-B irradiance in Tennessee. 2 Rates of photosynthesis and photosynthetic pigments were significantly reduced by irradiances greater than ambient during the 13-day experiment, suggesting that food supply rates to grazers would be depressed by increases in current UV-B levels. Effects on community structure were minor, but mean diatom cell size decreased at higher UV-B irradiances. 3 Irradiated periphyton was fed in surplus to juvenile snails (Physella gyrina) in the first experiment as a bioassay for food quality. Snail growth was the same on all four diets, suggesting that UV-B did not affect food quality. Nitrogen and phosphorus content of the periphyton were not affected by UV-B, either. 4 Photosynthesis by low-biomass periphyton in the second experiment was significantly depressed by irradiances above ambient after only 4 h. Photosynthesis by the high biomass periphyton was not significantly affected by UV-B, suggesting that self-shading reduced UV-B effects. [ABSTRACT FROM AUTHOR]

Nutrient-diffusing substrates were used to investigate nutrient limitation of attached algal assemblages in a shaded stream and an unshaded stream in northern California. Water front both streams contained low levels of nitrogen (<14 μg · L[SUP-1] and very low N:P ratios (<2). After 31 days of colonization and growth, attached algal biomass on nitrate-diffusing substrates was significantly greater than on control substrates in the unshaded stream. Nitrate-diffusing substrates also supported larger numbers of grazing insects in the unshaded stream. The prostrate diatoms. Achnanthes lanceolata Bech. and Cocconeis placentula Ehr. displayed the most consistent positive responses to nitrate enrichment. Nutrient enrichment did not increase the accrual of algal biomass in the shaded stream, but algal biomass was significantly greater at sites located under openings in the tree canopy, implicating light as a limiting factor in this stream. Several Navicula and Nitzschia species, and one unidentified Gomphonema species, were positively associated with higher light levels in the shaded stream. Shade appears to be the primary factor limiting algal growth in small northern California streams, but when its effect is reduced by logging, the inherently low levels of nitrogen in these streams can become limiting. [ABSTRACT FROM AUTHOR]

Examines the response of periphyton and grazing snails (Elimia clavaeformis) to summer shade in a Tennessee forest. Effects of light limitation and grazing; Shade adaptation; Snail growth versus irradiance; Snail growth; Lipid accumulation; Photosynthesis-irradiance curves.

This paper provides information concerning the flexibility of leader behavior. Subjects were 48 black and 48 white undergraduate students. Twelve black and 12 white older students were selected as leaders while the remaining subjects were assigned randomly as members of 12 black, 12 white, and 12 mixed dyads. Results indicate that leader behavior as measured by Bales Interaction Process Analysis changed as the group performed different tasks. There were no behavior differences as a function of leader's race although an interaction was found in the directive categories between task and dyad composition. [ABSTRACT FROM AUTHOR]

This paper measured subordinates' perceptions of their leaders' ability to use different leadership styles. Subjects were 124 middle and first level supervisors from accounting and R & D departments in the United Kingdom. The results indicate that subjects did not believe that their supervisors would either use the same style (irrespective of what style) or randomly employ styles to deal with four typical but hypothetical problems. These results persisted when respondents were subdivided on the basis of both organizational level and functional area. Some evidence suggests that perceived patterns of style usage may be associated, at least in the mind of subordinates, with the general task dimension, interpersonal-technical. [ABSTRACT FROM AUTHOR]

A letter to the editor is presented in response to the article "The Data Doughnut and the Software Hole" in the June 2006 issue of "The Profession."

In the wake of experts' failure to predict the outcome of the 2016 presidential election, a rigorous analysis of what went right and wrong is needed to improve future polling. Despite claims that "data is dead," low-tech factors such as sampling errors and inaccurate likely-voter models were probably most responsible. [ABSTRACT FROM PUBLISHER]

Stable isotopes are widely used to infer trophic relationships with little attention paid to temporal variability at the base of the food web. We examined changes in the carbon-stable isotope composition during periphyton development, sampling periphyton that accumulated on ceramic tiles at four stream sites over a 2-month period. Periphyton 13C rose and fell in general concordance with rising and falling biomass at all four sites, resulting in significant correlations between periphyton δ13C and chlorophyll a (Chl a). Mean δ13C values at one site rose from -26% to -20% in 2 weeks, falling back to -24% the next week after a large scouring spate. Periphyton 13C also underwent a smaller, longer-term increase that correlated with a gradual rise in stream temperature. Multiple regression analysis with both Chl a and temperature as independent variables accounted for up to 88% of the temporal variability in δ13C, with Chl a the largest source of variability. Water velocity, measured on each sampling occasion, was unrelated to temporal changes in 13C. Depletion of inorganic carbon within the periphyton matrix is the probable cause of increasing 13C in periphyton as biomass develops. Rising δ13C values during periphyton biomass development suggest the possibility of carbon-limited periphyton growth, even in alkaline waters. The strong link between biomass and periphyton 13C helps explain the large range of δ13C reported for periphyton in streams, where temporal and spatial variability in periphyton biomass are notorious. [ABSTRACT FROM AUTHOR]

Abstract: The penultimate stem–loop of 16S ribosomal RNA (rRNA), helix 44, plays a central role in ribosome function. Using time-resolved dimethyl sulfate (DMS) probing, we have analyzed time-dependent modifications that occur at specific bases in this helix near the decoding region, resulting from the binding of elongation factor G (EF-G) in various forms. When EF-G-GTP is bound to 70S ribosomes, bases A1492 and A1493 are immediately protected, while other bases in the region show either no change or enhanced modification. When apo-EF-G is bound to 70S ribosomes and GTP is added, substantial transient time-dependent enhancement occurs at bases A1492 and A1493, with somewhat less enhancement occurring at base A1483, all in the first 45 ms. When mRNA and deacylated tRNAs are bound to the 70S ribosome and EF-G-GTP is added, bases A1492 and A1493 again show substantial and continued enhancement, while bases A1408, A1413, and A1418 all show time-dependent protection. These results provide primary, real-time evidence that EF-G induces direct or indirect structural changes in this region as EF-G is bound and as GTP is hydrolyzed. [Copyright &y& Elsevier]

Some expected changes in climate resulting from human greenhouse gas emissions are clear and well documented, but others may be harder to predict because they involve extreme weather events or heretofore unusual combinations of weather patterns. One recent example of unusual weather that may become more frequent with climate change occurred in early spring 2007 when a large Arctic air mass moved into the eastern United States following a very warm late winter. In this paper, we document effects of this freeze event on Walker Branch, a well-studied stream ecosystem in eastern Tennessee. The 2007 spring freeze killed newly grown leaf tissues in the forest canopy, dramatically increasing the amount of light reaching the stream. Light levels at the stream surface were sustained at levels considerably above those normal for the late spring and summer months due to the incomplete recovery of canopy leaf area. Increased light levels caused a cascade of ecological effects in the stream beginning with considerably higher (two–three times) rates of gross primary production (GPP) during the late spring and summer months when normally low light levels severely limit stream GPP. Higher rates of stream GPP in turn resulted in higher rates of nitrate (NO3−) uptake by the autotrophic community and lower NO3− concentrations in stream water. Higher rates of stream GPP in summer also resulted in higher growth rates of a dominant herbivore, the snail Elimia clavaeformis. Typically, during summer months net NO3− uptake and snail growth rates are zero to negative; however, in 2007 uptake and growth were maintained at moderate levels. These results show how changes in forest vegetation phenology can have dramatic effects on stream productivity at multiple trophic levels and on nutrient cycling as a result of tight coupling of forest and stream ecosystems. Thus, climate change-induced changes in canopy structure and phenology may lead to large effects on stream ecosystems in the future. [ABSTRACT FROM AUTHOR]

Headwater streams are key sites of nutrient and organic matter processing and retention, but little is known about temporal variability in gross primary production (GPP) and ecosystem respiration (ER) rates as a result of the short duration of most metabolism measurements in lotic ecosystems. We examined temporal variability and controls on ecosystem metabolism by measuring daily rates continuously for 2 years in Walker Branch, a first-order deciduous forest stream. Four important scales of temporal variability in ecosystem metabolism rates were identified: (1) seasonal, (2) day-to-day, (3) episodic (storm-related), and (4) inter-annual. Seasonal patterns were largely controlled by the leaf phenology and productivity of the deciduous riparian forest. Walker Branch was strongly net heterotrophic throughout the year with the exception of the open-canopy spring when GPP and ER rates were co-equal. Day-to-day variability in weather conditions influenced light reaching the streambed, resulting in high day-to-day variability in GPP particularly during spring (daily light levels explained 84% of the variance in daily GPP in April). Episodic storms depressed GPP for several days in spring, but increased GPP in autumn by removing leaves shading the streambed. Storms depressed ER initially, but then stimulated ER to 2–3 times pre-storm levels for several days. Walker Branch was strongly net heterotrophic in both years of the study, with annual GPP being similar (488 and 519 g O2 m−2 y−1 or 183 and 195 g C m−2 y−1) but annual ER being higher in 2004 than 2005 (−1,645 vs. −1,292 g O2 m−2 y−1 or −617 and −485 g C m−2 y−1). Inter-annual variability in ecosystem metabolism (assessed by comparing 2004 and 2005 rates with previous measurements) was the result of the storm frequency and timing and the size of the spring macroalgal bloom. Changes in local climate can have substantial impacts on stream ecosystem metabolism rates and ultimately influence the carbon source and sink properties of these important ecosystems. [ABSTRACT FROM AUTHOR]

Two sweetpotato [Ipomoea batatas (L.) Lam] genotypes (TU-82-155 and NCC-58) were grown hydroponically and subjected to a temporary loss of lighting in the form of 14 days of prolonged darkness compared with a lighted control under standard daily light periods to determine the impact on growth responses and storage root yield. Vine cuttings of both genotypes were grown in rectangular channels. At 65 days after planting, lights were turned off in the treatment chambers and replaced by a single incandescent lamp, providing between 7 and 10 µmol-m-2·s-1 photosynthetic photon flux (PPF) for 18 hours, and the temperature lowered from 28/22 °C light/dark, to a constant 20 °C. Plants remained under these conditions for 14 days after which the original light level was restored. Growth chamber conditions predark included, a PPF mean provided by 400-W metal halide lamps, of 600 ± 25 pimol·m-2-s-1, an 18-hour light/6-hour dark cycle and a relative humidity of 70% ± 5%. The nutrient solution used was a modified half-Hoagland with pH and electrical conductivity (EC) maintained between 5.5-6.0 and 1000-1200 µS·cm-1, respectively, and was adjusted weekly. Storage root number and fresh weight were similar regardless of treatments. Plants exposed to prolonged darkness produced 10.5% and 25% lower fibrous root fresh and dry mass, respectively, but similar foliage yield and harvest index (HI). 'NCC-58' produced an average of 31% greater storage root yield than that of 'TU-82-155' but the number of storage roots as well as % dry matter (%DM) were similar. 'NCC-58' also produced 31% greater fibrous root dry weight, whereas 'TU-82-155' produced a 44% greater HI. The significant interaction between prolonged darkness and cultivars for %DM of the storage roots showed that DM for 'TU-82-155' was 18.4% under prolonged darkness and 17.9% in the light. That for 'NCC-58' was 16.4% under prolonged darkness compared with 19.4% (14.8% greater) for plants that were not subjected to prolonged darkness. The evidence that there were no adverse impacts on storage root yield following the exposure to prolonged darkness suggests that the detrimental effects were below the detectable limits for these cultivars in response to the short perturbation in the available light and that sweetpotatoes would be hardy under short-term failure situations. [ABSTRACT FROM AUTHOR]

Abstract: Time-dependent chemical modification and cleavage results have provided intriguing insights into structural changes that occur in the distal loop of helix 11 in 16S ribosomal RNA (rRNA). Located distant from the decoding region, between proteins S17 and S20, the results of this study suggest that this region of rRNA may act as a buffer or a spring between these two proteins during protein biosynthesis. During the assembly process, protein S17 apparently produces the major structural deformations in this region, causing it to be folded in a spring-like structure. Base C264 in this region shows erratic behavior during assembly and also shows time-dependent enhancement when elongation factor G with GTP is added to 70S ribosomes. Evidence is presented to suggest that this region of rRNA may be used to allow relative motion to occur between proteins S17 and S20 during translocation. [Copyright &y& Elsevier]

The use of a DNA-based identification system (DNA barcoding) founded on the mitochondrial gene cytochrome c oxidase subunit I (COl) was investigated for updating the U.S. Food and Drug Administration Regulatory Fish Encyclopedia (RFE; http://www.cfsan.fda.gov/∼frf/rfe0.html). The RFE is a compilation of data used to identify fish species. It was compiled to help regulators identify species substitution that could result in potential adverse health consequences or could be a source of economic fraud. For each of many aquatic species commonly sold in the United States, the RFE includes high-resolution photographs of whole fish and their marketed product forms and species-specific biochemical patterns for authenticated fish species. These patterns currently include data from isoelectric focusing studies. In this article, we describe the generation of DNA barcodes for 172 individual authenticated fish representing 72 species from 27 families contained in the RFE. These barcode sequences can be used as an additional identification resource. In a blind study, 60 unknown fish muscle samples were barcoded, and the results were compared with the RFE barcode reference library. All 60 samples were correctly identified to species based on the barcoding data. Our study indicates that DNA barcoding can be a powerful tool for species identification and has broad potential applications. [ABSTRACT FROM AUTHOR]

Summary: Translation initiation factor IF3 is an essential bacterial protein, consisting of two domains (IF3C and IF3N) separated by a linker, which interferes with ribosomal subunit association, promotes codon-anticodon interaction in the P site, and ensures translation initiation fidelity. Using time-resolved chemical probing, we followed the dynamic binding path of IF3 on the 30S subunit and its release upon 30S-50S association. During binding, IF3 first contacts the platform (near G700) of the 30S subunit with the C domain and then the P-decoding region (near A790) with its N domain. At equilibrium, attained within less than a second, both sites are protected, but before reaching binding equilibrium, IF3 causes additional transient perturbations of both the platform edge and the solvent side of the subunit. Upon 30S-50S association, IF3 dissociates concomitantly with the establishment of the 30S-50S bridges, following the reverse path of its binding with the IF3N-A790 interaction being lost before the IF3C-G700 interaction. [Copyright &y& Elsevier]

The notion that the ribosome is dynamic has been supported by various biochemical techniques, as well as by differences observed in high-resolution structures of ribosomal complexes frozen in various functional states. Yet, the mechanisms and extent of rRNA dynamics are still largely unknown. We have used a novel, fast chemical-modification technique to provide time-resolved details of 16S rRNA structural changes that occur as bridges are formed between the ribosomal subunits as they associate. Association of different 16S rRNA regions was found to be a sequential, multi-step process involving conformational rearrangements within the 30S subunit. Our results suggest that key regions of 16S rRNA, necessary for decoding and tRNA A-site binding, are structurally altered in a time-dependent manner by association with the 50S ribosomal subunits. [Copyright &y& Elsevier]

Ribosomal protein L11 and the L11 binding region of ribosomal RNA constitute an important domain involved in active functions of the ribosome during translation. We studied the effects of L11 knock-out and truncation mutations on the structure of the rRNA in this region and on its interactions with a translation elongation factor and the antibiotic thiostrepton. The results indicated that the structure of the L11-binding rRNA becomes conformationally flexible when ribosomes lack the entire L11 protein, but not when the C-terminal domain is present on ribosomes. Probing wild type and mutant ribosomes in the presence of the antibiotic thiostrepton and elongation factor-G (EF-G) rigorously localized the binding cleft of thiostrepton and suggested a role for the rRNA in the L11-binding domain in modulating factor binding. Our results also provide evidence that the structure of the rRNA stabilized by the C-terminal domain of L11 is necessary to stabilize EF-G binding in the post-translocation state, and thiostrepton may modulate this structure in a manner that interferes with the ribosome-EF-G interaction. The implications for recent models of thiostrepton activity and factor interactions are discussed. [ABSTRACT FROM AUTHOR]

Bacterial translation initiation factor IF2 was localized on the ribosome by rRNA cleavage using free Cu(II):1,10-orthophenanthroline. The results indicated proximity of IF2 to helix 89, to the sarcin–ricin loop and to helices 43 and 44, which constitute the “L11/thiostrepton” stem-loops of 23 S rRNA. These findings prompted an investigation of the L11 contribution to IF2 activity and a re-examination of the controversial issue of the effect on IF2 functions of thiostrepton, a peptide antibiotic known primarily as a powerful inhibitor of translocation. Ribosomes lacking L11 were found to have wild-type capacity to bind IF2 but a strongly reduced ability to elicit its GTPase activity. We found that thiostrepton caused a faster recycling of this factor on and off the 70 S ribosomes and 50 S subunits, which in turn resulted in an increased rate of the multiple turnover IF2-dependent GTPase. Although thiostrepton did not inhibit the P-site binding of fMet-tRNA, the A-site binding of the EF-Tu-GTP-Phe-tRNA or the activity of the ribosomal peptidyl transferase center (as measured by the formation of fMet-puromycin), it severely inhibited IF2-dependent initiation dipeptide formation. This inhibition can probably be traced back to a thiostrepton-induced distortion of the ribosomal-binding site of IF2, which leads to a non-productive interaction between the ribosome and the aminoacyl-tRNA substrates of the peptidyl transferase reaction. Overall, our data indicate that the translation initiation function of IF2 is as sensitive as the translocation function of EF-G to thiostrepton inhibition. [Copyright &y& Elsevier]

Presents a study pertaining to the RNA bound at the P-site of Escherichia coli ribosomes. Information on the transfer of RNA interactions with the ribosomes; Materials and methods used in the study; Reference to cleavage approach considerations in the study; Results from the study.

Studies the cleavage of 16S rRNA within the ribosome by the mRNA modified in the A-site codon with phenanthroline-Cu(II). Cleavage of proximal nucleotides; Extension analysis of 16S rRNA fragments; Evidence in the translation process.

A greenhouse study was conducted to evaluate the influence of harvest intervals on biomass yield and omega fatty acids of 'Golden purslane' (Portulaca oleracea). Nutrients were supplied as a modified full-strength Hoagland solution two to three times weekly. Plants were harvested sequentially at 20, 40, and 60 days after transplanting (DAT) corresponding to 42, 63, and 84 days after sowing. Fatty acids were determined using a gas chromatography-mass spectrometry. Harvest intervals significantly influenced foliage fresh and dry weight, leaf number and plant height, and root length and fresh weight and were greatest at 60 DAT. Fatty acid analysis verified the presence of myristate, palmitate, linoleate, and linolenate at 20 DAT and in all three harvests, whereas stearate and oleate were detected only in the last two harvests (40 and 60 DAT). Linoleate, palminate, and linolenate were the most abundant fatty acids in purslane with levels in excess of 300 mg⋅kg-1. Those for myristate, stearate, and oleate were in excess of 200 mg⋅kg-1. The ratio of omega-6/omega-3 ranged from 0.44 for Harvest 1 to 1.1 for Harvest 3, whereas ratios for harvest intervals two and three were equal to or greater than the recommended daily human requirement. Results showed qualitative and quantitative differences of harvest intervals of purslane, suggesting that an optimal ratio of omega-6 to omega-3 fatty acids can be achieved ≈20 DAT. [ABSTRACT FROM AUTHOR]