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Here we present a novel application of stage-structured population modelling to explore the properties of neuronal dendrites with spines. Dendritic spines are small protrusions that emanate from the dendritic shaft of several functionally important neurons in the cerebral cortex. They are the postsynaptic sites of over 90% of excitatory synapses in the mammalian brain. Here, we formulate a stage-structured population model of a passive dendrite with activity-dependent spines using a continuum approach. This computational study models three dynamic populations of activity-dependent spine types, corresponding to the anatomical categories of stubby, mushroom, and thin spines. In this stage-structured population model, transitions between spine type populations are driven by calcium levels that depend on local electrical activity. We explore the influence of the changing spine populations and spine types on the development of electrical propagation pathways in response to repetitive synaptic input, and which input frequencies are best for facilitating these pathways. [ABSTRACT FROM AUTHOR]

There is limited biogeochemical rate data from the oligotrophic central Indian Ocean, but it is known that there are geographical gradients in the physical and chemical conditions that may lead to unique biogeochemical regimes. As participants on a GO-SHIP repeat hydrography cruise, a transect was completed in spring of 2016 from 28 °S to 18 °N in the Indian Ocean, roughly along the 95 °E meridion. Cell count samples (phytoplankton and heterotrophic bacteria) analyzed by flow cytometry, and samples for carbon and nitrogen productivity incubations, assessed by stable isotopic tracers, were obtained from 20 m at approximately every other degree of latitude. Microbial cell counts by flow cytometry indicate that Prochlorococcus was the principal autotroph, but with increasing contributions of Synechococcus around the equator. Large eukaryotes (> 20 μm), imaged using FlowCAM, were generally absent or in very low abundance (< 0.5 μg C L−1) from near-surface waters. Heterotrophic bacteria contributed 28.4 ± 4.6% of the microbial community biomass and were positively correlated to autotrophic biomass. Dissolved nitrate and phosphate concentrations were below detection limits throughout the surface 50 m, while ammonium was undetectable at almost all stations and depths. In the 50–200 m depth range, nitrate and phosphate concentrations steadily increased from south to north, reaching a maximum of 31 and 2.2 μM respectively north of 6 °N. The ratio of dissolved NO 3 -:PO 4 - standing stocks was always below 15. Urea concentrations (0.02–0.18 μmol L−1) showed an increasing trend towards the north and dissolved organic phosphorus concentrations (0.25–0.37 μmol L−1) had a maxima between 5 and 15 °S. Absolute uptake rates of nitrate, ammonium and urea were all less than 1.5 nmol N L−1 h−1 south of 15 °S, and increased until local maxima of 3.4, 9.0, and 21 nmol N L−1 h−1 for nitrate, ammonium, and urea respectively, were reached between 1.5 and 6.5 °N. Ammonium and urea uptake rates were consistently 3–8 times higher than nitrate uptake rates, with a mean f-ratio of 0.23 for the entire transect. Primary production followed the same trend of nitrogen uptake, with a local maximum of 7.4–10.8 nmol C L−1 h−1 found 1.5–6.5 °N. These data represent some of the first reported measurements of primary production, nitrogen uptake, and phytoplankton diversity across biogeochemical provinces in the central oligotrophic Indian Ocean a large but understudied region of the global ocean. • Surface plankton assemblage dominated by heterotrophic bacteria and Prochlorococcus. • Low absolute uptake rates of C and N, but specific rates suggest an actively growing community. • The f-ratio was considerably higher than other oligotrophic regions of the global ocean. • Dissolved organic compounds are the dominant biogeochemical currency in this region. [ABSTRACT FROM AUTHOR]

In the North Atlantic Ocean, we found that natural populations of Prochlorococcus adhered to Redfield ratio dimensions when comparing cell quotas of carbon to nitrogen, but had flexible composition under nutrient and light stress, allowing for a broad range of cellular carbon- and nitrogen-to-phosphorus ratios. Synechococcus populations also exhibited a wide range of elemental stoichiometry, including carbon-to-nitrogen ratios and increased their carbon-to-phosphorus ratios in response to low dissolved phosphorus availability. Small eukaryotic populations tended to have lower carbon-to-phosphorus ratios than single cell cyanobacterial groups, with the exception of one group of samples, which highlights the importance of community composition when determining how biological diversity influences bulk particle stoichiometry. The ratio of dissolved nitrogen:phosphorus fluxes into the euphotic zone was not correlated to nitrogen:phosphorus cellular quotas. The lack of a homeostatic relationship implies that other mechanisms, such as species-specific adaptation to oligotrophic phosphorus concentrations, control elemental particle ratios. [ABSTRACT FROM AUTHOR]

Biogeochemical rate processes in the Arctic are not currently well constrained, and there is very limited information on how rates may change as the region warms. Here we present data on the sensitivity of ammonium (NH4+) uptake and nitrification rates to short-term warming. Samples were collected from the Chukchi Sea off the coast of Barrow, Alaska, during winter, spring, and summer and incubated for 24 h in the dark with additions of 15NH4+ at −1.5, 6, 13, and 20°C. Rates of NH4+ uptake and nitrification were measured in conjunction with bacterial production. In all seasons, NH4+ uptake rates were highest at temperatures similar to current summertime conditions but dropped off with increased warming, indicative of psychrophilic (i.e., cold-loving) microbial communities. In contrast, nitrification rates were less sensitive to temperature and were higher in winter and spring compared to summer. These findings suggest that as the Arctic coastal ecosystem continues to warm, NH4+ assimilation may become increasingly important, relative to nitrification, although the magnitude of NH4+ assimilation would be still be lower than nitrification. [ABSTRACT FROM AUTHOR]

How Arctic climate change might translate into alterations of biogeochemical cycles of carbon (C) and nitrogen (N) with respect to inorganic and organic N utilization is not well understood. This study combined 15N uptake rate measurements for ammonium, nitrate, and urea with 15N- and 13C-based DNA stable-isotope probing (SIP). The objective was to identify active bacterial and archeal plankton and their role in N and C uptake during the Arctic summer and winter seasons. We hypothesized that bacteria and archaea would successfully compete for nitrate and urea during the Arctic winter but not during the summer, when phytoplankton dominate the uptake of these nitrogen sources. Samples were collected at a coastal station near Barrow, AK, during August and January. During both seasons, ammonium uptake rates were greater than those for nitrate or urea, and nitrate uptake rates remained lower than those for ammonium or urea. SIP experiments indicated a strong seasonal shift of bacterial and archaeal N utilization from ammonium during the summer to urea during the winter but did not support a similar seasonal pattern of nitrate utilization. Analysis of 16S rRNA gene sequences obtained from each SIP fraction implicated marine group I Crenarchaeota (MGIC) as well as Betaproteobacteria, Firmicutes, SARI 1, and SAR324 in N uptake from urea during the winter. Similarly, 13C SIP data suggested dark carbon fixation for MGIC, as well as for several proteobacterial lineages and the Firmicutes. These data are consistent with urea-fueled nitrification by polar archaea and bacteria, which may be advantageous under dark conditions. [ABSTRACT FROM AUTHOR]

Our eyes move constantly, even when we try to fixate our gaze. Fixational eye movements prevent and restore visual loss during fixation, yet the relative impact of each type of fixational eye movement remains controversial. For over five decades, the debate has focused on microsaccades, the fastest and largest fixational eye movements. Some recent studies have concluded that microsaccades counteract visual fading during fixation. Other studies have disputed this idea, contending that microsaccades play no significant role in vision. The disagreement stems from the lack of methods to determine the precise effects of microsaccades on vision versus those of other eye movements, as well as a lack of evidence that microsaccades are relevant to foveal vision. Here we developed a novel generalized method to determine the precise quantified contribution and efficacy of human microsaccades to restoring visibility compared with other eye movements. Our results indicate that microsaccades are the greatest eye movement contributor to the restoration of both foveal and peripheral vision during fixation. Our method to calculate the efficacy and contribution of microsaccades to perception can determine the strength of connection between any two physiological and/or perceptual events, providing a novel and powerful estimate of causal influence; thus, we anticipate wide-ranging applications in neuroscience and beyond. [ABSTRACT FROM AUTHOR]

The article offers a look on the growing demand for life cycle assessments (LCAs) in North America. The growth is focused on the integration of life cycle approaches into greener buildings, the development of life cycle-based carbon footprint protocols, and the rapid development of requirements from retail companies needing environmental performance of consumer goods. Also described are the LCA requirements being asked of suppliers of retail firms like Walter-Mart and Tesco. Efforts on studying the demand for LCA studies for policy are discussed.

The retina is a part of the central nervous system that is accessible, well documented, and studied by researchers spanning the clinical, experimental, and theoretical sciences. Here, we mathematically model the subcircuits of the outer plexiform layer of the retina on two spatial scales: that of an individual synapse and that of the scale of the receptive field (hundreds to thousands of synapses). To this end we formulate a continuum spine model (a partial differential equation system) that incorporates the horizontal cell syncytium and its numerous processes (spines) within cone pedicles. With this multiscale modeling approach, detailed biophysical mechanisms at the synaptic level are retained while scaling up to the receptive field level. As an example of its utility, the model is applied to study background-induced flicker enhancement in which the onset of a dim background enhances the center flicker response of horizontal cells. Simulation results, in comparison with flicker enhancement data for square, slit, and disk test regions, suggest that feedback mechanisms that are voltage-axis modulators of cone calcium channels (for example, ephaptic and/or pH feedback) are robust in capturing the temporal dynamics of background-induced flicker enhancement. The value and potential of this continuum spine approach is that it provides a framework for mathematically modeling the input-output properties of the entire receptive field of the outer retina while implementing the latest models for transmission mechanisms at the synaptic level. [ABSTRACT FROM AUTHOR]

The article focuses on the life cycle assessments (LCA) of buildings in North America. It reports that the consideration of life cycle environmental, economic and social aspects of buildings is in vogue. In addition to this, multistakeholder forums are creating demand for LCA for buildings in North America and according to officials, on the supply side, they look to the need for additional practitioners. They believe that there are not enough LCA practitioners to meet the growing demand particularly in the business sector.

The Indian Ocean accounts for about one fifth of global ocean net primary production but remains undersampled relative to other major ocean basins. The eastern tropical Indian Ocean is characterized by extremely low concentrations of both macronutrients and the micronutrient iron. We measured concentrations of dissolved and particulate trace metals (Fe, Mn, Zn, Pb) in the upper ocean along the GO-SHIP IO9N transect (28˚S to 17˚N, mostly along the 95˚E meridian) during a cruise in April 2016. Cellular quotas (metal/C) of Fe, Mn, Co, Ni, Cu, and Zn were measured in small eukaryotic flagellates (2–7 μm). Deckboard bottle incubation experiments were conducted at one station in each of three putative biogeochemical regions: southern Indian Ocean gyre (SIO, 28-10˚S); equatorial Indian Ocean (EqIO, 10˚S - 5˚N); Bay of Bengal (BoB, 5-17˚N). Nitrate and phosphate were below detection limits in surface waters across the transect. Dissolved and particulate Fe were <0.2 nM south of 10˚N and lowest in the EqIO. Cellular Fe/C quotas were approximately 6 μmoL/mol and did not vary along the transect, nor did cellular Mn/C or Co/C quotas. Cellular Ni/C and Zn/C quotas were significantly higher at the southern terminus. Nutrient addition experiments indicated that N was the primary limiting nutrient for autotrophs using chlorophyll a as a proxy, but biomass measurements of specific phytoplankton groups pointed to a more complex nutrient limitation mosaic. Prochlorococcus was limited by N in the EqIO but by multiple nutrients (N, P, and/or Fe) in the BoB. Synechococcus was limited by N in the EqIO and BoB, while small (<20 μm) eukaryotic phytoplankton were limited by N in the EqIO and by multiple nutrients in the BoB. Stoichiometric comparisons of cells and underlying source waters indicate a gradient of N and Fe stress along the transect. These data demonstrate that autotroph communities are poised near multiple nutrient limitation horizons in extremely oligotrophic waters far from micronutrient sources. [ABSTRACT FROM AUTHOR]

Background In cat, the onset of dim backgrounds can enhance small-spot flicker responses of retinal horizontal cells. This is called background-induced flicker enhancement. A biophysical model involving neural subcircuits in the outer-plexiform layer (OPL) of the retina was proposed to explain the enhancement effect. The model states that hyperpolarization of horizontal cell dendritic terminals, conducted centripetally through gap junctions from rod-induced hyperpolarization of peripheral horizontal cells, increases the flow of calcium into the cone synaptic pedicle, stimulating transmitter release by the cone presynaptic apparatus, and resulting in an enhanced postsynaptic cone response. It remains controversial as to what causes the increased calcium entry in response to horizontal cell hyperpolarization, and two hypotheses which are hotly debated have received the most attention. The first hypothesis proposes that hyperpolarization of the horizontal cell reduces the release of a blocking agent (most likely gamma-aminobutyric acid (GABA)) from the horizontal cell, allowing more calcium to enter the cone synaptic terminal. The second hypothesis contends that the rise of calcium current is GABA independent and is caused by an ephaptic mechanism in which hemichannels (half of a gap junction) and a relatively high intersynaptic resistance play the key role. It speculates that hyperpolarization of horizontal cells leads to an increased hemichannel-mediated current flow through the intersynaptic cleft into the horizontal cell. As a result, the potential drop over the intersynaptic resistance will increase, and the nearby voltage-dependent calcium channels in the cone membrane will sense a local depolarization, giving rise to an augmented calcium influx. Although several experiments seem to support the ephaptic hypothesis, some recent experimental and computational analyses question its significance. However, it is still premature to dismiss the ephaptic hypothesis based on these recent results. Computational study A partial differential equation model for the subcircuits in the retinal OPL that incorporates both the GABA and ephaptic mechanisms is formulated, in which the collection of horizontal cells is treated as a two-dimensional conductive sheet due to the presence of numerous gap junctions between horizontal cells. Simulations are conducted for different test regions and compared to experimental data for the enhancement effect in cat retinal horizontal cells. The results provide insight into which of the competing hypotheses for calcium entry is appropriate, and may suggest the possibility of a hybrid mechanism combining both the GABA and ephaptic hypotheses. [ABSTRACT FROM AUTHOR]

Introduction Dendritic spines can change in number and shape in response to various physiological, behavioral or pathological states. This activity-dependent structural plasticity exists over a vast range of time scales, from minutes to days or weeks. Although spines may have a continuum of shapes, evidence suggests that spines may cluster in defined groups by their shapes. Further evidence suggests that spine morphology correlates with distance from the cell body; proximal to the soma, where the dendrite has the largest diameter, spines have short necks and appear stubby, whereas distally, where the dendrite is thinner, spines have thinner and longer stems. At intermediate distances, a variety of spine types are seen including the intermediate mushroom-shaped spines. We formulate a stage-structured population model for activity-dependent spines of three types: stubby (type-I), mushroom (type-II), and thin (type-III). Each spine type is characterized electrically by a spine stem resistance and chemically by a parameter that controls the level of calcium accumulation in the spine head. Transitions between spine types are driven by intraspine calcium levels that depend on local electrical activity. A continuum formulation based on the cable equation represents a dendritic branch and includes the stage transitions between stubby, mushroom, and thin dendritic spines. The model allows for a study of the interaction between the many activity-dependent (active or passive) spines and for an investigation of the impact of their individual and collective dynamics on the output properties of the dendrite. Simulations are run for both passive and active spines to investigate how dendritic diameter and synaptic input frequency influence spine morphology along the dendrite. In the passive case, the model is reduced to a system of ordinary differential equations because evidence shows spatially uniform repetitive synaptic input to passive spines, within an input region, drive spine restructuring within that region, with little or no effect on dendritic output and structural change outside the input region. This is in stark contrast to excitable spines where synaptic activity can drive restructuring and dendritic output far from the input region. Finally, we discuss how this model can be extended to study the formation and loss of spines as well as how to handle different input frequencies. [ABSTRACT FROM AUTHOR]