184 research outputs found
Massive Gravity Simplified: A Quadratic Action
We present a simplified formulation of massive gravity where the Higgs fields
have quadratic kinetic term. This new formulation allows us to prove in a very
explicit way that all massive gravity theories considered so far inevitably
have Boulware-Deser ghost in non-trivial fluctuations of background metric.Comment: 8 pages, paragraph added proving that Bianchi identity does not imply
vanishing of linearized curvatur
Effect of Light/Dark Transition on Carbon Translocation in Eelgrass Zostera marina Seedlings
Carbon translocation in the marine macrophyte Zostera marina L. (eelgrass) was investigated to elucidate the impact of light/dark transitions on sucrose partitioning between roots and shoots. After exposure of leaves to C-14-bicarbonate, the level of C-14-labelled photoassimilates increased monotonically in both leaves and fully aerobic roots of plants maintained in the light. Accumulation of C-14 in roots and leaves ceased abruptly when plants were transferred to darkness that induced root anaerobiosis even though C-14 levels remained high in the dark-exposed leaves. Thus, translocation of C-14 photoassimilates from shoots to roots was inhibited when roots became anoxic. Anoxia induced by light limitation of photosynthesis, whether due to day/night transitions or periods of extreme light attenuation in the water column, can have an impact on carbon availability in subterranean tissues of eelgrass. As a consequence, light availability is likely to control the productivity and distribution of eelgrass in highly variable and light-limited coastal environments through its effects on carbon partitioning between shoots and roots, in addition to whole-plant carbon balance
Modeling Daily Production of Aquatic Macrophytes from Irradiance Measurements: A Comparative Analysis
The importance of submerged aquatic macrophytes to coastal ecosystems has generated a need for knowledge of minimum light levels that will support the maintenance and restoration of healthy populations. Our goals were (1) to evaluate the sensitivity to natural, non-sinusoidal fluctuations in irradiance I of analytical integration techniques for calculating daily carbon gain, (2) to evaluate the Hsat (the daily period of I-saturated photosynthesis) model of daily production relative to models based on instantaneous photosynthesis vs irradiance (P vs I) and (3) to provide some guidance for the temporal density of irradiance data required for accurate estimation of daily carbon gain. Monthly measures of the P vs I response of an eelgrass Zostera marina L. population were used to predict rates of daily carbon gain from continuous in situ recordings of I. Daily integrated I was not a reliable predictor of daily production. Numerical (iterative) integration of Hsat was much more reliable but required repeated measures of I within a day, as did numerical integration of P vs I. Analytical (non-iterative) models based only on observations of Im (noon) could not predict daily production accurately. Analytical models of P vs I and Hsat agreed with each other, however, indicating that the analytical models may be useful where the daily pattern of I is sinusoidal. Given the high degree oi temporal variability in coastal light environments, continuous monitoring of light availability may be required for calculation of daily production and reliable management of aquatic macrophyte populations
Molecular and Physiological Responses of Diatoms to Variable Levels of Irradiance and Nitrogen Availability: Growth of Skeletonema Costatum in Simulated Upwelling Conditions
Molecular mechanisms that drive metabolic acclimation to environmental shifts have been poorly characterized in phytoplankton. In this laboratory study. the response of light- and N-limited Skeletonema costatum cells to an increase in light and NO3 availability was examined. C assimilation was depressed relative to N assimilation early in enrichment, and the photosynthetic quotient (O2: CO2) increased, consistent with the shunting of reducing equivalents from CO2 fixation to NO3- reduction. The concomitant increase in dark respiration was consistent with the increased energetic demand associated with macromolecular synthesis. The accelerations of N-specific rates of NO3- uptake and nitrate reductase activity (NRA) over the first 24 h were comparable to observations for coastal upwelling systems. Increases in cell-specific rates of these processes, however, were confined to the first 8 h of enrichment. The abundance of 18S ribosomal ribonucleic acid (rRNA) increased immediately after the environmental shift, followed by increases in levels of NR-specific mRNA that coincided with the acceleration in NO3- assimilation. NRA, however, exhibited a diurnal rhythm that did not correspond to changes in NR protein abundance, suggesting that enzyme activity was also regulated by direct modulation of existing NR protein by light and NO3- availability
Is Growth of Eelgrass Nitrogen Limited? A Numerical Simulation of the Effects of Light and Nitrogen on the Growth Dynamics of Zostera marina
A numerical model of nitrogen uptake and growth was developed for the temperate seagrass Zostera marina L. Goals were to evaluate the relative effects of light and nitrogen availability on nitrogen uptake and partitioning between leaf and root tissue, and to estimate nitrogen concentrations in the sedment and water column required to saturate growth. Steady-state predictions are quite robust with respect to a range of parameter values justified by available data The calculations indicated that roots are probably more important in overall nitrogen acquisition in most light and nitrogen environments encountered in situ, but may contribute less than 50 % of the total uptake in low light. The model also predicted ammonium to be a much more important source of nitrogen than nitrate. Nitrogen concentrations required to saturate growth (even for nitrate) were estimated to be at least 50 % below concentrations commonly reported in situ, an indication that nitrogen limitation of Z. marina is probably very rare in nature
Resource allocation and sucrose mobilization in light-limited eelgrass Zostera marina
Este artículo contiene 12 páginas, 9 figuras, 4 tablas.This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation. A quantitative understanding of the mobilization process and its limitations is essential to the development of robust models predicting minimum light levels required to maintain healthy seagrass populations. Plants were grown in running seawater tanks under 2 light regimes. One treatment was provided with 2 h irradiance-saturated photosynthesis (Hsat) to produce severe Light Limitation, while control plants were grown under 7 h Hsat, simulating the typical wintertime condition in Monterey Bay, California, USA. Although plants maintained under 2 h Hsat were more severely carbon limited than plants grown under 7 h Hsat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both Hsat treatments. The eelgrass studied here responded to negative carbon balances by suppressing the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of carbon stress. The 7 h Hsat plants survived the 45 d course of the experiment while the plants grown under 2 h Hsat died within 30 d, even though one-third of their carbon reserves remained immobilized in the rhizome. Thus. extreme Light limitation can prevent full mobilization of carbon reserves stored in below-ground tissues, probably through the effects of anoxia on translocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatments. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines.Financial support was provided by a CIRIT (Comissionat
per a Universitats i Recerca)Trainlng Grant to T.A, and
by grant OCE-9223265 from the US National Science Foundation
to R.C.Z. and R.Peer reviewe
Resource Allocation and Sucrose Mobilization In Light Limited Eelgrass Zostera marina
This study evaluated the ability of Zostera marina L. (eelgrass) to balance the daily photosynthetic deficit by mobilization of carbon reserves stored in below-ground tissues during a period of extreme winter light limitation. A quantitative understanding of the mobilization process and its limitations is essential to the development of robust models predicting minimum light levels required to maintain healthy seagrass populations. Plants were grown in running seawater tanks under 2 light regimes. One treatment was provided with 2 h irradiance-saturated photosynthesis (Hsat) to produce severe Light Limitation, while control plants were grown under 7 h Hsat, simulating the typical wintertime condition in Monterey Bay, California, USA. Although plants maintained under 2 h Hsat were more severely carbon limited than plants grown under 7 h Hsat, whole-plant carbon balance calculated from metabolic needs and growth rates was negative for both Hsat treatments. The eelgrass studied here responded to negative carbon balances by suppressing the production of new roots, depleting sucrose reserves, and effecting a gradual decrease in growth rate and an increase in the activity of sucrose synthase (SS, E.C. 2.4.1.13) in sink tissues in the terminal stages of carbon stress. The 7 h Hsat plants survived the 45 d course of the experiment while the plants grown under 2 h Hsat died within 30 d, even though one-third of their carbon reserves remained immobilized in the rhizome. Thus. extreme Light limitation can prevent full mobilization of carbon reserves stored in below-ground tissues, probably through the effects of anoxia on translocation. Metabolic rates, particularly photosynthesis and respiration of the shoot, were unaffected by prolonged carbon limitation in both treatments. The patterns observed here can provide useful indices for assessing the state and fate of seagrass ecosystems in advance of catastrophic declines
Massive Gravity: Exorcising the Ghost
We consider Higgs massive gravity [1,2] and investigate whether a nonlinear
ghost in this theory can be avoided. We show that although the theory
considered in [10,11] is ghost free in the decoupling limit, the ghost
nevertheless reappears in the fourth order away from the decoupling limit. We
also demonstrate that there is no direct relation between the value of the
Vainshtein scale and the existence of nonlinear ghost. We discuss how massive
gravity should be modified to avoid the appearance of the ghost.Comment: 16 page
Note About Hamiltonian Structure of Non-Linear Massive Gravity
We perform the Hamiltonian analysis of non-linear massive gravity action
studied recently in arXiv:1106.3344 [hep-th]. We show that the Hamiltonian
constraint is the second class constraint. As a result the theory possesses an
odd number of the second class constraints and hence all non physical degrees
of freedom cannot be eliminated.Comment: 15 page
Top-Down Impact Through a Bottom-Up Mechanism. In Situ Effects of Limpet Grazing on Growth, Light Requirements and Survival of the Eelgrass Zostera marina
Temporal changes in abundance, size, productivity, resource allocation and light requirements of a subtidal eelgrass (Zostera marina L.) population were followed for 2 yr after the September 1993 appearance of a previously rare oval form of the commensal limpet Tectura depicta (Berry) in Monterey Bay, California, USA, By exclusively targeting the epidermis, limpet grazing impaired photosynthetic performance but left respiratory demand, meristematic growth and more than 90 % of the leaf biomass intact, The resulting low P:R ratios of grazed plants raised the light requirements for the maintenance of positive carbon balance almost 2-fold relative to healthy ungrazed plants and prevented the summertime accumulation of internal carbon reserves. Shoot density in this once-continuously vegetated 30 ha meadow declined from more than 50 shoots m-2 (2230 g fresh wt [FW] m-2) to sparse patches supporting an average of 16 shoots m-2 (380 g FW m-2). More than 50 % of the continuously vegetated meadow was converted to bare sand despite ambient light availability and water temperatures that were favorable for growth of healthy, ungrazed plants. Plant size declined by 50 % and internal sugar reserves declined more than 4-fold within 6 mo after the appearance of T. depicta, Plant losses were most extensive during winter, when internal carbon reserves were minimal, The dramatic decline in eelgrass vigor and abundance reported here, despite a physical environment that was favorable for healthy eelgrass survival, illustrates the amplification of top-down control by this relatively inconspicuous limpet through a feeding mechanism that specifically impairs photosynthesis, a bottom-up process
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