Mercury in Florida Bay fish: spatial distribution of elevated concentrations and possible linkages to Everglades restoration

Health advisories are now posted in northern Florida Bay, adjacent to the Everglades, warning of high mercury concentrations in some species of gamefish. Highest concentrations of mercury in both forage fish and gamefish have been measured in the northeastern corner of Florida Bay, adjacent to the dominant freshwater inflows from the Everglades. Thirty percent of spotted seatrout (Cynoscion nebulosus Cuvier, 1830) analyzed exceeded Florida’s no consumption level of 1.5 μg g−1 mercury in this area. We hypothesized that freshwater draining the Everglades served as the major source of methylmercury entering the food web supporting gamefish. A lack of correlation between mercury concentrations and salinity did not support this hypothesis, although enhanced bioavailability of methylmercury is possible as freshwater is diluted with estuarine water. Stable isotopes of carbon, nitrogen, and sulfur were measured in fish to elucidate the shared pathways of methylmercury and nutrient elements through the food web. These data support a benthic source of both methylmercury and nutrient elements to gamefish within the eastern bay, as opposed to a dominant watershed source. Ecological characteristics of the eastern bay, including active redox cycling in near-surface sediments without excessive sulfide production are hypothesized to promote methylmercury formation and bioaccumulation in the benthos. Methylmercury may then accumulate in gamefish through a food web supported by benthic microalgae, detritus, pink shrimp (Farfantepenaeus duorarum Burkenroad, 1939), and other epibenthic feeders. Uncertainty remains as to the relative importance of watershed imports of methylmercury from the Everglades and in situ production in the bay, an uncertainty that needs resolution if the effects of Everglades restoration on mercury levels in fish are to be modeled and managed

Distributional Equivalence and Structure Learning for Bow-free Acyclic Path Diagrams

We consider the problem of structure learning for bow-free acyclic path diagrams (BAPs). BAPs can be viewed as a generalization of linear Gaussian DAG models that allow for certain hidden variables. We present a first method for this problem using a greedy score-based search algorithm. We also prove some necessary and some sufficient conditions for distributional equivalence of BAPs which are used in an algorithmic ap- proach to compute (nearly) equivalent model structures. This allows us to infer lower bounds of causal effects. We also present applications to real and simulated datasets using our publicly available R-package

Finite-Size-Scaling at the Jamming Transition: Corrections to Scaling and the Correlation Length Critical Exponent

We carry out a finite size scaling analysis of the jamming transition in frictionless bi-disperse soft core disks in two dimensions. We consider two different jamming protocols: (i) quench from random initial positions, and (ii) quasistatic shearing. By considering the fraction of jammed states as a function of packing fraction for systems with different numbers of particles, we determine the spatial correlation length critical exponent $\nu\approx 1$, and show that corrections to scaling are crucial for analyzing the data. We show that earlier numerical results yielding $\nu<1$ are due to the improper neglect of these corrections.Comment: 5 pages, 4 figures -- slightly revised version as accepted for Phys. Rev. E Rapid Communication

Spontaneous Breaking of Translational Invariance in One-Dimensional Stationary States on a Ring

We consider a model in which positive and negative particles diffuse in an asymmetric, CP-invariant way on a ring. The positive particles hop clockwise, the negative counterclockwise and oppositely-charged adjacent particles may swap positions. Monte-Carlo simulations and analytic calculations suggest that the model has three phases; a "pure" phase in which one has three pinned blocks of only positive, negative particles and vacancies, and in which translational invariance is spontaneously broken, a "mixed" phase with a non-vanishing current in which the three blocks are positive, negative and neutral, and a disordered phase without blocks.Comment: 7 pages, LaTeX, needs epsf.st

Stochastic Models on a Ring and Quadratic Algebras. The Three Species Diffusion Problem

The stationary state of a stochastic process on a ring can be expressed using traces of monomials of an associative algebra defined by quadratic relations. If one considers only exclusion processes one can restrict the type of algebras and obtain recurrence relations for the traces. This is possible only if the rates satisfy certain compatibility conditions. These conditions are derived and the recurrence relations solved giving representations of the algebras.Comment: 12 pages, LaTeX, Sec. 3 extended, submitted to J.Phys.

Wavelet-Based Image Registration and Segmentation Framework for the Quantitative Evaluation of Hydrocephalus

Hydrocephalus, characterized by increased fluid in the cerebral ventricles, is traditionally evaluated by a visual assessment of serial CT scans. The complex shape of the ventricular system makes accurate visual comparison of CT scans difficult. The current research developed a quantitative method to measure the change in cerebral ventricular volume over time. Key elements of the developed framework are: adaptive image registration based on mutual information and wavelet multiresolution analysis; adaptive segmentation with novel feature extraction based on the Dual-Tree Complex Wavelet Transform; volume calculation. The framework, when tested on physical phantoms, had an error of 2.3%. When validated on clinical cases, results showed that cases deemed to be normal/stable had a calculated volume change less than 5%. Those with progressive/treated hydrocephalus had a calculated change greater than 20%. These findings indicate that the framework is reasonable and has potential for development as a tool in the evaluation of hydrocephalus

Competitive effect of iron(III) on metal complexation by humic substances : characterisation of ageing processes

Aiming at an assessment of counteractive effects on colloid-borne migration of actinides in the event of release from an underground repository, competition by Fe(III) in respect of metal complexation by dissolved organic matter was investigated for the example of Eu(III) as an analogue of trivalent actinides. Complexation with different humic materials was examined in cation exchange experiments, using 59Fe and 152Eu as radioactive tracers for measurements in dilute systems as encountered in nature. Competitive effects proved to be significant when Fe is present at micromolar concentrations. Flocculation as a limiting process was attributed to charge compensation of humic colloids. Fe fractions bound to humic acids (HA) were higher than 90%, exceeding the capacity of binding sites at high Fe concentrations. It is thus concluded that the polynuclear structure of hydrolysed Fe(III) is maintained when bound to HA, which is also inferred from UV-Vis spectrometry. The competitive effect was found to be enhanced if Fe and HA were contacted before Eu was added. Depending on the time of Fe/HA pre-equilibration, Eu complexation decreased asymptotically over a time period of several weeks, the amount of bound Fe being unchanged. Time-dependent observations of UV-Vis spectra and pH values revealed that the ageing effect was due to a decline in Fe hydrolysis rather than structural changes within HA molecules. Fe polycations are slowly degraded in contact with humic colloids, and more binding sites are occupied as a consequence of dispersion. The extent of degradation as derived from pH shifts depended on the Fe/HA ratio

A Method for Calculating the Structure of (Singular) Spacetimes in the Large

A formalism and its numerical implementation is presented which allows to calculate quantities determining the spacetime structure in the large directly. This is achieved by conformal techniques by which future null infinity (\Scri{}^+) and future timelike infinity ($i^+$) are mapped to grid points on the numerical grid. The determination of the causal structure of singularities, the localization of event horizons, the extraction of radiation, and the avoidance of unphysical reflections at the outer boundary of the grid, are demonstrated with calculations of spherically symmetric models with a scalar field as matter and radiation model.Comment: 29 pages, AGG2

How coenzyme B12 radicals are generated: the crystal structure of methylmalonyl-coenzyme A mutase at 2 å resolution

AbstractBackground: The enzyme methylmalonyl-coenzyme A (CoA) mutase, an αβ heterodimer of 150 kDa, is a member of a class of enzymes that uses coenzyme B12 (adenosylcobalamin) as a cofactor. The enzyme induces the formation of an adenosyl radical from the cofactor. This radical then initiates a free-radical rearrangement of its substrate, succinyl-CoA, to methylmalonyl-CoA.Results Reported here is the crystal structure at 2 å resolution of methylmalonyl-CoA mutase from Propionibacterium shermanii in complex with coenzyme B12 and with the partial substrate desulpho-CoA (lacking the succinyl group and the sulphur atom of the substrate). The coenzyme is bound by a domain which shares a similar fold to those of flavodoxin and the B12-binding domain of methylcobalamin-dependent methionine synthase. The cobalt atom is coordinated, via a long bond, to a histidine from the protein. The partial substrate is bound along the axis of a (β/α)8 TIM barrel domain.Conclusion The histidine–cobalt distance is very long (2.5 å compared with 1.95–2.2 å in free cobalamins), suggesting that the enzyme positions the histidine in order to weaken the metal–carbon bond of the cofactor and favour the formation of the initial radical species. The active site is deeply buried, and the only access to it is through a narrow tunnel along the axis of the TIM barrel domain

Flexible Lipid Bilayers in Implicit Solvent

A minimalist simulation model for lipid bilayers is presented. Each lipid is represented by a flexible chain of beads in implicit solvent. The hydrophobic effect is mimicked through an intermolecular pair potential localized at the ``water''/hydrocarbon tail interface. This potential guarantees realistic interfacial tensions for lipids in a bilayer geometry. Lipids self assemble into bilayer structures that display fluidity and elastic properties consistent with experimental model membrane systems. Varying molecular flexibility allows for tuning of elastic moduli and area/molecule over a range of values seen in experimental systems.Comment: 5 pages, 5 figure