Effects of coastal managed retreat on mercury biogeochemistry

We investigated the impact of managed retreat on mercury (Hg) biogeochemistry at a site subject to diffuse contamination with Hg. We collected sediment cores from an area of land behind a dyke one year before and one year after it was intentionally breached. These sediments were compared to those of an adjacent mudflat and a salt marsh. The concentration of total mercury (THg) in the sediment doubled after the dyke was breached due to the deposition of fresh sediment that had a smaller particle size, and higher pH. The concentration of methylmercury (MeHg) was 27% lower in the sediments after the dyke was breached. We conclude that coastal flooding during managed retreat of coastal flood defences at this site has not increased the risk of Hg methylation or bioavailability during the first year. As the sediment becomes vegetated, increased activity of Hg-methylating bacteria may accelerate Hg-methylation rate

Gaseous mercury flux from salt marshes is mediated by solar radiation and temperature

Salt marshes are ecologically sensitive ecosystems where mercury (Hg) methylation and biomagnification can occur. Understanding the mechanisms controlling gaseous Hg flux from salt marshes is important to predict the retention of Hg in coastal wetlands and project the impact of environmental change on the global Hg cycle. We monitored Hg flux from a remote salt marsh over 9 days which included three cloudless days and a 4 mm rainfall event. We observed a cyclical diel relationship between Hg flux and solar radiation. When measurements at the same irradiance intensity are considered, Hg flux was greater in the evening when the sediment was warm than in the morning when the sediment was cool. This is evidence to suggest that both solar radiation and sediment temperature directly influence the rate of Hg(II) photoreduction in salt marshes. Hg flux could be predicted from solar radiation and sediment temperature in sub-datasets collected during cloudless days (R2 = 0.99), and before (R2 = 0.97) and after (R2 = 0.95) the rainfall event, but the combined dataset could not account for the lower Hg flux observed after the rainfall event that is in contrast to greater Hg flux from soils after rainfall events

Pharmacological Analysis of the Activation and Receptor Properties of the Tonic GABACR Current in Retinal Bipolar Cell Terminals

GABAergic inhibition in the central nervous system (CNS) can occur via rapid, transient postsynaptic currents and via a tonic increase in membrane conductance, mediated by synaptic and extrasynaptic GABAA receptors (GABAARs) respectively. Retinal bipolar cells (BCs) exhibit a tonic current mediated by GABACRs in their axon terminal, in addition to synaptic GABAAR and GABACR currents, which strongly regulate BC output. The tonic GABACR current in BC terminals (BCTs) is not dependent on vesicular GABA release, but properties such as the alternative source of GABA and the identity of the GABACRs remain unknown. Following a recent report that tonic GABA release from cerebellar glial cells is mediated by Bestrophin 1 anion channels, we have investigated their role in non-vesicular GABA release in the retina. Using patch-clamp recordings from BCTs in goldfish retinal slices, we find that the tonic GABACR current is not reduced by the anion channel inhibitors NPPB or flufenamic acid but is reduced by DIDS, which decreases the tonic current without directly affecting GABACRs. All three drugs also exhibit non-specific effects including inhibition of GABA transporters. GABACR ρ subunits can form homomeric and heteromeric receptors that differ in their properties, but BC GABACRs are thought to be ρ1-ρ2 heteromers. To investigate whether GABACRs mediating tonic and synaptic currents may differ in their subunit composition, as is the case for GABAARs, we have examined the effects of two antagonists that show partial ρ subunit selectivity: picrotoxin and cyclothiazide. Tonic and synaptic GABACR currents were differentially affected by both drugs, suggesting that a population of homomeric ρ1 receptors contributes to the tonic current. These results extend our understanding of the multiple forms of GABAergic inhibition that exist in the CNS and contribute to visual signal processing in the retina

Editorial For \u201cWetlands in a changing World\u201d

Wetland ecosystems are some of themost threatened ecosystems in theWorld. These systems are of great ecological importance since they providemany ecosystemservices including high biological productivity, hydrological flux regulation, shoreline protection against sea-level rise, phytoremediation of contaminated areas, storage and recycling of carbon, nutrients and trace-elements, and provide habitat for aquatic and terrestrial flora and fauna. Special attention is warranted for these unique ecosystems and their examination through both laboratory and field-based studies. This special issue of STOTEN presents a collection of multidisciplinary research topics to help better understand the role of wetlands in the environment and how they are altered under the pressure of human activities. The manuscripts represent wetland research around the world with a wide diversity of ecosystems such as Arctic wetlands, Italian coastal lagoons, the Florida Everglades, and Egyptian saltmarshes. Topics covered include both spatial and temporal studies on nutrient and contaminant biogeochemistry, food web development, coastal ecology, and policy implications

A comparison of mercury biomagnification through lacustrine food webs supporting Brook Trout (Salvelinus fontinalis) and other salmonid fishes

Methyl mercury (MeHg) bioaccumulation in lower-trophic-level organisms and its subsequent biomagnification through food webs differs in magnitude among lakes and results in intraspecific variability of MeHg in top predator fishes. Understanding these differences is critical given the reproductive and neurotoxic effects of MeHg on fishes and their predators, including humans. In this study we characterized the food webs of five lakes in New Brunswick, Canada, supporting Brook Trout (Salvelinus fontinalis) using measures of relative trophic position (δ15N) and carbon sources (δ13C), determined the concentrations of MeHg in invertebrates and total Hg (THg) in fishes, and quantified MeHg biomagnification from primary to tertiary consumers. Methyl Hg and THg concentrations were highest in biota from lakes with lower pH. The trophic magnification slopes (TMS; log Hg versus δ15N) varied significantly among lakes (0.12-0.20; ANCOVA, p=0.031). When combined with data from other salmonid lakes in temperate and Arctic Canada (n=36), among-system variability in TMS was best, but weakly, positively predicted by aqueous total phosphorous (p = 0.028, R2adj = 0.109). These results suggest that lake productivity directly or indirectly influences the biomagnification of MeHg through diverse food webs supporting salmonids

Mercury in Soils of Seabird Nesting Islands in West Iceland

Seabirds are globally recognized vectors of marine-derived materials, which get deposited on land at their breeding colonies, potentially altering local soil chemistry. We studied mercury (Hg) in soil cores on two islands in west Iceland that host thousands of nesting seabirds, predicting that Hg subsidies from nesting birds would result in elevated Hg in local soils. However, despite clear evidence from nitrogen isotopes of marine influence (seabird faeces) on coastal soil cores, O horizon Hg concentrations averaged 223 nanograms per gram (ng/g), were similar between reference and seabird-nesting sites, and were within the range of soils elsewhere in Europe and the Arctic. The concentration of Hg declined for samples deeper in the core, mirroring declines in organic content and concomitant increases in stable isotopes of nitrogen. A more detailed analysis of local pedogenic processes is required to determine the relative contribution of lithogenic, atmospheric, and anthropogenic Hg, but our data do not suggest that seabirds are markedly increasing local soil Hg through ornithogenic subsidies. À l’échelle mondiale, les oiseaux de mer sont reconnus en tant que vecteurs de matières d’origine marine, celles-ci étant déposées à leurs colonies de nidification de la terre ferme, ce qui peut avoir pour effet de modifier la chimie du sol local. Nous avons étudié le mercure (Hg) se trouvant dans des carottes de sol de deux îles de l’ouest de l’Islande où nichent des milliers d’oiseaux de mer, prédisant que les bonifications en Hg des oiseaux nicheurs donneraient lieu à des taux de Hg élevés dans les sols locaux. Cependant, malgré la preuve évidente d’isotopes d’azote d’influence marine (déjections d’oiseaux de mer) dans les carottes de sol côtier, les concentrations de mercure de l’horizon O atteignaient en moyenne 223 nanogrammes par gramme (ng/g), étaient semblables entre le point de référence et les sites de nidification des oiseaux de mer, et se situaient dans la même gamme de sols que ceux se trouvant ailleurs en Europe et dans l’Arctique. La concentration de Hg diminuait dans le cas des échantillons prélevés plus en profondeur dans les carottes, reflétant des diminutions du contenu organique et des augmentations concomitantes des isotopes stables d’azote. Bien qu’une analyse plus détaillée des processus pédogénétiques locaux s’avère nécessaire dans le but de déterminer la contribution relative du mercure lithogénétique, atmosphérique et anthropique, nos données ne suggèrent pas que les oiseaux de mer fassent augmenter considérablement la teneur en Hg du sol local au moyen de leurs bonifications ornithogéniques.