Rapid blood acid-base regulation by European sea bass (Dicentrarchus labrax) in response to sudden exposure to high environmental CO2.

Fish in coastal ecosystems can be exposed to acute variations in CO2 of between 0.2 and 1 kPa CO2 (2000-10,000 µatm). Coping with this environmental challenge will depend on the ability to rapidly compensate for the internal acid-base disturbance caused by sudden exposure to high environmental CO2 (blood and tissue acidosis); however, studies about the speed of acid-base regulatory responses in marine fish are scarce. We observed that upon sudden exposure to ∼1 kPa CO2, European sea bass (Dicentrarchus labrax) completely regulate erythrocyte intracellular pH within ∼40 min, thus restoring haemoglobin-O2 affinity to pre-exposure levels. Moreover, blood pH returned to normal levels within ∼2 h, which is one of the fastest acid-base recoveries documented in any fish. This was achieved via a large upregulation of net acid excretion and accumulation of HCO3- in blood, which increased from ∼4 to ∼22 mmol l-1. While the abundance and intracellular localisation of gill Na+/K+-ATPase (NKA) and Na+/H+ exchanger 3 (NHE3) remained unchanged, the apical surface area of acid-excreting gill ionocytes doubled. This constitutes a novel mechanism for rapidly increasing acid excretion during sudden blood acidosis. Rapid acid-base regulation was completely prevented when the same high CO2 exposure occurred in seawater with experimentally reduced HCO3- and pH, probably because reduced environmental pH inhibited gill H+ excretion via NHE3. The rapid and robust acid-base regulatory responses identified will enable European sea bass to maintain physiological performance during large and sudden CO2 fluctuations that naturally occur in coastal environments.

Increasing Blood Pressure Variability Predicts Poor Functional Outcome Following Acute Stroke.

INTRODUCTION: Increasing blood pressure variability has been reported following acute stroke, but there is uncertainty about how best to measure it and about the impact on prognosis following acute ischaemic stroke and transient ischaemic attack. METHODS: Enhanced casual blood pressure and ambulatory blood pressure monitoring were completed at baseline (≤48 h post symptom onset). Blood pressure variability was defined by standard deviation and coefficient of variation of systolic, diastolic, mean arterial pressure, and pulse pressure. Modified Rankin scale score ≥3 described poor functional outcome assessed at 1- and 12-months post-stroke. Multivariable logistic regression models incorporating blood pressure variability measurement and other factors were performed, and odds ratio and 95% confidence intervals reported. RESULTS: 232 patients were recruited; 45 were dependent at 1-month, and 37 at 12-months. Dependent patients were more likely to be older, with a higher burden of pre-morbid conditions, and with increased blood pressure variability. Enhanced casual standard deviations of diastolic blood pressure [1.19 (1.02 to 1.39)] and mean arterial pressure [1.20 (1.00 to 1.43)] predicted dependency at 1-month. Predictors of 12-month dependency included: enhanced casual standard deviation of mean arterial pressure [1.21 (1.0-1.46)]; 24 h ambulatory monitor standard deviations of diastolic blood pressure [2.30 (1.08-4.90)] and mean arterial pressure [1.72 (1.09-2.72)], and the coefficient of variation of mean arterial pressure [1.76 (1.05-2.94)]; day-time ambulatory monitor coefficient of variation of systolic blood pressure [1.44 (1.02-2.03)] and mean arterial pressure [1.46 (1.02-2.08)]; and night-time ambulatory standard deviation of diastolic blood pressure [1.65 (1.03 -2.63)], and the coefficient of variation of mean arterial pressure and [1.38 (1.00- 1.90)] and pulse pressure [1.29 (1.00-1.65)]. CONCLUSION: Increasing blood pressure variability is independently and modestly associated with poor functional outcome at 1- and 12-months following acute stroke.

Advances in Understanding Mobilization Processes of Trace Metals in Marine Sediments.

Different mobilization mechanisms control the metal distribution in surface sediments of the Belgium coastal zone (BCZ) and the anoxic Gotland basin (GB). This mobilization was studied using DGT (diffusive gradients in thin films): vertical one-dimensional (1D) profiles of Cd, Co, Cu, Fe, Mn, Ni, Pb, and Zn were measured at 5 mm intervals, while two-dimensional (2D) high-resolution (100 µm) images of smaller zones of the sediment profile were obtained on separate DGT probes. Removal of dissolved Cd, Cu, and Pb in BCZ sediments caused steep vertical gradients at the sediment-water interface that were well replicated in 1D profiles and 2D images. While 1D profiles showed apparent coincident maxima of Co, Mn, and Fe, 2D images revealed mutually exclusive Co and Fe mobilization. Correlation analysis supported this observation and showed a consistent linkage between Co and Mn. Sharp maxima of some metals in the vertical 1D profiles of GB sediment were attributed to localized mobilization in microniches. Examination of an ∼1 mm diameter Cu and Ni maximum in 2D, defined by ∼300 data points, showed that the metals were supplied from localized decomposition of reactive organic material, rather than from reductively dissolving Fe or Mn oxides, and that they were removed as their sulfides.

Can cardiovascular risk management be improved by shared care with general practice to prevent cognitive decline following stroke/TIA? A feasibility randomised controlled trial (SERVED memory).

BACKGROUND: Cognitive impairment and dementia following cerebrovascular disease are increasingly common in the UK. One potential strategy to prevent post-stroke cognitive decline is multimodal vascular risk factor management. However, its efficacy remains uncertain and its application in vulnerable patients with incident cerebrovascular disease and early cognitive impairment has not been assessed. The primary aim of this study was to assess the feasibility of recruitment and retention of patients with early cognitive impairment post-stroke or transient ischaemic attack (TIA) to a trial of enhanced vascular risk factor management combining primary and secondary care. METHODS: In this single centre, open label trial adults with a recent stroke or TIA and mild cognitive impairment (MCI) were randomised 1:1 to a three-monthly multimodal vascular risk factor intervention jointly delivered by the trial team and General Practitioner (GP), or control (defined as usual care from the GP). Chosen risk factors were blood pressure (BP), total cholesterol, blood glucose (HbA1C) in those with diabetes, and heart rate and adequacy of anticoagulation in those with atrial fibrillation (AF). Similar patients with normal cognition were enrolled in an embedded observational cohort and also received usual care from the GP. Repeat cognitive screening was undertaken in all participants after 12 months. RESULTS: Seventy three participants were recruited to the randomised trial and 94 to the observational cohort (21.8% of those screened). From the randomised trial 35/73 (47.9%) dropped out before final follow-up. In all groups guideline based rates of risk factor control were mostly poor at baseline and did not significantly improve during follow-up. The observational cohort demonstrated greater decline in cognitive test scores at 12 months, with no difference between the randomised groups. CONCLUSIONS: Recruitment to such a study was feasible, but retention of participants was difficult and generally poor rates of risk factor control suggested insufficient application of the intervention. Consequently, successful scaling up of the trial would require protocol changes with less reliance on primary care services. Any future trial should include participants with normal cognition post-stroke as they may be at greatest risk of cognitive decline. TRIAL REGISTRATION: ISRCTN, ISRCTN42688361 . Registered 16 April 2015.

Understanding the gastrointestinal physiology and responses to feeding in air-breathing Anabantiform fishes.

The Mekong Delta is host to a large number of freshwater species, including a unique group of facultative air-breathing Anabantiforms. Of these, the striped snakehead (Channa striata), the climbing perch (Anabas testudineus), the giant gourami (Osphronemus goramy) and the snakeskin gourami (Trichogaster pectoralis) are major contributors to aquaculture production in Vietnam. The gastrointestinal responses to feeding in these four species are detailed here. Relative intestinal length was lowest in the snakehead, indicating carnivory, and 5.5-fold greater in the snakeskin, indicating herbivory; climbing perch and giant gourami were intermediate, indicating omnivory. N-waste excretion (ammonia-N + urea-N) was greatest in the carnivorous snakehead and least in the herbivorous snakeskin, whereas the opposite trend was observed for net K+ excretion. Similarly, the more carnivorous species had a greater stomach acidity than the more herbivorous species. Measurements of acid-base flux to water indicated that the greatest postprandial alkaline tide occurred in the snakehead and a potential acidic tide in the snakeskin. Additional findings of interest were high levels of both PCO2 (up to 40 mmHg) and HCO3 - (up to 33 mM) in the intestinal chyme of all four of these air-breathing species. Using in vitro gut sac preparations of the climbing perch, it was shown that the intestinal net absorption of fluid, Na+ and HCO3 - was upregulated by feeding but not net Cl- uptake, glucose uptake or K+ secretion. Upregulated net absorption of HCO3 - suggests that the high chyme (HCO3 - ) does not result from secretion by the intestinal epithelium. The possibility of ventilatory control of PCO2 to regulate postprandial acid-base balance in these air-breathing fish is discussed.

Blood pressure differences between home monitoring and daytime ambulatory values and their reproducibility in treated hypertensive stroke and TIA patients.

BACKGROUND: Guidelines recommend ambulatory or home blood pressure monitoring to improve hypertension diagnosis and monitoring. Both these methods are ascribed the same threshold values, but whether they produce similar results has not been established in certain patient groups. METHODS: Adults with mild/moderate stroke or transient ischemic attack (N = 80) completed 2 sets of ambulatory and home blood pressure monitoring. Systolic and diastolic blood pressure values from contemporaneous measurements were compared, and the limits of agreement were assessed. Exploratory analyses for predictive factors of any difference were conducted. RESULTS: Daytime ambulatory blood pressure values were consistently lower than home values, the mean difference in systolic blood pressure for initial ambulatory versus first home monitoring was -6.6 ± 13.5 mm Hg (P≤.001), and final ambulatory versus second home monitoring was -7.1 ± 11.0mm Hg (P≤.001). Mean diastolic blood pressure differences were -2.1 ± 8.5mm Hg (P=.03) and -2.0 ± 7.2mm Hg (P=.02). Limits of agreement for systolic blood pressure were -33.0 to 19.9mm Hg and -28.7 to 14.5mm Hg for the 2 comparisons and for DBP were -18.8 to 14.5mm Hg and -16.1 to 12.2mm Hg, respectively. The individual mean change in systolic blood pressure difference was 11.0 ± 8.3mm Hg across the 2 comparisons. No predictive factors for these differences were identified. CONCLUSIONS: Daytime ambulatory systolic and diastolic blood pressure values were significantly lower than home monitored values at both time points. Differences between the 2 methods were not reproducible for individuals. Using the same threshold value for both out-of-office measurement methods may not be appropriate in patients with cerebrovascular disease.

Fish polyomaviruses belong to two distinct evolutionary lineages.

The Polyomaviridae is a diverse family of circular double-stranded DNA viruses. Polyomaviruses have been isolated from a wide array of animal hosts. An understanding of the evolutionary and ecological dynamics of these viruses is essential to understanding the pathogenicity of polyomaviruses. Using a high throughput sequencing approach, we identified a novel polyomavirus in an emerald notothen (Trematomus bernacchii) sampled in the Ross sea (Antarctica), expanding the known number of fish-associated polyomaviruses. Our analysis suggests that polyomaviruses belong to three main evolutionary clades; the first clade is made up of all recognized terrestrial polyomaviruses. The fish-associated polyomaviruses are not monophyletic, and belong to two divergent evolutionary lineages. The fish viruses provide evidence that the evolution of the key viral large T protein involves gain and loss of distinct domains.

Does self-monitoring and self-management of blood pressure after stroke or transient ischemic attack improve control? TEST-BP, a randomized controlled trial.

The therapeutic benefit of self-monitoring blood pressure in stroke patients is uncertain. We investigated the effect of self-monitoring with or without guided antihypertensive management compared with usual care in patients with a recent cerebrovascular event. No between-group differences in blood pressure at outcome were found, but blood pressure self-monitoring and management was well tolerated.

The Ancient Evolutionary History of Polyomaviruses.

Polyomaviruses are a family of DNA tumor viruses that are known to infect mammals and birds. To investigate the deeper evolutionary history of the family, we used a combination of viral metagenomics, bioinformatics, and structural modeling approaches to identify and characterize polyomavirus sequences associated with fish and arthropods. Analyses drawing upon the divergent new sequences indicate that polyomaviruses have been gradually co-evolving with their animal hosts for at least half a billion years. Phylogenetic analyses of individual polyomavirus genes suggest that some modern polyomavirus species arose after ancient recombination events involving distantly related polyomavirus lineages. The improved evolutionary model provides a useful platform for developing a more accurate taxonomic classification system for the viral family Polyomaviridae.

Plant Induced Changes to Rhizosphere Characteristics Affecting Supply of Cd to Noccaea caerulescens and Ni to Thlaspi goesingense.

Changes in soil rhizosphere properties after growing the Cd hyperaccumulator Noccaea caerulescens and the Ni hyperaccumulator Thlaspi goesingense were investigated. Dissolved organic carbon content increased in the rhizosphere, but there were no significant changes in the solution concentrations of Cd and Ni. Concentrations of these metals extracted by NH4Cl and EDTA decreased in the rhizosphere, as did DGT-measured concentrations, indicating a depletion of labile metal in the solid phase. The results could be explained by the increased DOC in the rhizosphere maintaining a higher proportion of the labile metal in solution through complexation, with the overall depletion of metals only manifest in the solid phase. The DGT induced fluxes in soils and sediments (DIFS) model was used to provide key kinetic information on soil processes and labile pool size. These data showed that the more limited metal supply in the rhizosphere after the growth of hyperaccumulators was due to both depletion of the solid phase pool and a lower rate constant of supply from solid phase to solution. The effect on the rate constant, which could be rationalized by the plant sequentially accessing and consuming the more labile pools of metal, was most marked for Cd, which had the highest accumulation factors.

Extending the Use of Diffusive Gradients in Thin Films (DGT) to Solutions Where Competition, Saturation, and Kinetic Effects Are Not Negligible.

DGT (Diffusion Gradients in Thin films) was designed to sample trace metals in situ at their natural concentrations. The setup and the experimental deployment conditions were established to allow interpretation of a linear accumulation of metal with time, using a simple expression based on a steady-state flux under perfect sink conditions. However, the extension of DGT to a wide range of analytes and its use under varied conditions has shown that, in some situations, these conditions are not fulfilled, so that accumulations with time are nonlinear. Previously, when such curvature was observed, concentrations in solution could not be reliably calculated. Here, we present fundamentally derived equations that reproduce the time accumulation for three situations: (i) kinetic limitations in the binding to the resin, (ii) saturation or equilibrium effects, or (iii) non-negligible competitive effects. We show how the accumulations can be quantified, in terms of the required kinetic and thermodynamic constants, and provide practical guidance for their use to obtain reliable estimates of solution concentrations. Solutions containing Mg or Mn, where all three situations can prevail, are used as examples. Calculated concentrations show reasonable agreement with the experimentally known values and with the results of a numerical model of the system, significantly improving the estimations based on perfect sink conditions. Such an approach opens up the possibility of using DGT more widely in challenging systems and allows DGT data to be interpreted more fully.

Development and Application of the Diffusive Gradients in Thin Films Technique for the Measurement of Nitrate in Soils.

Nitrate (NO3-N), the main plant/microbial nitrogen source, has a fast turnover in soil driven by species transformation (nitrification/denitrification) and phyto/microbiota assimilation. The technique of diffusive gradients in thin films (DGT) is capable of a robust, low disturbance measurement of NO3-N but has not been implemented due to the absence of a binding layer suitable for deployment in soils. In this study, a new styrene divinylbenzene-based absorbent with amine functional groups (SIR-100-HP) was cast into an agarose gel support. The NO3-N ion selectivity of the SIR-100-HP/agarose binding layer was retained in the presence of high multivalent ion concentrations and was used successfully to acquire in situ NO3-N measurements in bulk soil. The kinetics of binding and the maximum binding capacity were determined. The total capacity of the DGT containing the SIR-100-HP/agarose binding phase was 667 µg of NO3-N. The performance of DGT was not affected by varying pH (3-8) or ionic strength (0-0.018 mol L-1), while anion competition effects at concentrations reflecting those in common agricultural soils were found to be negligible. Complete elution (100% efficiency) of NO3-N from the binding phase was achieved using a solution of 5% NaCl. This technique was validated in three contrasting soils. CDGT measurements were in excellent agreement with pore water NO3-N values. Two-dimensional NO3-N mapping of a profile of flooded rice paddy soil demonstrated the potential of this novel methodology for improved characterization of in situ N speciation for further understanding of bioavailability and biogeochemical processes of NO3-N in soils.

Morphology and hydro-sensory role of superficial neuromasts in schooling behaviour of yellow-eyed mullet (Aldrichetta forsteri).

The lateral line system is a mechanosensory organ found in all fish species and located on the skin or in subdermal canals. The basic functional units are superficial and canal neuromasts, which are involved in hydrodynamic sensing and cohesion in schooling fish. Yellow-eyed mullet (Aldrichetta forsteri) are an obligate schooling species found commonly in shallow coastal areas of New Zealand and Australia. Schooling is a fundamental part of their behavioural repertoire, yet little is known about the structure or functionality of the lateral line in this species. We used scanning electron microscopy to characterise the morphology of trunk superficial neuromasts. We then took a multi-sensory approach and conducted behavioural experiments comparing school structure in groups of fish with and without fully functioning lateral lines, under photopic and scotopic conditions. A highly developed hydro-sensing system exists on the trunk of yellow-eyed mullet consisting of superficial neuromasts containing hundreds of hair cells aligned, with respect to their most sensitive axis, in a rostrocaudal direction. Without functioning superficial neuromasts, schooling behaviour was disrupted under both photopic and scotopic conditions and the ability to detect stationary objects decreased. Results highlight the importance of this component of the lateral line system to schooling behaviour.

Does sex really matter? Explaining intraspecies variation in ocean acidification responses.

Ocean acidification (OA) poses a major threat to marine ecosystems globally, having significant ecological and economic importance. The number and complexity of experiments examining the effects of OA has substantially increased over the past decade, in an attempt to address multi-stressor interactions and long-term responses in an increasing range of aquatic organisms. However, differences in the response of males and females to elevated pCO2 have been investigated in fewer than 4% of studies to date, often being precluded by the difficulty of determining sex non-destructively, particularly in early life stages. Here we highlight that sex can significantly impact organism responses to OA, differentially affecting physiology, reproduction, biochemistry and ultimately survival. What is more, these impacts do not always conform to ecological theory based on differential resource allocation towards reproduction, which would predict females to be more sensitive to OA owing to the higher production cost of eggs compared with sperm. Therefore, non-sex-specific studies may overlook subtle but ecologically significant differences in the responses of males and females to OA, with consequences for forecasting the fate of natural populations in a near-future ocean.

Activated charcoal based diffusive gradients in thin films for in situ monitoring of bisphenols in waters.

Widespread use of bisphenols (BPs) in our daily life results in their elevated concentrations in waters and the need to study their environmental impact, which demands reliable and robust measurement techniques. Diffusive gradients in thin films (DGT) is an in situ passive sampling approach which provides time-integrated data. In this study we developed a new methodology, based on DGT with activated charcoal (AC) as a binding agent, for measuring three BPs (BPA, BPB, and BPF) which incorporated and tested its performance characteristics. Consistent elution efficiencies were obtained using methanol when concentrations of BPs were low and a methanol-NaOH mixture at high concentrations. The diffusion coefficients of BPA, BPB, and BPF in the diffusive gel, measured using an independent diffusion cell, were 5.03 × 10(-6), 5.64 × 10(-6), and 4.44 × 10(-6) cm(2) s(-1) at 25 °C, respectively. DGT with an AC binding gel had a high capacity for BPA, BPB, and BPF at 192, 140, and 194 µg/binding gel disk, respectively, and the binding performance did not deteriorate with time, up to 254 d after production. Time-integrated concentrations of BPs measured in natural waters using DGT devices with AC gels deployed in situ for 7 d were comparable to concentrations measured by an active sampling method. This study demonstrates that AC-based DGT is an effective tool for in situ monitoring of BPs in waters.

Elevated temperature causes metabolic trade-offs at the whole-organism level in the Antarctic fish Trematomus bernacchii.

As a response to ocean warming, shifts in fish species distribution and changes in production have been reported that have been partly attributed to temperature effects on the physiology of animals. The Southern Ocean hosts some of the most rapidly warming regions on earth and Antarctic organisms are reported to be especially temperature sensitive. While cellular and molecular organismic levels appear, at least partially, to compensate for elevated temperatures, the consequences of acclimation to elevated temperature for the whole organism are often less clear. Growth and reproduction are the driving factors for population structure and abundance. The aim of this study was to assess the effect of long-term acclimation to elevated temperature on energy budget parameters in the high-Antarctic fish Trematomus bernacchii. Our results show a complete temperature compensation for routine metabolic costs after 9 weeks of acclimation to 4°C. However, an up to 84% reduction in mass growth was measured at 2 and 4°C compared with the control group at 0°C, which is best explained by reduced food assimilation rates at warmer temperatures. With regard to a predicted temperature increase of up to 1.4°C in the Ross Sea by 2200, such a significant reduction in growth is likely to affect population structures in nature, for example by delaying sexual maturity and reducing production, with severe impacts on Antarctic fish communities and ecosystems.

Novel Speciation Method Based on Diffusive Gradients in Thin-Films for in Situ Measurement of Cr(VI) in Aquatic Systems.

Hexavalent chromium (Cr(VI)) is much more toxic and mobile than the trivalent species (Cr(III)) and consequently, in situ monitoring of Cr(VI) can improve the understanding of Cr biogeochemistry and toxicity in ecosystems. The passive diffusive gradients in thin-films (DGT) technique is a powerful tool for determining metal(loid) speciation, but a binding phase that absorbs only one specific species of Cr is needed. N-Methyl-d-glucamine (NMDG) functional resin was incorporated into the DGT binding phase for selective measurement of Cr(VI). This NMDG-DGT sampler exhibited a theoretically linear accumulation of Cr(VI), with negligible accumulation (<5%) of Cr(III), even after 72 h deployment. The good prediction of Cr(VI) concentration in synthetic freshwater with NMDG-DGT, even in the presence of 10-time more Cr(III), further indicated the sampler's reliability in selective detection of Cr(VI). Moreover, its high capacity for Cr(VI), which exceeded 230 µg cm(-2), facilitates measurement of Cr(VI) in both uncontaminated natural waters and in slightly and heavily contaminated (ppm level) waters. Field deployment of the NMDG-DGT sampler in such waters allowed accurate measurement of time-averaged Cr(VI) concentration, indicating its robustness for in situ measurements of Cr speciation and its potential for further application in the risk assessment of Cr.

Measurement of metals using DGT: impact of ionic strength and kinetics of dissociation of complexes in the resin domain.

As the measurement of metals by DGT (diffusion gradients in thin films) in low salinity media has been controversial, a thorough study of the impact of ionic strength (I) is timely. DGT accumulations of Cd, Co, and Ni in the presence of NTA at pH 7.5 with I in the range from 10(-4) to 0.5 M were obtained. An observed decrease in the metal accumulation as the ionic strength of the system decreased is partially explained by the electrostatic repulsion between the negatively charged resin domain and the dominant negatively charged complex species M-NTA. This electrostatic effect reduces the complex penetration into the resin domain, especially for nonlabile complexes, which do not fully dissociate in the gel domain. Analytical expressions, based on the Donnan model, were able to quantify these electrostatic effects. Additionally, the data indicate that the kinetic dissociation constant of M-NTA complexes in the resin layer is higher than Eigen predictions, suggesting a ligand-assisted dissociation mechanism. As the ionic strength decreases, the rate of reaction in the resin layer decreases due to the repulsion between the negatively charged resin sites and the complex species. This decrease contributes to the decrease in metal accumulation. These novel, previously unconsidered, effects of ionic strength and the ligand-assisted dissociation mechanism in the resin domain will affect DGT measurements made in freshwaters and soils.

Mechanistic insights from DGT and soil solution measurements on the uptake of Ni and Cd by radish.

This work tests the previously proposed hypothesis that plant uptake of metals is determined dominantly by diffusional controlled or plant limiting uptake mechanisms at, respectively, low and high metal concentrations. Radish (Raphanus sativus) was grown in 13 soils spiked with Ni (10 and 100 mg kg(-1)) and Cd (0.5 and 4 mg kg(-1)) for 4 weeks to investigate the mechanisms affecting plant uptake. Soil solution concentrations, Css, of Ni and Cd were measured, along with the DGT interfacial concentration, CDGT, and the derived effective concentration in soil solution, CE. Free ion activities, aNi(2+) and aCd(2+), were obtained using WHAM 6. Although there was a poor relationship between Ni in radish roots and either Css or aNi(2+) in unamended soils, the distribution of data could be rationalized in terms of the extent of release of Ni from the soil solid phase, as identified by DGT and soil solution measurements. By contrast Ni in radish was linearly related to CE, demonstrating diffusion limited uptake. For soils amended with high concentrations of Ni, linear relationships were obtained for Ni in radish plotted against, Css, aNi(2+), and CE, consistent with the plant controlling uptake. For Ni the hypothesis concerning dominant diffusional and plant limiting uptake mechanisms was demonstrated. Poor relationships between Cd in radish and Css, aCd(2+), and CE, irrespective of amendment by Cd, showed the importance of factors other than diffusional supply, such as rhizosphere and inhibitory processes, and that fulfilment of this hypothesis is plant and metal specific.

Localized flux maxima of arsenic, lead, and iron around root apices in flooded lowland rice.

In wetland-adapted plants, such as rice, it is typically root apexes, sites of rapid entry for water/nutrients, where radial oxygen losses (ROLs) are highest. Nutrient/toxic metal uptake therefore largely occurs through oxidized zones and pH microgradients. However, the processes controlling the acquisition of trace elements in rice have been difficult to explore experimentally because of a lack of techniques for simultaneously measuring labile trace elements and O2/pH. Here, we use new diffusive gradients in thin films (DGT)/planar optode sandwich sensors deployed in situ on rice roots to demonstrate a new geochemical niche of greatly enhanced As, Pb, and Fe(II) mobilization into solution immediately adjacent to the root tips characterized by O2 enrichment and low pH. Fe(II) mobilization was congruent to that of the peripheral edge of the aerobic root zone, demonstrating that the Fe(II) mobilization maximum only developed in a narrow O2 range as the oxidation front penetrates the reducing soil. The Fe flux to the DGT resin at the root apexes was 3-fold higher than the anaerobic bulk soil and 27 times greater than the aerobic rooting zone. These results provide new evidence for the importance of coupled diffusion and oxidation of Fe in modulating trace metal solubilization, dispersion, and plant uptake.