Robert J.G. Mortimer

Microbial influence on the oxygen isotopic composition of diagenetic siderite

Abstract Numerous early diagenetic siderite concretions previously described have been interpreted as the results of microbially mediated reactions. Interpretation of oxygen isotope data for such material requires an understanding of the effect of temperature on fractionation processes. However, whilst equilibrium fractionation of oxygen isotopes between siderite and water has been measured down to 33°C, extrapolation to lower temperatures may be invalid. Furthermore, inorganic measurements may not be applicable to microbial systems. The specific iron-reducing microorganism, Geobacter metallireducens, has been cultured anaerobically in the laboratory using acetate as an organic substrate and amorphous FeOOH as an electron acceptor. The acetate is oxidised to CO2, with concurrent iron reduction and extracellular siderite precipitation. Rhombohedral siderite crystals up to 25 μm in size have been precipitated over a range of temperatures (18–40°C). Stable isotopic analysis of these crystals and the solutions from which they precipitate shows that this microbial precipitation of siderite has an associated isotopic fractionation different from the published equilibrium, and which is not simply a function of temperature. In all cases, δ18O values of siderite are lower than predicted by inorganic equilibrium fractionation data. This may explain the numerous anomalously-low δ18O values reported for early diagenetic marine siderites and previously attributed to mechanisms such as mixing with meteoric water, sediment-water interaction, recrystallisation, or variable isotopic fractionation, despite a lack of supporting evidence.

Localised remobilization of metals in a marine sediment

Trace metals and Fe and Mn were measured at vertical spatial resolutions of 2.5 and 5 mm in the top 35 cm of the profundal sediment of a Scottish sea-loch using DGT (diffusive gradients in thin films) technique. DGT probes lower adjacent metal concentrations in pore waters and induce a flux of metal from the solid phase to porewater. The concentrations of metals in porewaters at the interface of the probe were measured during its deployment in a box core. These measurements reflect porewater concentrations of metals and their rates of resupply from the local solid phase of a very small volume (25 microl) of sediment. There was pronounced horizontal and vertical structure in the interfacial concentrations. Horizontal variations were shown by results from adjacent DGT assemblies being markedly different in detail, while vertical structure was measured directly by the DGT-depth profiles. Iron and Mn varied systematically with depth, with both broad and detailed features of Co aligning with those of Mn. There was, however, evidence of additional localised sources of Co that were apparently unrelated to the redox behaviour that Mn typifies, but associated with the remobilization of Ni, possibly from mineral dissolution. Arsenic(III) was remobilized in well-defined zones. Detailed correspondence of As(II) with some Fe features suggest that its release is mechanistically-related to iron oxide dissolution, but the 3 orders of magnitude higher concentrations of Fe may sometimes obscure the association. These results demonstrate that, within sediments, metals may be released in discrete locations that are not measured by conventional porewater sampling techniques due to their horizontal averaging.

Sediment–Water Exchange of Nutrients in the Intertidal Zone of the Humber Estuary, UK

Abstract Nutrient pore-water profiles, sediment–water exchange and sediment characteristics were measured for six intertidal mudflat sites throughout the Humber Estuary over the different seasons. The Humber is a highly dynamic, non steady state system and hence neither the biogeochemical zones nor the macrofaunal communities were sustained for long periods of time. Sediment mixing and resuspension on tidal, episodic or seasonal timescales was the predominant control on nutrients. Over an annual cycle, mean measured fluxes were −8.7 mmoles/m 2 /day nitrate, 3.7 mmoles/m 2 /day ammonia and 0.2 mmoles/m 2 /day nitrite. Net phosphate and silicate fluxes were very small. The intertidal mudflats were a sink for nitrate (−1000 kmol/day), a major source of ammonia (430 kmol/day) and a minor source of nitrite (25 kmol/day). Nitrate influxes decreased in a seaward direction (−13.4 mmoles/m 2 /day in inner estuary, −11.0 mmoles/m 2 /day in mid-estuary, −5.2 mmoles/m 2 /day in outer estuary), but when the area of the mudflats in each area is taken into account, both the mid and outer estuary were sinks for approximately 40% of the total nitrate taken up. In contrast, the outer estuary was the source of c. 90% of the ammonia and 105% of the nitrite (the inner estuary being a minor sink of nitrite).

Predatory Functional Response and Prey Choice Identify Predation Differences between Native/Invasive and Parasitised/Unparasitised Crayfish

Background Invasive predators may change the structure of invaded communities through predation and competition with native species. In Europe, the invasive signal crayfish Pacifastacus leniusculus is excluding the native white clawed crayfish Austropotamobius pallipes. Methodology and Principal Findings This study compared the predatory functional responses and prey choice of native and invasive crayfish and measured impacts of parasitism on the predatory strength of the native species. Invasive crayfish showed a higher (>10%) prey (Gammarus pulex) intake rate than (size matched) natives, reflecting a shorter (16%) prey handling time. The native crayfish also showed greater selection for crustacean prey over molluscs and bloodworm, whereas the invasive species was a more generalist predator. A. pallipes parasitised by the microsporidian parasite Thelohania contejeani showed a 30% reduction in prey intake. We suggest that this results from parasite-induced muscle damage, and this is supported by a reduced (38%) attack rate and increased (30%) prey handling time. Conclusions and Significance Our results indicate that the per capita (i.e., functional response) difference between the species may contribute to success of the invader and extinction of the native species, as well as decreased biodiversity and biomass in invaded rivers. In addition, the reduced predatory strength of parasitized natives may impair their competitive abilities, facilitating exclusion by the invader.

Behavior of Aluminum, Arsenic, and Vanadium during the Neutralization of Red Mud Leachate by HCl, Gypsum, or Seawater

Red mud leachate (pH 13) collected from Ajka, Hungary is neutralized to < pH 10 by HCl, gypsum, or seawater addition. During acid neutralization >99% Al is removed from solution during the formation of an amorphous boehmite-like precipitate and dawsonite. Minor amounts of As (24%) are also removed from solution via surface adsorption of As onto the Al oxyhydroxides. Gypsum addition to red mud leachate results in the precipitation of calcite, both in experiments and in field samples recovered from rivers treated with gypsum after the October 2010 red mud spill. Calcite precipitation results in 86% Al and 81% As removal from solution, and both are nonexchangeable with 0.1 mol L(-1) phosphate solution. Contrary to As associated with neoformed Al oxyhydroxides, EXAFS analysis of the calcite precipitates revealed only isolated arsenate tetrahedra with no evidence for surface adsorption or incorporation into the calcite structure, possibly as a result of very rapid As scavenging by the calcite precipitate. Seawater neutralization also resulted in carbonate precipitation, with >99% Al and 74% As removed from solution during the formation of a poorly ordered hydrotalcite phase and via surface adsorption to the neoformed precipitates, respectively. Half the bound As could be remobilized by phosphate addition, indicating that As was weakly bound, possibly in the hydrotalcite interlayer. Only 5-16% V was removed from solution during neutralization, demonstrating a lack of interaction with any of the neoformed precipitates. High V concentrations are therefore likely to be an intractable problem during the treatment of red mud leachates.

Use of gel probes for the determination of high resolution solute distributions in marine and estuarine pore waters

Pore water profiles were obtained at high resolution (millimeter scale) in marine sediments using a DET (diffusive equilibration in thin films) gel probe. A plastic probe which holds a 1–2 mm thick polyacrylamide gel covered by a 0.45 μm Millipore filter is inserted into the sediment. The gel was prehydrated in water of similar salinity to the in situ sampling conditions. Chloride and sulphate needed 24 and 48 h for complete front and back equilibration, respectively, while 6–8 h (front) and 4–6 h (back) were used for calcium and alkalinity. Ammonia-N and total CO2 were back-equilibrated for 2 h. Probes were sectioned immediately after sampling, stored for up to 1–2 days (NH4+, ∑CO2) or for up to 1–2 weeks (Ca, alkalinity, Cl, SO4, NO3), before they were back-equilibrated into Milli-Q water or 0.7 M NaCl (calcium and alkalinity). The gel retains between 3–7% of the total sulphate. A simple procedure has been developed to correct for this incomplete recovery. Recovery tests using seawater and 50% seawater spiked to concentrations found in nearshore pore waters, showed recoveries of 101.4±0.5% (chloride), 101.4±0.7% (bromide), 100.3±0.2% (nitrate), 96.6±0.7% (sulphate), 99.7±0.8% (ammonia-N), 99.1±1.2% (∑CO2), 96.9±0.8% (calcium) and 96.8±1.4% (alkalinity) (mean±standard error). Pore water profiles obtained simultaneously using gel probes and conventional techniques (box core-anoxic slicing followed by centrifugation) showed excellent comparability at cm resolution though features which required higher resolution which could only be seen in the gel profiles.

Metal Speciation (Cu, Zn, Pb, Cd) and Organic Matter in Oxic to Suboxic Salt Marsh Sediments, Severn Estuary, Southwest Britain

Abstract Pollutant metals (Cu, Pb, Zn and Cd) in Late Flandrian mudflat/saltmarsh sediments in the Severn Estuary are predominantly bound in the carbonate, Fe/Mn oxide and organic phases. Comparison of the oxic Rumney Formation with the suboxic Wentlooge Formation shows that redox conditions control iron cycling, and that this in turn controls trace metal associations. An important fraction of the trace metals present in the Fe/Mn oxide phase in oxic sediments (such as the Rumney Formation) is remobilised during iron reduction and reprecipitated in other phases such as authigenic carbonates (as in the Wentlooge). Since many metal pollutants are highly redox sensitive, a better understanding of the effects of redox on metal speciation in estuarine sediments is useful for management of the coastal environment.

Stimulation of Microbially Mediated Chromate Reduction in Alkaline Soil-Water Systems

Acetate was added to two closed soil-water systems that are representative of the subsurface environment close to chromium ore processing residue disposal sites; one had a pH of 7.7, the other 9.3. Cr(VI) reduction occurred in both systems as part of a cascade of microbially mediated terminal electron-accepting processes, occurring between nitrate and iron reduction. Cr(VI) and subsequently iron reduction took longer to start and were slower in the more alkaline system. At the point when Cr(VI) reduction was essentially complete, the microbial populations in both systems showed an increase in species closely related to β-proteobacteria that are capable of nitrate reduction.

Chromate reduction in Fe(II)-containing soil affected by hyperalkaline leachate from chromite ore processing residue

Highly alkaline (pH 12.2) chromate contaminated leachate (990 μmol L(-1)) has been entering soils below a chromite ore processing residue disposal (COPR) site for over 100 years. The soil immediately beneath the waste has a pH of 11→12.5, contains 0.3→0.5% (w/w) chromium, and 45→75% of the microbially available iron is Fe(II). Despite elevated pH, a viable microbial consortium of Firmicutes dominated iron reducers was isolated from this COPR affected soil. Soil pH and Cr concentration decrease with distance from the waste. XAS analysis of soil samples indicated that Cr is present as a mixed Cr(III)-Fe(III) oxy-hydroxide phase, suggesting that the elevated soil Cr content is due to reductive precipitation of Cr(VI) by Fe(II). Microcosm results demonstrate the capacity of COPR affected soil to abiotically remove all Cr(VI) from the leachate within 40 days. In air oxidation experiments less than 2% of the total Cr in the soil was remobilised despite significant Fe(II) oxidation. XAS analysis after air oxidation showed no change in Cr-speciation, indicating the Cr(III)-containing phase is a stable long term host for Cr. This work suggests that reductive precipitation of Cr(VI) is an effective method of contaminant immobilisation in soils where microbially produced Fe(II) is present.

Internal loading of phosphorus in a sedimentation pond of a treatment wetland: Effect of a phytoplankton crash

Sedimentation ponds are widely believed to act as a primary removal process for phosphorus (P) in nutrient treatment wetlands. High frequency in-situ P, ammonium (NH(4)(+)) and dissolved oxygen measurements, alongside occasional water quality measurements, assessed changes in nutrient concentrations and productivity in the sedimentation pond of a treatment wetland between March and June. Diffusive equilibrium in thin films (DET) probes were used to measure in-situ nutrient and chemistry pore-water profiles. Diffusive fluxes across the sediment-water interface were calculated from the pore-water profiles, and dissolved oxygen was used to calculate rates of primary productivity and respiration. The sedimentation pond was a net sink for total P (TP), soluble reactive P (SRP) and NH(4)(+) in March, but became subject to a net internal loading of TP, SRP and NH(4)(+) in May, with SRP concentrations increasing by up to 41μM (1300μl(-1)). Reductions in chlorophyll a and dissolved oxygen concentrations also occurred at this time. The sediment changed from a small net sink of SRP in March (average diffusive flux: -8.2μmolm(-2)day(-1)) to a net source of SRP in June (average diffusive flux: +1324μmolm(-2)day(-1)). A diurnal pattern in water column P concentrations, with maxima in the early hours of the morning, and minima in the afternoon, occurred during May. The diurnal pattern and release of SRP from the sediment were attributed to microbial degradation of diatom biomass, causing reduction of the dissolved oxygen concentration and leading to redox-dependent release of P from the sediment. In June, 2.7mol-Pday(-1) were removed by photosynthesis and 23mol-Pday(-1) were supplied by respiration in the lake volume. SRP was also released through microbial respiration within the water column, including the decomposition of algal matter. It is imperative that consideration to internal recycling is given when maintaining sedimentation ponds, and before the installation of new ponds designed to treat nutrient waste.