Anoxic nitrification: Evidence from Humber Estuary sediments (UK)

Conventional understanding of the nitrogen cycle in marine sediments has changed in recent years with the discovery of an alternative pathway for ammonia oxidation via the reduction of manganese oxides (during anoxic nitrification). In anoxic sediments, the potential for manganese oxides to serve as oxidant for nitrification may be considerable yet previous work on manganese-rich sediments has suggested anoxic nitrification may not be significant. In this study, the potential for anoxic nitrification in a range of sediment types was investigated. Laboratory incubation of sediment from three sites on the Humber Estuary, a microbially diverse environment, showed anoxic accumulation of nitrate, nitrite and dinitrogen gas, with and without the addition of synthetic manganese oxides. Incubation experiments confirmed anoxic nitrification as microbially mediated, with heat-killed controls yielding negative results. The anoxic nitrification reaction significantly depleted ammonia concentrations, and occurred simultaneously with manganese-, iron- and sulphate reduction, and methanogenesis. Taken in conjunction with other studies, results suggest anoxic nitrification may not only be dependent on total manganese concentrations but on manganese dynamics. Anoxic nitrification may be explained as a non-steady state reaction, dependent on the recent stability of a sediment system. Physical perturbation of sediments may cause the redistribution and/or introduction of manganese oxides and promote anoxic nitrification. The significance and persistence of anoxic nitrification is likely to depend on the frequency and magnitude of sediment perturbation, which explains why the reaction varies so widely across studied sites, and why it may not occur in some manganese-rich sediment.     

An improved specific interaction model for seawater at 25°C and 1 atmosphere total pressure

Equations recently developed by Pitzer are applied to a seawater model on the assumption that only interactions between ions of opposite charge sign are significant. This model is shown to give good agreement with available experimental data for the osmotic coefficient and for the mean-ion activity coefficients of the major electrolyte components. The results are compared with the predictions of the ion-association model and with a number of models using the concept of specific ionic interaction. The relative merits of the various models are discussed. Pitzer's original equations are modified to enable the calculation of conventional single-ion activity coefficients. These values are compared with those obtained on the basis of alternative conventions and an encouraging measure of agreement is observed. It is concluded that the new model introduces a fluency into the specific interaction approach that should greatly facilitate the incorporation of trace components into future models.     

Stress-induced apoptosis in sea urchin embryogenesis

We find that early sea urchin embryos have the capability to induce programmed cell death, or apoptosis, in response to chemical and physical stress. Strongylocentrotus purpuratus embryos (fertilized, 4 cell, 16 cell, 64 cell, and early blastula) were exposed to known cytotoxins, in order to determine when apoptosis occurs naturally and in response to stress. Using cell permeability as an indicator of early stage apoptosis, caspase activation as a mid-stage indicator, and DNA fragmentation as a late stage indicator, we find that during the cleavage stage of embryogenesis apoptosis is almost completely absent. However, a statistically significant (p<0.001) rise in apoptosis in stressed embryos is evident around 24 h after fertilization, during the early blastula stage and shortly after hatching. Before this stage, exposed embryos show no statistically significant increases in apoptosis in comparison to the controls. This pattern of apoptosis in development is similar to that seen in lower vertebrate models in which stress-induced apoptosis occurs only around the mid-blastula transition. We conclude that apoptosis may be used to rid embryos of aberrant or damaged cells in early development, but this response is stage-dependant. Repair, rather than apoptosis, may be utilized during earlier stages, or alternatively, embryos exposed to such stressors may continue development with damaged cells and perhaps damaged DNA. Our continued studies will focus on these alternative hypotheses.     

A Stable Carbon Isotope Study of the Food-web in a Freshwater-deprived South African Estuary, with Particular Emphasis on the Ichthyofauna

The importance of macrophytes as food sources for estuarine nekton is unclear. Previous carbon isotope investigations in the macrophyte-dominated, freshwater-deprived Kariega Estuary showed that the bivalveSolen cylindraceusdid not utilize the dominant estuarine macrophytes found within the estuary as a primary food source. This finding prompted questions as to what the nekton of this estuary utilize as primary energy sources. δ¹³C analyses of the principal autochthonous and allochthonous primary carbon sources, as well as the dominant invertebrate and fish species, indicate that there are two main carbon pathways within the Kariega Estuary. The littoral community, which incorporates the majority of crustaceans, gobies, mullet and a sparid, utilizes δ¹³C enriched primary food sources namelySpartina maritima,Zosteracapensis and epiphytes. The channel fauna, which includes the zooplankton, zooplanktivorous and piscivorous fish, utilizes a primary food source depleted in δ¹³C, which is most likely a mixture of phytoplankton, terrestrial plant debris and C₄macrophyte detritus. The C₃saltmarsh macrophytesSarcocornia perennisandChenolea diffusa, as well as benthic microalgae, appear to be less important as primary food sources to the nekton of the Kariega Estuary.     

The Southern Antarctic Circumpolar Current Front: physical and biological coupling at South Georgia

The coupling of physics and biology was examined along a 160 km long transect running out from the north coast of South Georgia Island and crossing the Southern Antarctic Circumpolar Current Front (SACCF) during late December 2000. Surface and near surface potential TS properties indicated the presence of three water types: a near-shore group of stations characterised by water which became progressively warmer and fresher closer to South Georgia, an offshore grouping in which sea surface temperatures and those at the winter water level were relatively warm (1.8°C and 0.5°C, respectively), and a third in which surface and winter water temperatures were cooler and reflected the presence of the SACCF. The transect bisected the SACCF twice, revealing that it was flowing in opposite directions, north-westward closest to South Georgia and south-eastwards at its furthest point from the island. The innermost limb was a narrow intense feature located just off the shelf break in 2000–3500 m of water and in which rapid surface baroclinic velocities (up to 35 cm s⁻¹) were encountered. Offshore in the outermost limb, shown subsequently to be a mesoscale eddy that had meandered south from the retroflected limb of the SACCF, flow was broader and slower with peak velocities around 20 cm s⁻¹. Chlorophyll a biomass was generally low (<1 mg m⁻³) over much of the transect but increased dramatically in the region of the innermost limb of the SACCF, where a deepening of the surface mixed layer was coincident with a subsurface chlorophyll maximum (7.4 mg m⁻³) and elevated concentrations down to 100 m. The bloom was coincident with depleted nutrient concentrations, particularly silicate, nitrate and phosphate, and although ammonium concentrations were locally depleted the bloom lay within an elevated band (up to 1.5 mmol m⁻³) associated with the frontal jet. Increased zooplankton abundance, higher copepod body carbon mass and egg production rates all showed a strong spatial integrity with the front. The population structure of the copepods Calanoides acutus and Rhincalanus gigas at stations within the front suggested that rather than simply resulting from entrainment and concentration within the jet, increased copepod abundance was the result of development in situ. Estimates of bloom duration, based on silicate and carbon budget calculations, set the likely duration between 82 and 122 d, a figure supported by the development schedule of the two copepod species. Given this timescale, model outputs from FRAM and OCCAM indicated that particles that occurred on the north side of South Georgia in December would have been in the central-southern Scotia Sea 2–3 months earlier, probably in sea ice affected regions.     

The partial molar volume, thermal expansivity, and compressibility of H2O in NaAlSi3O8 liquid: new measurements and an internally consistent model

The molar volumes of 19 hydrous albitic liquids (1.9 to 6.1 wt% H₂O^total) were determined at one bar and 505–765 K. These volume data were derived from density measurements on hydrous glasses at 298 K, followed by measurements of the thermal expansion of each glass from 298 K to its respective glass transition temperature. The technique exploits the fact that the volume of a glass is equal to that of the corresponding liquid at the limiting fictive temperature (T _f′), and that T _f′ can be approximated as the temperature near the onset of the rapid increase in thermal expansion that occurs in the glass transition interval. The volume data of this study were combined with available volume data for anhydrous, Na₂O-Al₂O₃-SiO₂ liquids to derive the partial molar volume (±1) of the H₂O component in an albitic melt at ∼565 K and one bar. To extend the determination of to higher temperatures and pressures, the molar volumes of the hydrous albitic liquids determined in this study were combined with those measured by previous authors at 1023–1223 K and 480–840 MPa, leading to the following fitted values (±1) at 1673 K and one bar: (±0.46)×10⁻³ cm⁻³/mol-K, and dVˉ ^H ₂ ^{O total} /dP=−3.82 (±0.36)×10⁻⁴ cm³/mol-bar. The measured molar volumes of this study and those of previous authors can be recovered with a standard deviation of 0.5%, which is within the respective experimental errors. There is a significant difference between the values for derived in this study as a function of temperature and pressure and those obtained from an existing polynomial, primarily caused by the previous absence of accurate density measurements on anhydrous silicate liquids. The coefficients of thermal expansion (=4.72×10⁻⁴/K) and isothermal compressibility ( _T =1.66×10⁻⁵/bar) for the H₂O component at 1273 K and 100 MPa, indicate that H₂O^total is the single most expansive and compressible component in silicate liquids. For example, at 1473 K and 70 MPa (conditions of a mid-ocean ridge crustal magma chamber), the presence of just 0.4 wt% H₂O will decrease the density of a basaltic liquid by more than one percent. An equivalent decrease in melt density could be achieved by increasing the temperature by 175 degrees or the decreasing pressure by 230 MPa. Therefore, even minor quantities of dissolved water will have a marked effect on the dynamic properties of silicate liquids in the crustal environment. Received: 20 August 1996 / Accepted: 15 March 1997     

Molecular simulation of humic substance-Ca-montmorillonite complexes

An understanding of the processes that lead to long-term stabilization of organic matter in soils is essential to the effective implementation of strategies designed to mitigate CO₂ loss from the soil carbon reservoir in temperate climatic zones. Decomposition studies indicate that montmorillonite, a smectite that often forms with interlayers rich in Ca²⁺, greatly retards the microbial mineralization of soil organic matter. We performed a series of atomistic simulations designed to identify favorable molecular-scale organo-mineral interactions within nanoscale, hydrated complexes consisting of a humic substance and Ca-montmorillonite. Both protonated and Ca-saturated forms of the model humic molecule, representing acidic and circumneutral solution conditions, respectively, were studied within the hydrated interlayer region of a rigid-atom model of Ca-montmorillonite. The protonated humic substance formed direct hydrophobic and hydrogen bonding (H-bonding) interactions with the clay mineral. A few polar organic groups adsorbed via water bridging interactions. The Ca-saturated humic substance adsorbed via numerous cation bridges, less numerous water bridges, and indirect H-bonding interactions mediated by water molecules. Application of molecular modeling techniques to this complex organo-mineral system thus allowed identification of interactions favorable to carbon sequestration under both acidic and circumneutral conditions.     

Application of a multimetric fish index to assess the environmental condition of South African estuaries

A multimetric fish index, the Estuarine Fish Community Index (EFCI) of Harrison and Whitfield (2004), was applied to data collected for 190 South African estuaries. Estuaries spanned three biogeographic regions and included three distinct estuarine typologies. The EFCI is based on 14 metrics or measures that represent four broad fish community attributes: species diversity and composition, species abundance, nursery function, and trophic integrity. Metric reference conditions and scoring criteria were developed separately for each estuary type within each zoogeographic region. The final EFCI was applied to each estuary by comparing its fish community with the appropriate reference. Index values ranged between 18 (very poor) and 66 (very good). A comparison of the EFCI with independent measures of estuarine condition revealed that the index was able to effectively differentiate between poor and good quality sites. Applying the EFCI to estuaries in which multiple samples were taken also showed that the index is reproducible. The EFCI is both a robust and sensitive method for assessing the ecological condition of estuarine systems; it is also an effective communication tool for converting ecological information into an easily understood format for managers, policy makers, and the general public.