Bacterial Nitrate Reduction Research Articles

Discovery of a Gut Bacterial Metabolic Pathway that Drives α-Synuclein Aggregation.

Parkinson's disease (PD) etiology is associated with aggregation and accumulation of α-synuclein (α-syn) proteins in midbrain dopaminergic neurons. Emerging evidence suggests that in certain subtypes of PD, α-syn aggregates originate in the gut and subsequently spread to the brain. However, mechanisms that instigate α-syn aggregation in the gut have remained elusive. In the brain, the aggregation of α-syn is induced by oxidized dopamine. Such a mechanism has not been explored in the context of the gastrointestinal tract, a niche harboring 46% of the body's dopamine reservoirs. Here, we report that Enterobacteriaceae, a bacterial family prevalent in human gut microbiotas, induce α-syn aggregation. More specifically, our in vitro data indicate that respiration of nitrate by Escherichia coli K-12, which results in production of nitrite that mediates oxidation of Fe2+ to Fe3+, creates an oxidizing redox potential. These oxidizing conditions enabled the formation of dopamine-derived quinones and α-syn aggregates. Exposing nitrite, but not nitrate, to enteroendocrine STC-1 cells induced aggregation of α-syn that is natively expressed in these cells, which line the intestinal tract. Taken together, our findings indicate that bacterial nitrate reduction may be critical for initiating intestinal α-syn aggregation.

Characteristics of microbial communities and antibiotic resistance genes in typical rivers of the western Qinghai Lake basin

Environmental factors and anthropogenic activity are key factors that shape the distribution of bacteria and antibiotic resistance genes (ARGs) in natural environments. However, few studies have focused on the occurrence of bacteria and ARGs in remote and pristine environments. In this study, the distribution of bacteria and ARGs in two typical rivers of the western Qinghai Lake basin was investigated. Results showed that cold-resistant Planomicrobium sp. was the predominant genus due to the low temperature, followed by unclassified_f_Planococcaceae. High nitrogen nutrients increased the abundance and diversity of the bacteria community and denitrification was the dominant means of bacterial nitrate reduction. Tetracycline resistance genes including tetA, tetB, and tetC were the dominant ARGs in the western Qinghai Lake basin, ranging from 2.30 ​× ​103 to 1.91 ​× ​108 copies/L, while the low abundances of intI1, and ARGs such as sul1 indicated low anthropogenic activity in the western Qinghai Lake basin. Finally, the strong positive correlation between ARGs and intI1 highlights the potential transmission risk of ARGs through cross- or co-selection by horizontal gene transfer. Our study emphasized the adaptation of bacteria to the environment and the facilitation of anthropogenic activity in the propagation of ARGs in natural environments.

Biogenic Fe(II-III) Hydroxycarbonate Green Rust Enhances Nitrate Removal and Decreases Ammonium Selectivity during Heterotrophic Denitrification

Nitrification-denitrification is the most widely used nitrogen removal process in wastewater treatment. However, this process can lead to undesirable nitrite accumulation and subsequent ammonium production. Biogenic Fe(II-III) hydroxycarbonate green rust has recently emerged as a candidate to reduce nitrite without ammonium production under abiotic conditions. The present study investigated whether biogenic iron(II-III) hydroxycarbonate green rust could also reduce nitrite to gaseous nitrogen during bacterial nitrate reduction. Our results showed that biogenic iron(II-III) hydroxycarbonate green rust could efficiently decrease the selectivity of the reaction towards ammonium during heterotrophic nitrate reduction by native wastewater-denitrifying bacteria and by three different species of Shewanella: S. putrefaciens ATCC 12099, S. putrefaciens ATCC 8071 and S. oneidensis MR-1. Indeed, in the absence of biogenic hydroxycarbonate green rust, bacterial reduction of nitrate converted 11–42% of the initial nitrate into ammonium, but this value dropped to 1–28% in the presence of biogenic hydroxycarbonate green rust. Additionally, nitrite accumulation did not exceed the 2–13% in the presence of biogenic hydroxycarbonate green rust, versus 0–28% in its absence. Based on those results that enhance the extent of denitrification of about 60%, the study proposes a water treatment process that couples the bacterial nitrite production with the abiotic nitrite reduction by biogenic green rust.

Effects of combined magnetic fields on bacteria Rhodospirillum rubrum VKM B-1621.

Bacteria are the simplest model of living organisms and thus are a convenient object for magnetobiological research. This paper describes some effects of combined magnetic fields (CMFs) on the bacterium Rhodospirillum rubrum strain VKM B-1621, which is not a pathogen but was selected due to its wide spectrum of growth abilities. The authors chose magnetic field-resonant phosphorus and iron (Fe3+ ) because P-containing biochemical compounds (standard abbreviations PP1 , AMP, ADP, ATP) provide energy flows in bacteria while iron could take part in formation of magnetosensitive intracellular inclusions. CMFs were produced by interaction of a geomagnetic field (ВDС ) and an alternating electromagnetic field (ВАС ), which were similar in their intensities. Their magnetic characteristics were as follows: (CMF-1) ВDC = 46.80 µТ, ВАС = 86.11 µT, f = 807.0 Hz; (CMF-2) ВDC = 46.80 µТ, ВАС = 86.11 µT, f = 38.3 Hz; that is, the frequencies of applied alternating electromagnetic fields coincided with cyclotron frequencies of phosphorus or ferric ions, respectively. The blank variants were exposed to the geomagnetic field. The CMFs increased bacterial consumption of dissolved iron as measured by residual concentrations of iron in the medium (P > 99%). An increase of bacterial nitrate reduction in the CMFs was statistically insignificant (P > 90%) when measured by residual concentrations of nitrate. Application of CMFs can influence bacterial activity and metabolism. Bioelectromagnetics. 2018;39:485-490, 2018. © 2018 Wiley Periodicals, Inc.

Ex situ groundwater treatment triggering the mobilization of geogenic uranium from aquifer sediments

Uranium (U) concentrations in groundwater extracted for drinking water usage from a Quaternary fluvial aquifer partly exceed the German drinking water guideline of 10μgL−1. Responsible sources and mobilization processes were unknown and gave rise to this study. Land use of the watershed is mainly agricultural leading to groundwater nitrate concentrations >50mgL−1 and a need for water treatment prior to utilization as drinking water. This is successfully accomplished by addition of nutrients triggering bacterial nitrate reduction, followed by the addition of NaOH for water softening and CO2 for pH adjustment, with subsequent reinfiltration into the aquifer. Three boreholes were drilled to obtain a total of 127 solid samples from Quaternary and underlying Tertiary sediments. Geochemistry and mineralogy were assessed using elemental analysis (CS, ICP-MS), X-ray diffraction and scanning electron microscopy to complement hydrochemical data and unravel U occurrence in the subsurface. Solid phase U fractionation was characterized by a sequential extraction procedure, U remobilization potential by a 137days column experiment. Shallow Quaternary sediments yielded low U contents <1μgg−1, higher values were found in depths of more than 20m below ground surface. Here, strata of elevated Corg and Cinorg contain up to 14μgg−1 U, mainly bound in organic and carbonate fractions. Groundwater U concentrations >10μgL−1 almost exclusively appear in this same depth range, and only in wells influenced by water treatment runoff. Results suggest that the applied water treatment approach triggers U remobilization from geogenic sources in the aquifer. The most probable mechanism is dissolution of U bearing calcite induced by CO2 application; redox reactions and pH-driven desorption appear to play a minor role in mobilization. We conclude that groundwater treatment should carefully account for unwanted hydrogeochemical side effects triggering the mobilization of geogenic trace elements such as uranium.

Surface and bacterial reduction of nitrate at alkaline pH: Conditions comparable to a nuclear waste repository

This study investigates the reactivity of nitrates in abiotic and biotic conditions at alkaline pH in the context of a repository for long-lived inter-mediate-level radioactive waste. The work, carried out under environmental conditions comparable to those prevailing in the repository: alkaline pH, no oxygen, solid materials (cementitious material, steel), aims to identify the by-products of the nitrate reduction and to evaluate reaction kinetics of these reactions with and without the presence of denitrifying microorganisms. This paper demonstrates that, even at the high pH characteristic of nuclear waste repositories, nitrate reduction may be a likely scenario, with biotic catalysis in the presence of microorganisms and surface catalysis in the presence of steel and/or corrosion products.

Nitrate reduction to nitrite, nitric oxide and ammonia by gut bacteria under physiological conditions.

The biological nitrogen cycle involves step-wise reduction of nitrogen oxides to ammonium salts and oxidation of ammonia back to nitrites and nitrates by plants and bacteria. Neither process has been thought to have relevance to mammalian physiology; however in recent years the salivary bacterial reduction of nitrate to nitrite has been recognized as an important metabolic conversion in humans. Several enteric bacteria have also shown the ability of catalytic reduction of nitrate to ammonia via nitrite during dissimilatory respiration; however, the importance of this pathway in bacterial species colonizing the human intestine has been little studied. We measured nitrite, nitric oxide (NO) and ammonia formation in cultures of Escherichia coli, Lactobacillus and Bifidobacterium species grown at different sodium nitrate concentrations and oxygen levels. We found that the presence of 5 mM nitrate provided a growth benefit and induced both nitrite and ammonia generation in E.coli and L.plantarum bacteria grown at oxygen concentrations compatible with the content in the gastrointestinal tract. Nitrite and ammonia accumulated in the growth medium when at least 2.5 mM nitrate was present. Time-course curves suggest that nitrate is first converted to nitrite and subsequently to ammonia. Strains of L.rhamnosus, L.acidophilus and B.longum infantis grown with nitrate produced minor changes in nitrite or ammonia levels in the cultures. However, when supplied with exogenous nitrite, NO gas was readily produced independently of added nitrate. Bacterial production of lactic acid causes medium acidification that in turn generates NO by non-enzymatic nitrite reduction. In contrast, nitrite was converted to NO by E.coli cultures even at neutral pH. We suggest that the bacterial nitrate reduction to ammonia, as well as the related NO formation in the gut, could be an important aspect of the overall mammalian nitrate/nitrite/NO metabolism and is yet another way in which the microbiome links diet and health.

Synchrotron-Based Chemical Nano-Tomography of Microbial Cell-Mineral Aggregates in their Natural, Hydrated State

Chemical nano-tomography of microbial cells in their natural, hydrated state provides direct evidence of metabolic and chemical processes. Cells of the nitrate-reducing Acidovorax sp. strain BoFeN1 were cultured in the presence of ferrous iron. Bacterial reduction of nitrate causes precipitation of Fe(III)-(oxyhydr)oxides in the periplasm and in direct vicinity of the cells. Nanoliter aliquots of cell-suspension were injected into custom-designed sample holders wherein polyimide membranes collapse around the cells by capillary forces. The immobilized, hydrated cells were analyzed by synchrotron-based scanning transmission X-ray microscopy in combination with angle-scan tomography. This approach provides three-dimensional (3D) maps of the chemical species in the sample by employing their intrinsic near-edge X-ray absorption properties. The cells were scanned through the focus of a monochromatic soft X-ray beam at different, chemically specific X-ray energies to acquire projection images of their corresponding X-ray absorbance. Based on these images, chemical composition maps were then calculated. Acquiring projections at different tilt angles allowed for 3D reconstruction of the chemical composition. Our approach allows for 3D chemical mapping of hydrated samples and thus provides direct evidence for the localization of metabolic and chemical processes in situ.

Finite-element model simulation of nitrate transport behaviour in saturated fractured porous media

Abstract A finite-element reactive transport model is developed and applied to study the transport behaviour of nitrate under the effect of denitrification in saturated fractured media. The reduction of nitrate is considered to be undertaken by a heterotrophic population of bacteria present on the fracture walls and in the pore space of the porous matrix. The bacterial metabolic activity is controlled by the availability of an electron donor (organic matter) and electron acceptors (nitrate and oxygen). Model equations are developed assuming 1D advective–dispersive transport in a set of parallel fractures with 2D diffusive transport in the adjacent porous matrix. The bacterial reaction in these equations is represented by dual Monod kinetic terms and the bacterial growth is represented by a single linear kinetic equation. The equations are discretized by the finite-element Galerkin method and the resultant set of non-linear algebraic equations is solved by an iterative scheme. The model is used to explore the effect of denitrification on fracture and porous matrix nitrate concentration profiles for different values of fracture aperture size, advective velocity, porous matrix width, porosity, and dispersion and diffusion coefficients. The effect of the presence of oxygen in limiting rates of bacterial nitrate reduction is also assessed. Overall, it is concluded that in fractured media with good porous matrix diffusion properties and favourable properties for bacterial growth, denitrification in the porous matrix can be more significant than in the fracture set in the reduction of nitrate mass.

Enhanced xanthine oxidoreductase expression and tissue nitrate reduction in germ free mice

The nitrate–nitrite–NO pathway is emerging as an alternative to the l-arginine/NO-synthase pathway for the generation of NO in mammals. Bioactivation of the stable nitrate anion involves initial reduction to nitrite by commensal bacteria in the gastrointestinal tract. Nitrite is then further metabolized in blood and tissues to form nitric oxide (NO) and other bioactive nitrogen oxides. In addition to nitrate reduction by bacteria, a functional mammalian nitrate reductase activity was recently explored. It was demonstrated that xanthine oxidoreductase (XOR) and possibly other enzymes can catalyze nitrate reduction under normoxic conditions in vivo. In the present study, we compared nitrate reduction in germ free (GF) and conventional mice. One aim was to see if the complete lack of bacterial nitrate reduction in the GF mice would be associated with an upregulation of mammalian nitrate reductase activity. Sodium nitrate (NaNO 3) or placebo (NaCl) was injected intraperitoneally and blood and tissues were collected 1.5–2 h later for measurements of nitrate and nitrite and in some cases analyses of protein expression. Tissue and plasma levels of nitrate increased to a similar extent in conventional and GF animals after nitrate administration. Plasma nitrite was 3-fold higher in GF mice receiving nitrate compared to placebo while this effect of nitrate was absent in the conventional mice. In GF mice pretreated with the xanthine oxidase inhibitor allopurinol the increase in nitrite was attenuated. The levels of nitrite in the liver and small intestine increased after the nitrate load in GF mice but not in the conventional mice. Anaerobic nitrate reduction to nitrite in intestinal tissue homogenates was also accelerated in GF mice. Studies of tissue protein levels revealed increased expression of XOR in the livers of GF animals. We conclude that XOR expression in tissues is enhanced in germ free mice and this may explain the apparently greater tissue nitrate reductase activity observed in these animals. Future studies will reveal if this represents a compensatory functional response to uphold nitrite homeostasis in the absence of commensal bacteria.

Oral bacteria – The missing link to ambiguous findings of exhaled nitrogen oxides in cystic fibrosis

Nitrite in exhaled breath condensate (EBC) has been shown to be elevated in cystic fibrosis (CF), while exhaled nitric oxide (FENO) is paradoxically low. This has been argued to reflect increased metabolism of NO while its diffusion is obstructed by mucus. However, we wanted to study the possible influence of salivary nitrite and bacterial nitrate reduction on these parameters in CF patients by the intervention of an anti-bacterial mouthwash. EBC and saliva were collected from 15 CF patients (10-43 years) and 15 controls (9-44 years) before and 5 min after a 30s chlorhexidine mouthwash, in parallel with measurements of FENO. Nitrite and nitrate concentrations were measured fluorometrically. EBC nitrite, but not nitrate, was significantly higher in the CF patients (median 3.6 vs 1.3 microM in controls, p<0.05) and decreased after mouthwash in both groups (3.6-1.4 microM, p<0.01; 1.3-0.5 microM, p<0.01). Salivary nitrite correlated significantly to EBC nitrite (r=0.60, p<0.001) and decreased correspondingly after chlorhexidine, whereas salivary nitrate increased. FENO was lower in CF and the difference between patients and controls was accentuated after mouthwash (5.4 vs 8.4 ppb in controls, p<0.05). EBC nitrite mainly originates in the pharyngo-oral tract and its increase in CF is possibly explained by a regional change in bacterial activity. The limited lower airway contribution supports the view of a genuinely impaired formation and metabolism of NO in CF, rather than poor diffusion of the molecule.

Effect of estuarine sediment resuspension on early diagenesis, sulfide oxidation and dissolved molybdenum and uranium distribution in the Gironde estuary, France

Vertical distributions of dissolved major redox parameters, Mo and U from the water to the sediment were sampled in the salinity gradient of the Gironde estuary. In the water, sulfate and Mo are conservatively transported throughout the salinity gradient. In contrast, U shows a significant removal rate of 85 μmol m − 2 a − 1 related to reductive sequestration of U in the anoxic sediment. Early diagenesis is highly transient and characterized by a three-zone regime related to three density layers functioning at different timescales. Excess of sulfate (up to 1 mM) is observed in the soft mud layer and is assigned to sulfide oxidation by both abiotic Mn- and Fe-oxide reduction and bacterial nitrate reduction. During early diagenesis, Mo and U are released in the sediment porewater from Mn- and Fe-oxides. In not-dredged anoxic sediments, authigenic precipitation of Mo and U with iron sulfides is proposed to explain their removal from solution. In contrast, in dredged sediments, authigenic precipitation of U is inhibited probably owing to the less effective reduction of soluble U(VI) to insoluble U(IV) due to oxidizing transient conditions. Mixing of anoxic sediment porewater with oxygenated water, due to (i) cyclical resuspension of the soft mud layer and (ii) dredging operations in the navigation channel, induces addition of dissolved sulfate (produced from diagenetic sulfide oxidation), Mo and U to the water. This addition is equivalent to 2% (sulfate), 5.5% (Mo) and 0.5% (U) of the dissolved fluvial inputs into the estuary for the natural resuspension, and 0.4% (Mo), less than 0.2% (sulfate, U) for dredge-induced resuspension. Addition of dissolved Mo and U, produced by oxidation of sulfide phases in the dredged sediments redeposited in dynamic environments, are equivalent to 12% (Mo) and 0.8% (U) of their respective fluvial inputs. Additional Mo and U inputs into the water originate from diffusive outflow from muddy sediments and are equivalent to 2% (Mo) and 0.3% (U) of their respective fluvial inputs. However, we show that all these dissolved inputs are scavenged by solid phases.

Southern Ocean control on the extent of denitrification in the southeast Pacific over the last 70 ka

Temporal changes in oceanic denitrification, the bacterial reduction of nitrate under suboxic conditions, highlight the potential importance of N inventory changes and the production of N 2O on the climate system. At the same time, the cause of the globally observed variation in denitrification remains unclear. High-resolution benthic foraminiferal oxygen isotope and bulk sediment nitrogen isotope records from ODP Site 1234 on the Chile Margin record integrated denitrification changes within the Peru–Chile Upwelling system over the last ∼70 ka. Denitrification changes in the southeast Pacific are coherent with Antarctic climate changes recorded by the Byrd ice core δ 18O record, and lead northern hemisphere climate events. The southern-hemisphere character of the Chile margin δ 15N record suggests that episodes of reduced denitrification in the SE Pacific represent times when more oxygen was supplied as the result of changes in the ventilation and preformed nutrient content of Subantarctic Mode Water (SAMW), which forms in the Subantarctic zone of the Southern Ocean and feeds into the low-latitude thermocline.

Evaluation of bacterial nitrate reduction in the human oral cavity

It is hypothesized that the enterosalivary nitrate circulation encourages nitrate reducing bacteria to reside within the oral cavity. Nitrite production may then limit the growth of acidogenic bacteria as a result of the production of antimicrobial oxides of nitrogen, including nitric oxide. This study was carried out with 10 subjects to characterize oral nitrate reduction and identify the bacteria responsible. Nitrate reduction varied between individuals (mean 85.4 +/- 15.9 nmol nitrite min(-1) with 10 ml 1 mm KNO(3) mouth wash) and was found to be concentrated at the rear of the tongue dorsal surface. Nitrate reductase positive isolates identified, using 16S rDNA sequencing, from the tongue comprised Veillonella atypica (34%), Veillonella dispar (24%), Actinomyces odontolyticus (21%), Actinomyces naeslundii (2%), Rothia mucilaginosa (10%), Rothia dentocariosa (3%) and Staphylococcus epidermidis (5%). Nitrite production rates, using intact and permeabilized cells, of the major tongue nitrate reducers were determined in the presence of methyl and benzyl viologen. Under anaerobic conditions in the presence of nitrate, rates in decreasing order were: A. odontolyticus > R. mucilaginosa > R. dentocariosa > V. dispar > V. atypica. In conclusion, Veillonella spp. were found to be the most prevalent taxa isolated and thus may make a major contribution to nitrate reduction in the oral cavity.

N2 production by the anammox reaction in the anoxic water column of Golfo Dulce, Costa Rica.

In oxygen-depleted zones of the open ocean, and in anoxic basins and fjords, denitrification (the bacterial reduction of nitrate to give N2) is recognized as the only significant process converting fixed nitrogen to gaseous N2. Primary production in the oceans is often limited by the availability of fixed nitrogen such as ammonium or nitrate, and nitrogen-removal processes consequently affect both ecosystem function and global biogeochemical cycles. It was recently discovered that the anaerobic oxidation of ammonium with nitrite--the 'anammox' reaction, performed by bacteria--was responsible for a significant fraction of N2 production in some marine sediments. Here we show that this reaction is also important in the anoxic waters of Golfo Dulce, a 200-m-deep coastal bay in Costa Rica, where it accounts for 19-35% of the total N2 formation in the water column. The water-column chemistry in Golfo Dulce is very similar to that in oxygen-depleted zones of the oceans--in which one-half to one-third of the global nitrogen removal is believed to occur. We therefore expect the anammox reaction to be a globally significant sink for oceanic nitrogen.

Amperometric Detection with Microelectrodes in FIA: Studies on the Conversion of Nitrate to Nitrite in Human Saliva

A flow injection method for the determination of nitrate and nitrate in human saliva is described. It is based on the reaction of nitrite with iodide in acidic medium, the triiodide formed being amperometrically monitored at +0.2 V with a platinum microelectrode (r = 5μm) in a wall-jet cell. The method is very sensitive owing to the association of hydrodynamic transport in a flowing solution and the high mass transport rate at a microelectrode, the limit of detection for nitrite and nitrate being 0.07 and 0.25 μM respectively. The sample throughput was 60 h−1 and the standard deviation for 5 injections of 1 μM nitrite was 1.9% (1.3% for nitrate). The method was used in the study of the conversion of nitrate to nitrite in the oral cavity by bacteria, experimental data concerning the salivary concentration of both ions being obtained before and after treatment with an antiseptic solution. The results showed the decline of nitrite concentration after the oral treatment followed by a proportional increase in the nitrate concentration, proving that the majority on salivary nitrite comes from bacterial reduction of nitrate.

Nitric Oxide Is Generated on the Skin Surface by Reduction of Sweat Nitrate

Nitric oxide (NO) is known to be synthesized by mammalian cells from L-arginine by a group of NO synthase enzymes. We now show that NO is generated from human skin and propose a different mechanism of production. Whereas enzymatic NO synthesis is inhibited by monomethyl L-arginine, this arginine analog, when infused into the brachial artery at concentrations sufficient to inhibit endothelial NO synthase activity, has little effect on hand skin NO production. Hand skin NO production is increased by topical acidification of the skin surface and greatly increased by the addition of nitrite solutions. We propose that NO generation from skin derives from sweat nitrite (the concentration of which was found to average 3.4 microM in six subjects) due to chemical reduction consequent to the acidic nature of sweat. Sweat contains nitrate in appreciable amounts, and skin commensal bacteria can synthesize nitrate reductase enzyme. Patients on long-term tetracycline antibiotics showed significantly reduced skin NO synthesis, although topical antiseptic and antibiotics had little effect on NO generation in the short-term. We propose that NO generation from skin is dependent on bacterial nitrate reduction to nitrite and subsequent reduction by acidification. We speculate that this has a physiologic role in the inhibition of infection by pathogenic fungi and other susceptible microorganisms and may affect cutaneous T-cell function, keratinocyte differentiation, and skin blood flow.

Diurnal variation of denitrification and nitrification in sediments colonized by benthic microphytes

Diurnal variation of denitrification in sediments with benthic microphytes was investigated by a 15N isotope-pairing technique in order to distinguish between coupled nitrification-denitrification and denitrification of nitrate supplied from the water column. Sediments were incubated in a continuous flowthrough system and exposed to diurnal light and dark cycles. Illumination of both limnetic and estuarine sediments doubled the rate of coupled nitrification-denitrification but reduced the rate of denitrification of nitrate supplied from the water column by -50%. Photosynthesis in the uppermost sediment layers during illumination led to deeper oxygen penetration and a resultant stimulation of nitrification. This stimulation may explain the increased rate of coupled nitrification-denitrification during illumination. The rate of denitrification of nitrate supplied from the water column was reduced during illumination due to a longer diffusion path between water and the anoxic denitrification zone. Denitrification, the bacterial reduction of nitrate to nitrogen gas via nitrite, is an important nitrogen sink in aquatic environments. There are two main sources of nitrate for sediment denitrification: nitrate diffusing into the sediment from the water column and nitrate produced by nitrification in the sediment (Jenkins and Kemp 1984; Seitzinger 1988). The processes of nitrification and denitrification are usually vertically separated within the sediment (Vanderborght and Billen 1975). Nitrification is strictly aerobic and therefore restricted to a thin oxic zone in the upper few millimeters of the sediment. Denitrification takes place in anoxic environments, and in many sediments, most activity is found just below the oxic-anoxic interface (Sweerts and de Beer 1989). The rates of nitrification and denitrification in the active microenvironments are controlled by the availability of substrates, which to a large extent are supplied by diffusion along

Inhibition of nitrosamine formation by ascorbic acid

Nitrosation occurs under a wide variety of conditions by reaction of most types of amines with any of a large number of nitrosating species. Nitrite can be formed in vivo via bacterial reduction of nitrate and by activated macrophages and endothelial cells. The mechanism of nitrite formation by mammalian cells is via enzymatic oxidation of arginine to NO followed by oxidation to N2O3 and N2O4. Nitrosatable amines are found in many foods and some, eg, dimethylamine, are synthesized in the body. Precursors of N-nitroso compounds are thus almost constantly present together under favorable reaction conditions in vivo and there is, consequently, considerable interest concerning possible human health risks arising from endogenous formation of this class of compounds. Among many nitrosation inhibitors, most attention has focused on ascorbic acid, which reacts with many nitrosating agents and which is virtually nontoxic. This presentation discusses the chemistry of ascorbic acid inhibition of nitrosation reactions.

Subsurface ammonium maxima in Northern Bay of Bengal

Chemical oceanographic data collected on RV Gaveshani cruises 181 and 182 in the Bay of Bengal during March–April, 1987 showed subsurface ammonium maxima in the water column in northern Bay of Bengal. While there is no regular trend either in horizontal or in vertical distribution of ammonium in the central Bay (Lat. 12–14°N), two prominent maxima, one in the euphotic zone near the oxycline (75–150 m) and the other in the oxygen deficient zone (400–600 m) are observed in the vertical distribution of ammonium in the northern Bay of Bengal (Lat. 18–20°N). The subsurface maximum (2·0–3·5 μmol dm −3) in the euphotic zone is attributed to the regeneration of ammonium by zooplankton excretion associated with the stratification of water column during summer (spring). The spatial distribution of recycled nitrogen (ammonium) along the transects showed an increasing trend of regenerated nitrogen from inshore to offshore regions in the northern Bay. The second ammonium maximum (4·0–6·0 μmol dm −3) observed in oxygen deficient region (< 50 μmol dm −3) is assumed to be due to the bacterial reduction of nitrate to ammonium in the denitrification process. Nitrate deficits ( ΔN = 9–12 μmol dm −3) derived from material balance calculations are in close agreement with those reported earlier. Nitrogen transformations in the water column are schematically represented by distinguishing three zones of biological activity on the basis of oxygen concentration levels.