Continuous Flow-through System Research Articles

Permeation of sumatriptan through human vaginal and buccal mucosa.

Continued interest in the various routes by which sumatriptan may be administered prompted us to investigate its passage through buccal mucosa. Because human buccal mucosa is scarce, we proposed using the relatively abundant vaginal mucosa, which has been shown to have comparable diffusion rates for a number of widely varying molecules, as a model of buccal mucosa. In addition, by comparing these two tissues with respect to their permeability to sumatriptan, the human vaginal/buccal mucosa model could be further evaluated. Clinically healthy human vaginal and buccal mucosa specimens were used in the permeability studies. Permeability to sumatriptan was determined using a continuous flow-through diffusion system in the presence and absence of permeation enhancers. No statistically significant differences in permeability could be demonstrated for both mucosae toward sumatriptan. Flux values obtained in the absence and presence of glycodeoxycholate and lauric acid (1:1 molar ratio) to sumatriptan of buccal and vaginal mucosa, respectively, were not significantly different. The results obtained further support the hypothesis of the vaginal/buccal mucosal in vitro permeability model and suggest that this model may be used in conjunction with various absorption enhancers. Further studies on the buccal route of absorption of sumatriptan are thus warranted.

Tuning the Performance of Room Temperature Phosphorescence Sensing Materials for Oxygen Using Factorial Designs

ABSTRACT The effects of three experimental factors (pH, precursors, alcohol) on the sensing characteristics of these materials were screened by means of two-level factorial designs. The resulting materials turned out to be useful as luminescent probes for the measurement of dissolved and gaseous oxygen. The photochemical properties and the analytical performance of the RTP sensing phases have been studied by using both gas flow-injection analysis and continuous liquid flow-through systems. The proposed sensing materials were particularly suitable for measuring dissolved oxygen in natural waters. The detection limit attained was 0.004 mg.ml−1 and a typical precision of ± 1.0% al a 0.6 mg.ml−1 oxygen level was achieved. Response time for 90% of the final RTP value was less than 90s in a continuous flow mode. No hysteresis effects were noticed.

Effect of kinetics of complexation by humic acid on toxicity of copper to Ceriodaphnia dubia

AbstractThe rate of reaction of trace metal ions is an important consideration when studying the chemistry of trace metals in natural waters. The application of speciation models to natural water systems requires knowledge of kinetics if reactions are slow. Most bioassay and toxicity tests conducted in static and flow‐through systems have not taken reaction kinetics into account. Therefore, results from these studies may overestimate the toxicity in the receiving waters. In the present study, the kinetics of the interaction of Cu(II) with humic acid (HA) and its influence on the toxicity of copper to Ceriodaphnia dubia were investigated by both chemical kinetic studies using a copper ion selective electrode and bioassay tests using a continuous flow‐through bioassay system. A two‐ligand site with first‐order rate constants model gave a very good description of experimental kinetic data of the change of free Cu2+ concentration. Average k1 was 1.85/h and average k2 was 0.094/h. Bioassay tests indicated that different reaction times of copper with HA solution produced different toxic effects to organisms. We determined the hydrodynamic characteristics of the bioassay chambers to better describe the exposure of the organisms to free Cu2+. The bioassays supported the free ion activity model that the bioavailability and therefore toxicity of copper was directly correlated to the free Cu2+ concentration rather than to the total copper concentration. It was further shown that conventional chemical kinetics can be used to predict the toxicity of copper in these bioassays. This study supports the importance of considering reaction kinetics when studying the chemistry of trace metals in natural waters.

A recirculating flow-through system for toxicity testing with stream-dwelling aquatic benthic invertebrates

The development and use of standardized toxicity test systems and methods applicable to stream dwelling benthic invertebrates have not received the same attention as test methods developed for still water and sediment associated invertebrates. As such, basic toxicological information concerning lotic organisms is generally lacking in comparison with lentic invertebrates. The requirement of flowing water in test systems with stream invertebrates has presented technical challenges uncommon to systems using still water or sediment organisms. To alleviate this problem, a new toxicity test system was developed for stream invertebrates. The new system, referred to as the `Trent tube', is small in size (17 cm×30 cm×9 cm deep), easily constructed from commonly available materials, relatively inexpensive to build and adaptable to many taxonomic orders of benthic invertebrates. These factors help to eliminate some of the technical challenges commonly associated with flowing water test systems and thereby increase the ease of testing and test replication. The standard design of the Trent tube is operated as a static renewal system with a pooled grouping of organisms, but is easily modified for use as a continuous flow-through system that can monitor the responses of individuals. A feature unique to the Trent tube is the ability to monitor multiple responses from individual organisms. When operated as a static renewal system, the Trent tube was successfully used to monitor the responses of individual Hydropsyche larvae during a 7 week test period. This system is easily adaptable to a variety of desired test end points. The basic design of the Trent tube system presents a model from which standard toxicity test methods could be developed.

Determination of ascorbic acid by use of a flow-through solid phase UV spectrophotometric system

A continuous flow-through solid phase spectrophotometric system was developed for the determination of ascorbic acid based on the measurement of its intrinsic absorbance in the UV region when retained on a 1 mm Sephadex QAE A-25 anion exchanger gel layer which is placed into an appropriate quartz flow-through cell, the absorbance exhibited by this solid phase being monitored at 267 nm. A monochannel manifold was used, the sample (300, 600 or 1000 μL) being injected into the carrier solution (acetate buffer). This solution also elutes the analyte after developing the analytical signal, and regenerates the resin layer which, therefore, remains ready for the next sample. The linear dynamic range and other analytical parameters vary according to the sample volume injected. Three calibration lines were established for 300, 600 and 1000 μL sample volume, which ranged from 1.0 to 20.0, 0.5 to 10.0 and 0.2 to 6.0 μg mL–1, respectively. The detection limits were 0.04 (300 μL), 0.03 (600 μL) and 0.02 μg mL–1 (1000 μL), the sampling rates 28, 24 and 21 h–1, and the RSDs (n = 10) 0.87%, 1.08% and 0.90%, respectively. The amount of ascorbic acid in various samples (pharmaceuticals, sweets and urine) were successfully determined with this method.

Simultaneous measurement of benthic denitrification, with the isotope pairing technique and the N 2 flux method in a continuous flow-through system

Denitrification rates in a freshwater sediment were measured simultaneously with the N 2 flux method and the 15N isotope pairing technique, using a continuous flow-through system designed for direct application of both techniques. With the N 2 flux method, denitrification was 151±4 μmol N m −2 h −1 and with the isotope pairing technique denitrification was 149±4 μmol N m −2 h −1. These identical results strongly suggested that both methods measured the true denitrification activity in the system. Using the conventional batch modes, the N 2 flux method has shown much larger denitrification rates than 15N isotope pairing. This discrepancy could be explained by the much longer incubation time and smaller water column, required by the N 2 flux method. Model simulations of a N 2 flux batch incubation showed how NO 3 − and NH 4 + accumulated in the water column and in turn increased nitrification and denitrification activity much above the initial level, as incubation proceeded.

Diffusion rates of vasopressin through human vaginal and buccal mucosa.

The permeability to several chemical compounds and the histology of vaginal and buccal mucosa are very similar. Because vaginal mucosa is more abundant, it may be used as a model for the latter. To further develop the vaginal/buccal mucosa model, the objective of the present study was to evaluate the passage of a small polypeptide, vasopressin, across fresh and frozen specimens of these two mucosae. Specimens of fresh buccal and vaginal mucosa were taken from excised tissue obtained following vaginal hysterectomies and various oral surgical procedures. Pieces of buccal and vaginal tissue specimens obtained were used fresh or were snap-frozen and stored at -85 degrees C for periods of up to 10 months. Biopsies from fresh and thawed specimens were mounted in flow-through diffusion cells and their permeability to tritiated vasopressin was determined using a continuous flow-through perfusion system. Specimens were examined histologically before and after freezing as well as before and after permeability experiments and similarities between vaginal and buccal tissues verified. No statistically significant differences between flux values for fresh and frozen vaginal and buccal mucosa, respectively, were found. These results demonstrate that the permeation of vasopressin across fresh and frozen human vaginal and buccal mucosa is for practical purposes similar. These results further support the human vaginal/buccal mucosa model for in vitro permeability studies on therapeutically active compounds.

In vivo inhibition of AChE activity in the European eel Anguilla anguilla exposed to technical grade fenitrothion

European eel ( Anguilla anguilla) were exposed to sublethal fenitrothion concentrations in a continuous flow-through system for 4 days. Plasma acetylcholinesterase (AChE) activity was evaluated after 2, 8, 12, 24, 32, 48, 56, 72 and 96 h pesticide exposure. AChE activity in the plasma of the eel decreased as concentration of fenitrothion increased. Pesticide induced significant inhibitory effects on the AChE activity of A. anguilla ranging from 51% inhibition at sublethal concentration of 0.02 ppm to 57% inhibition at sublethal concentration of 0.04 ppm. Eel were exposed to both fenitrothion concentrations for 96 h and then allowed a period of recovery in pesticide-free water. Following 1 week of recovery, the AChE activity for those animals previously exposed to fenitrothion was still different from the controls. Animals transferred to clean water showed plasma AChE activities reduced in a 34 and 51% when previously exposed to 0.02 and 0.04 ppm pesticide, respectively. This is probably due because regeneration of the enzyme is mainly by the novo synthesis after exposure to organophosphates and levels need a large extent to recover.

Penetration of human vaginal and buccal mucosa by 4.4-kd and 12-kd fluorescein-isothiocyanate-labeled dextrans

In a previous study we demonstrated that human vaginal mucosa was as permeable to water as was buccal mucosa. Water, however, is a very small molecule with a molecular weight of 18 d. To further explore similarities between these two types of mucosa with respect to permeability, it was decided to investigate the passage of two large, hydrophilic molecules across these epithelia. Specimens of fresh, clinically healthy human vaginal and buccal mucosa were taken from excised tissue obtained during vaginal hysterectomies and various oral surgical procedures. Seven biopsy materials from each specimen were mounted in flow-through diffusion cells (exposed area, 0.039 cm 2), and their permeability to 4.4- and 12-kd fluorescein-isothiocyanate-labeled dextrans was determined through use of a continuous flow-through perfusion system. Dextran was detected by means of a fluo-rospectrophotometric method at excitation and emission wave lengths of 498 and 520 nm, respectively. Specimens were examined histologically before and after permeability experiments, and similarities between vaginal and buccal tissues were verified. No statistically significant differences between the flux values of the 4.4-kd dextran across vaginal and buccal mucosa were found. However, for the 12-kd dextran the flux rate across buccal mucosa was significantly higher than the rate across vaginal mucosa. These results demonstrate that human vaginal mucosa is for practical purposes as permeable as buccal mucosa to 4.4-kd hydrophilic molecules. This further supports the hypothesis that vaginal mucosa may be a useful model for studying the passage across buccal mucosa of chemical compounds and therapeutic agents that are less than approximately 4.4 kd in molecular mass. For a 12-kd dextran the flux rate across buccal mucosa is significantly higher than the flux rate across vaginal mucosa, and the model becomes inaccurate.

Permeation of 17β-estradiol through human vaginal and buccal mucosa

Because of the relative scarcity of fresh human oral mucosa specimens for permeability studies, we investigated the use of human vaginal mucosa as a model of the former. In a previous study we demonstrated the comparable diffusion rate of water across human vaginal and buccal mucosa and proposed the use of the former as a suitable model of the latter for in vitro drug permeability studies. To further evaluate the human vaginal/buccal mucosa model, we decided to compare these two tissues with respect to their permeability to 17β-estradiol. Clinically healthy human vaginal and buccal mucosa specimens were obtained during vaginal hysterectomies and different oral surgical procedures. The permeability of each tissue specimen to 17β-estradiol was determined through the use of a continuous flow-through diffusion system. Specimens were examined histologically before and after experiments. Mean flux values for 17β-estradiol across human buccal mucosa tended to be slightly higher than those observed for vaginal tissue, but no statistically significant differences could be demonstrated. The results from this study further support our hypothesis that human vaginal mucosa may be a suitable model of human buccal mucosa for in vitro drug permeability studies.

Emission of N2O, nitrification and denitrification in a eutrophic lake sediment bioturbated by Chironomus plumosus

AME Aquatic Microbial Ecology Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsSpecials AME 14:289-299 (1998) - doi:10.3354/ame014289 Emission of N2O, nitrification and denitrification in a eutrophic lake sediment bioturbated by Chironomus plumosus Jonas M. Svensson* Department of Ecology/Limnology, Ecology Building, University of Lund, S-223 62 Lund, Sweden *E-mail: jonas.svensson@limnol.lu.se Emission of nitrous oxide and rates of nitrification and denitrification were determined in sediment from a eutrophic lake in southern Sweden. Effects of bioturbation on nitrogen transformation were investigated by incubating sediment at 10 and 15°C in a continuous flow-through system containing ca 2000 tube-dwelling larvae of Chironomus plumosus L. (Chironomidae, Diptera) m-2. N2O emission was found to be independent of nitrate concentration (33 to 267 μM) in the overlying water at both 10 and 15°C. It is shown that C. plumosus larvae significantly enhanced the emission of N2O from the sediment at 15°C, the highest emission recorded under these conditions was 0.45 ± 0.2 μmol N m-2 h-1. Since released N2O was proportional to the rate of nitrification, it is suggested to originate mainly from this process. Bioturbation stimulated the rate of nitrification by a factor of 1.8 at 15°C. However, no enhancement of nitrification by chironomids was observed at 10°C. In contrast to N2O emission and nitrification, denitrification in both bioturbated and non-bioturbated cores was significantly correlated to the concentration of nitrate in the overlying water. The stimulation of denitrification by bioturbation at 10 and 15°C was 3.5- and 4.3-fold respectively. Denitrification rates were mainly accounted for (98%) by changes in the concentration of nitrate in the water, the water temperature and the biomass of C. plumosus larvae. C. plumosus larvae were found to decrease the N2O:N2 ratio at both temperatures. This was explained by the water pumping activity of the animals, mobilizing released N2O to denitrifiers in or close to the animal burrows. The ratio of N2O:N2 did not exceed 0.5% in any of the treatments. Nitrous oxide · Nitrification · Denitrification · Lake · Sediment · Bioturbation · Chironomus plumosus Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in AME Vol. 14, No. 3. Publication date: April 06, 1998 Print ISSN: 0948-3055; Online ISSN: 1616-1564 Copyright © 1998 Inter-Research.

屋外実験池を用いたラン藻類優占に及ぼす魚の影響に関する研究

Experiments using six outdoor experimental ponds equipped with a continuous flow-through system of the same influx nitrogen and phosphorus concentrations and with different fish (goldfish) densities were conducted from Aug., 1993 to Sep., 1994 to investigate the factors influencing the dominance of blue-green algae in summer. In general, blue-green algae predominated in fish ponds, in contrast to green algae in fishless ponds. Blue-green algae started to flourish after the latter half of August, when the PO4-P concentrations decreased to less than 10 μg·l-1. The difference in dominated phytoplankton between fish and fishless ponds could be explained by the reduction of total phosphorus and subsequently the depletion of PO4-P in pond water due to the feces of fish and their growth, the condition of which is advantageous for blue-green algae than others.

Response and Recovery of Brain Acetylcholinesterase Activity in the European Eel,Anguilla anguilla,Exposed to Fenitrothion

European eel (Anguilla anguilla) were exposed to sublethal fenitrothion concentrations in a continuous flow-through system for 4 days. Brain acetylcholinesterase (AChE) activity was evaluated after 2, 8, 12, 24, 32, 48, 56, 72, and 96 h pesticide exposure. Results indicated that AChE activity in eel brains decreased as the concentration of fenitrothion increased. The pesticide induced significant inhibitory effects on the AChE activity ofA. anguilla,ranging from >40% inhibition at a sublethal concentration of 0.02 ppm to >60% inhibition at a sublethal concentration of 0.04 ppm. Eel were exposed to both fenitrothion concentrations for 96 h and then allowed a period of recovery in pesticide-free water. Samples were removed at 8, 12, 24, 48, 72, 96, 144, and 192 h and eel brain AChE activity was evaluated. Following 1 week of recovery, the AChE activity of those animals previously exposed to 0.02 and 0.04 ppm fenitrothion was still different from that of the controls. So, the AChE activity of eel brains at the end of the recovery phase remained significantly depressed.

Inhibition of Gill Na+,K+-ATPase Activity in the Eel,Anguilla anguilla,by Fenitrothion

European eels (Anguilla anguilla) were exposed to sublethal fenitrothion concentrations (0.02 and 0.04 mg/liter) in a continuous flow-through system for 4 days. Gill Mg2+- and Na+,K+-ATPase activities were evaluated after 2, 8, 12, 24, 32, 48, 56, 72, and 96 h of pesticide exposure. Results indicated that ATPase activity in gill tissue decreased as concentration of fenitrothion increased. Pesticide induced significant inhibitory effects on the Na+,K+-ATPase activity ofA. anguilla,ranging from >56% inhibition at a sublethal concentration of 0.02 ppm to >73% inhibition at a sublethal concentration of 0.04 ppm. Eels were exposed to both fenitrothion concentrations for 96 h and then allowed a period of recovery in pesticide-free water. Samples were removed at 8, 12, 24, 48, 72, 96, 144, and 192 h and eel gill ATPase activity was evaluated. Following 1 week of recovery, the Na+,K+-ATPase activity for those animals previously exposed to 0.04 ppm fenitrothion was still different from that of the controls.

Using Ultra-high Carbon Dioxide Levels Enhances Plantlet Growth In Vitro

A continuous CO2 flow system was used to study the growth of carrot (Daucus carota L.), citrus (Citrus macrophylla L.), kale (Brassica oleracea L.), lettuce (Lactuca sativa L.), radish (Raphanus sativus L.), and tomato (Lycopersicum esculentum L.) cultures in vitro under photoautotrophic, photomixotrophic, and heterotrophic conditions. Lettuce plantlets were grown on Murashige and Skoog medium with 0%, 0.3%, 1%, and 3% sucrose within flow chambers containing 350, 750, 1500, 3000, 10,000, 30,000, and 50,000 μL·L−1 CO2. Increasing the levels of CO2, especially at the ultra-high levels (i.e., ≥3,000 μL·L−1 CO2), increased fresh weight, shoot length, leaf number, leaf length, leaf width, root number, and root length for plantlets grown regardless of sucrose levels tested compared to plantlets grown at normal atmospheric CO2 levels, i.e., 350 μL·L-1. For example, fresh weights of lettuce plantlets grown on medium containing 0% or 3% sucrose increased 11- and 13-fold, respectively, when supplemented with 30,000 μL·L-1 CO2 compared to growth of lettuce plantlets grown on the same media without CO2 enrichment. Similar fold increases in growth responses were obtained with carrot, citrus, kale, radish, and tomato plantlets grown in atmospheres enriched with high CO2 levels, elevated from 3000 to 30,000 μL·L-1. Optimum CO2 concentration varied among species, suggesting a species-related response. Varying the rate of CO2 application between 250, 500, 1500, or 2000 mL·min-1 did not effect the rate of growth of lettuce plantlets. The passive diffusion continuous flow-through system presented in this paper is inexpensive, easily constructed, and allows for testing ultra-high CO2 levels on plant culture growth in vitro.

Nitrification and denitrification in the rhizosphere of the aquatic macrophyte Lobelia dortmanna L.

Nitrogen and O2 transformations were studied in sediments covered by Lobelia dortmanna L.; a combination of 15N isotope pairing and microsensor (O2, NO3−, and NH4+) techniques were used. Transformation rates and microprofiles were compared with data obtained in bare sediments. The two types of sediment were incubated in doublecompartment chambers connected to a continuous flow-through system. The presence of L. dortmanna profoundly influenced both the nitrification-denitrification activity and porewater profiles of O2, NO3−, and NH4+ within the sediment. The rate of coupled nitrification-denitrification was greater than sixfold higher in L. dortmnanna-vegetated sediment than in bare sediment throughout the light–dark cycle. Illumination of the Lobelia sediment reduced denitrification activity by ∼30%. In contrast, this process was unaffected by light–dark shifts in the bare sediment. Oxygen microprofiles showed that O2 was released from the L. dortmanna roots to the surrounding sediment both during illumination and in darkness. This release of O2 expanded the oxic sediment volume and stimulated nitrification, shown by the high concentrations of NO3− (∼30 µM) that accumulated within the rhizosphere. Both 15N2 isotope and microsensor data showed that the root-associated nitrification site was surrounded by two sites of denitrification above and below, and this led to a more efficient coupling between nitrification and denitrification in the Lobelia sediment than in the bare sediment.

Regulation of propylene-induced ripening and ethylene biosynthesis by oxygen in ‘Hayward’ kiwifruit

Abstract Kiwifruit, cv Hayward, were treated with 130 μl/l propylene under 1, 5, 10, 13, 16, and 21% O 2 concentrations in a continuous flow-through system at 20 °C. Control kiwifruit were kept in air (21%O 2 ). Flesh firmness, soluble solids content, internal ethylene concentration, l-aminocyclopropane-l-carboxylic acid concentration (ACC), malonyl-l-aminocyclopropane-l-carboxylic acid concentration (MACC), and EFE capacity were measured during the experiment. Ethylene biosynthesis and fruit ripening were stimulated when kiwifruit were exposed to a mixture of propylene in 21% O 2 . Reduction of oxygen concentration (

Continuous pollution monitoring using Photobacterium phosphoreum

Abstract Photobacterium phosphoreum is a marine bacterium which is used extensively as a bioluminescent indicator of pollutants, where the presence of toxicants diminishes light output. To evaluate the utility of cell immobilisation in continuous toxicity testing, the sensitivity of P. phosphoreum to five gelling agents was evaluated relative to the retention of bioluminescence in 3% NaCl-glycerol suspensions. Following storage at 4°C, the control cultures retained light output for up to 2 weeks before significant decline; alginate-glycerol suspensions were stable for up to 4 weeks and bioluminescence was detectable for up to 6 weeks. Cells stored in agar were no more stable than the control, whereas cells gelled in agarose and low-melting point agarose showed a significant decline in bioluminescence within 2 weeks of storage. Bioluminescence was totally retained in alginate-glycerol suspensions stored at −80°C for up to 12 weeks. P. phosphoreum was successfully immobilised in strontium alginate and showed a dose-related response to four of the five heavy metal ions, SDS and pentachlorophenol tested when responses were followed over a time-course. A flow-through system for Sr-alginate immobilised cells was developed and conditions for operation were optimised. When cells were exposed to a pulse of 4-nitrophenol or salicylate then the nutrient feed continued, bioluminescence declined in response (pulse of 4–6 min) to these pollutants then recovered to a new stable rate of decline which was faster than the pre-exposure rate. These results demonstrate the potential of using immobilised P. phosphoreum in a continuous flow-through system for real-time environmental monitoring of water quality.

A Flow-Injection System for Studying Heat Inactivation of Listeria monocytogenes and Salmonella enteritidis in Liquid Whole Egg

A flow-injection system was devised to mimic continuous flow-through pasteurization systems for laboratory thermal inactivation studies. Air bubbles were introduced into the sample stream to create separate moving segments (plugs) of liquid stream during pasteurization while residence time was adjusted by a combination of pump speed and column length. The method was used to obtain thermal inactivation data for Listeria monocytogenes Scott A and Salmonella enteritidis ATCC 13076 in liquid whole eggs at different temperatures and heating times. Thermal inactivation of L. monocytogenes using the capillary tube method (Zcap = 7.3°C) gave results comparable to those obtained with the flow-injection system (Zflow = 7.2°C). The flow-injection system also was used to examine thermal inactivation of S. enteritidis (SE) grown in either tryptic soy broth (TSB) or egg yolk medium (EYM) before inoculation into liquid whole egg (LWE). D-values were obtained by regression analysis and the data showed that SE grown in EYM gave D-values 15 to 120% higher than those obtained for SE grown in TSB. Thermal inactivation studies performed with S. enteritidis grown in commercial broth media may therefore inaccurately represent thermal resistance of S. enteritidis grown in liquid or shell raw egg as may occur in egg-associated outbreaks. The continuous flow-injection system described herein may be adapted to study continuous flow pasteurization processes not easily examined by the traditional capillary tube method.

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