Chloramphenicol inhibition of denitrifying enzyme activity in two agricultural soils

Comprehensive metagenomic and enzyme activity analysis reveals the inhibitory effects and potential toxic mechanism of tetracycline on denitrification in groundwater

The widespread use of tetracycline (TC) inevitably leads to its increasing emission into groundwater. However, the potential risks of TC to denitrification in groundwater remain unclear. In this study, the effects of TC on denitrification in groundwater were systematically investigated at both the protein and gene levels from the electron behavior aspect for the first time. The results showed that increasing TC from 0 to 10 µg·L-1 decreased the nitrate removal rate from 0.41 to 0.26 mg·L-1·h-1 while enhancing the residual nitrite concentration from 0.52 mg·L-1 to 50.60 mg·L-1 at the end of the experiment. From a macroscopic view, 10 µg·L-1 TC significantly inhibited microbial growth and altered microbial community structure and function in groundwater, which induced the degeneration of denitrification. From the electron behavior aspect (the electron production, electron transport and electron consumption processes), 10 µg·L-1 TC decreased the concentration of electron donors (nicotinamide adenine dinucleotide, NADH), electron transport system activity, and denitrifying enzyme activities at the protein level. At the gene level, 10 µg·L-1 TC restricted the replication of genes related to carbon metabolism, the electron transport system and denitrification. Moreover, discrepant inhibitory effects of TC on individual denitrification steps, which led to the accumulation of nitrite, were observed in this study. These results provide the information that is necessary for evaluating the potential environmental risk of antibiotics on groundwater denitrification and bring more attention to their effects on geochemical nitrogen cycles.

Permeability decides the effect of antibiotics on sedimentary nitrogen removal in Jiulong River Estuary

Sedimentary denitrification takes place beneath the oxic layer at the sediment-water interface, where nitrate and antibiotics need to diffuse through the overlying water. However, the antibiotics' effect on sedimentary N removal and associated N2O production has not been adequately investigated under in situ conditions. Here, isotope pairing techniques, including slurry incubations (potential) and intact core incubations (in situ), combined with metagenomic analysis were applied to investigate the impacts of two protein-inhibiting antibiotics (oxytetracycline and thiamphenicol) on sediment nitrogen removal in a subtropical estuary. Slurry incubations showed that the two antibiotics significantly inhibited denitrification (67-98%) and anammox (49-99%), while intact core incubations presented no antibiotic effect at upstream but significant inhibition (23%-52%) at downstream. Meanwhile, N2O yields were stimulated up to 20 folds in slurry incubations yet showing insignificant response in intact cores. Such contrasting results between up- and down-stream and between slurry and intact core incubations strongly indicated that permeability, which determines diffusion of antibiotics to microbes, is the key to exert the effect of antibiotics on in situ sedimentary nitrogen removal processes regardless the existence of antibiotics resistance genes. This diffusive obstruction may mitigate the toxic effect of antibiotics on nitrogen removal related microbes in natural environments.

Antibiotics sulfamethoxazole alter nitrous oxide production and pathways in estuarine sediments: Evidenced by the N15-O18 isotopes tracing

Estuarine antibiotic residues are profoundly impacting microbial nitrogen (N) cycling and associated N₂O production, but the response of N₂O production pathways to antibiotics remains poorly understood. Here, ¹⁵N-¹⁸O labeling technique combined with molecular methods were used to investigate the impacts of sulfamethoxazole on the contribution of ammonia oxidation (nitrifier nitrification, nitrifier denitrification, and nitrification-coupled denitrification) and heterotrophic denitrification (HD) to N₂O production in estuarine sediments. Results showed that environmental concentration of sulfamethoxazole (4 ng/g) promoted the total N₂O production by 17.1% through nitrifier denitrification. Environmentally relevant (40-4000 ng/g) and irrelevant (40,000 ng/g) concentration of sulfamethoxazole drove nitrification denitrification to gradually lose the dominant role in total N₂O production and ammonia oxidation-derived N₂O, replaced by HD and nitrifier nitrification, while total N₂O production were inhibited. Furthermore, when HD dominated the total N₂O production, the HD-derived N₂O increased by 63.6% with sulfamethoxazole concentration reaching 40,000 ng/g. The mechanistic investigation further showed that nitrifying bacteria were more susceptible to sulfamethoxazole than nitrifying archaea and denitrifiers. The increased expression of nirS gene carried by non-dominant denitrifiers improved the ratio of nirS:nosZ and hence increased HD-derived N₂O under high sulfamethoxazole stresses. Overall, our results provide a comprehensive view into how antibiotics regulate N₂O production and its pathways in estuarine sediments.

Trace Metal Availability Affects Greenhouse Gas Emissions and Microbial Functional Group Abundance in Freshwater Wetland Sediments

We investigated the effects of trace metal additions on microbial nitrogen (N) and carbon (C) cycling using freshwater wetland sediment microcosms amended with micromolar concentrations of copper (Cu), molybdenum (Mo), iron (Fe), and all combinations thereof. In addition to monitoring inorganic N transformations (NO₃ ^-, NO₂ ^-, N₂O, NH₄ ⁺) and carbon mineralization (CO₂, CH₄), we tracked changes in functional gene abundance associated with denitrification (nirS, nirK, nosZ), dissimilatory nitrate reduction to ammonium (DNRA; nrfA), and methanogenesis (mcrA). With regards to N cycling, greater availability of Cu led to more complete denitrification (i.e., less N₂O accumulation) and a higher abundance of the nirK and nosZ genes, which encode for Cu-dependent reductases. In contrast, we found sparse biochemical evidence of DNRA activity and no consistent effect of the trace metal additions on nrfA gene abundance. With regards to C mineralization, CO₂ production was unaffected, but the amendments stimulated net CH₄ production and Mo additions led to increased mcrA gene abundance. These findings demonstrate that trace metal effects on sediment microbial physiology can impact community-level function. We observed direct and indirect effects on both N and C biogeochemistry that resulted in increased production of greenhouse gasses, which may have been mediated through the documented changes in microbial community composition and shifts in functional group abundance. Overall, this work supports a more nuanced consideration of metal effects on environmental microbial communities that recognizes the key role that metal limitation plays in microbial physiology.

Effects of triclosan and triclocarban on denitrification and N2O emissions in paddy soil

Triclosan (TCS) and triclocarban (TCC) are two common antimicrobial compounds, which are widely used as ingredients in pharmaceuticals and personal care products. They occur ubiquitously in soil due to biosolid application as agricultural fertilizers, but their influence on microbially mediated soil biogeochemical processes is poorly understood. We tested the effects of varying concentrations of TCS and TCC applied both individually and together on denitrification and N₂O emissions in paddy soil. We also quantified denitrification functional gene abundances by q-PCR to elucidate the microbial mechanisms of TCS and TCC's effects. Our results showed that TCS and TCC exposure both individually and together significantly (p < 0.05) inhibited denitrification (7.0-36.7%) and N₂O emissions (15.4-86.4%) except for the 0.01 mg kg^-1 TCC treatment in which denitrification was slightly but significantly (p < 0.05) stimulated. The inhibitory effects of TCS and TCC exposure were mainly attributed to their negative net effects on denitrifying bacteria as suggested by the decrease in abundances of 16S rRNA, narG, nirK and clade I nosZ genes in the TCS and TCC treatments. Overall, we found that TCS and TCC exposure in paddy soil could substantially alter nitrogen cycling in rice paddy ecosystems by inhibiting denitrification and N₂O emissions. These effects should be taken into consideration when evaluating the environmental impacts of TCS and TCC.

Motility of Shewanella oneidensis MR-1 Allows for Nitrate Reduction in the Toxic Region of a Ciprofloxacin Concentration Gradient in a Microfluidic Reactor

Subsurface environments often contain mixtures of contaminants in which the microbial degradation of one pollutant may be inhibited by the toxicity of another. Agricultural settings exemplify these complex environments, where antimicrobial leachates may inhibit nitrate bioreduction, and are the motivation to address this fundamental ecological response. In this study, a microfluidic reactor was fabricated to create diffusion-controlled concentration gradients of nitrate and ciprofloxacin under anoxic conditions in order to evaluate the ability of Shewanella oneidenisis MR-1 to reduce the former in the presence of the latter. Results show a surprising ecological response, where swimming motility allow S. oneidensis MR-1 to accumulate and maintain metabolic activity for nitrate reduction in regions with toxic ciprofloxacin concentrations (i.e., 50× minimum inhibitory concentration, MIC), despite the lack of observed antibiotic resistance. Controls with limited nutrient flux and a nonmotile mutant (Δ flag) show that cells cannot colonize antibiotic rich microenvironments, and this results in minimal metabolic activity for nitrate reduction. These results demonstrate that under anoxic, nitrate-reducing conditions, motility can control microbial habitability and metabolic activity in spatially heterogeneous toxic environments.

Tetracycline and sulfamethazine alter dissimilatory nitrate reduction processes and increase N2O release in rice fields

Effects of antibiotics on the transformation of nitrate and the associated N₂O release in paddy fields are obscure. Using soil slurry experiments combined with ¹⁵N tracer techniques, the influence of tetracycline and sulfamethazine (applied alone and in combination) on the denitrification, anaerobic ammonium oxidation (anammox), dissimilatory nitrate reduction to ammonium (DNRA) and N₂O release rates in the paddy soil were investigated, while genes related to nitrate reduction and antibiotic resistance were quantified to explore the microbial mechanisms behind the antibiotics' effects. The potential rates of denitrification, anammox, and DNRA were significantly (p < 0.05) reduced, which were mainly attributed to the inhibitory effects of the antibiotics on nitrate-reducing microbes. However, the N₂O release rates were significantly (p < 0.05) stimulated by the antibiotic treatments (0.6-6000 μg kg^-1 soil dry weight), which were caused by the different inhibition effects of antibiotics on N₂O production and N₂O reduction as suggest by the changes in abundance of nirS (nitrite reduction step) and nosZ (N₂O reduction to N₂ step) genes. Antibiotic resistance gene (tetA, tetG, sulI, and sulIII) abundances were significantly (p < 0.05) increased under high antibiotic exposure concentrations (>600 μg kg^-1 soil dry weight). Our results suggest that the widespread occurrence of antibiotics in paddy soils may pose significant eco-environmental risks (nitrate accumulation and greenhouse effects) by altering nitrate transformation processes.

Effects of thiamphenicol on nitrate reduction and N2O release in estuarine and coastal sediments

Nitrate overload is an important driver of water pollution in most estuarine and coastal ecosystems, and thus nitrate reduction processes have attracted considerable attention. Antibiotics contamination is also an emerging environmental problem in estuarine and coastal regions as a result of growing production and usage of antibiotics. However, the effects of antibiotics on nitrate reduction remain unclear in these aquatic ecosystems. In this study, continuous-flow experiments were conducted to examine the effects of thiamphenicol (TAP, a common chloramphenicol antibiotic) on nitrate reduction and greenhouse gas N2O release. Functional genes involved in nitrogen transformation were also quantified to explore the microbial mechanisms of the TAP influence. Production of N2 were observed to be inhibited by TAP treatment, which implied the inhibition effect of TAP on nitrate reduction processes. As intermediate products of nitrogen transformation processes, nitrite and N2O were observed to accumulate during the incubation. Different TAP inhibition effects on related functional genes may be the microbial mechanism for the changes of nutrient fluxes, N2 fluxes and N2O release rates. These results indicate that the antibiotics residues in estuarine and coastal ecosystems may contribute to nitrate retention and N2O release, which could be a major factor responsible for eutrophication and greenhouse effects.

Gross nitrous oxide production drives net nitrous oxide fluxes across a salt marsh landscape

Sea level rise will change inundation regimes in salt marshes, altering redox dynamics that control nitrification - a potential source of the potent greenhouse gas, nitrous oxide (N2 O) - and denitrification, a major nitrogen (N) loss pathway in coastal ecosystems and both a source and sink of N2 O. Measurements of net N2 O fluxes alone yield little insight into the different effects of redox conditions on N2 O production and consumption. We used in situ measurements of gross N2 O fluxes across a salt marsh elevation gradient to determine how soil N2 O emissions in coastal ecosystems may respond to future sea level rise. Soil redox declined as marsh elevation decreased, with lower soil nitrate and higher ferrous iron in the low marsh compared to the mid and high marshes (P < 0.001 for both). In addition, soil oxygen concentrations were lower in the low and mid-marshes relative to the high marsh (P < 0.001). Net N2 O fluxes differed significantly among marsh zones (P = 0.009), averaging 9.8 ± 5.4 μg N m(-2)  h(-1) , -2.2 ± 0.9 μg N m(-2)  h(-1) , and 0.67 ± 0.57 μg N m(-2)  h(-1) in the low, mid, and high marshes, respectively. Both net N2 O release and uptake were observed in the low and high marshes, but the mid-marsh was consistently a net N2 O sink. Gross N2 O production was highest in the low marsh and lowest in the mid-marsh (P = 0.02), whereas gross N2 O consumption did not differ among marsh zones. Thus, variability in gross N2 O production rates drove the differences in net N2 O flux among marsh zones. Our results suggest that future studies should focus on elucidating controls on the processes producing, rather than consuming, N2 O in salt marshes to improve our predictions of changes in net N2 O fluxes caused by future sea level rise.

Do antibiotics have environmental side-effects? Impact of synthetic antibiotics on biogeochemical processes

Antibiotic use in the early 1900 vastly improved human health but at the same time started an arms race of antibiotic resistance. The widespread use of antibiotics has resulted in ubiquitous trace concentrations of many antibiotics in most environments. Little is known about the impact of these antibiotics on microbial processes or "non-target" organisms. This mini-review summarizes our knowledge of the effect of synthetically produced antibiotics on microorganisms involved in biogeochemical cycling. We found only 31 articles that dealt with the effects of antibiotics on such processes in soil, sediment, or freshwater. We compare the processes, antibiotics, concentration range, source, environment, and experimental approach of these studies. Examining the effects of antibiotics on biogeochemical processes should involve environmentally relevant concentrations (instead of therapeutic), chronic exposure (versus acute), and monitoring of the administered antibiotics. Furthermore, the lack of standardized tests hinders generalizations regarding the effects of antibiotics on biogeochemical processes. We investigated the effects of antibiotics on biogeochemical N cycling, specifically nitrification, denitrification, and anammox. We found that environmentally relevant concentrations of fluoroquinolones and sulfonamides could partially inhibit denitrification. So far, the only documented effects of antibiotic inhibitions were at therapeutic doses on anammox activities. The most studied and inhibited was nitrification (25-100 %) mainly at therapeutic doses and rarely environmentally relevant. We recommend that firm conclusions regarding inhibition of antibiotics at environmentally relevant concentrations remain difficult due to the lack of studies testing low concentrations at chronic exposure. There is thus a need to test the effects of these environmental concentrations on biogeochemical processes to further establish the possible effects on ecosystem functioning.

One-Time Addition of Nano-TiO2 Triggers Short-Term Responses in Benthic Bacterial Communities in Artificial Streams

Nano-TiO2 is an engineered nanomaterial whose production and use are increasing rapidly. Hence, aquatic habitats are at risk for nano-TiO2 contamination due to potential inputs from urban and suburban runoff and domestic wastewater. Nano-TiO2 has been shown to be toxic to a wide range of aquatic organisms, but little is known about the effects of nano-TiO2 on benthic microbial communities. This study used artificial stream mesocosms to assess the effects of a single addition of nano-TiO2 (P25 at a final concentration of 1 mg l(-1)) on the abundance, activity, and community composition of sediment-associated bacterial communities. The addition of nano-TiO2 resulted in a rapid (within 1 day) decrease in bacterial abundance in artificial stream sediments, but bacterial abundance returned to control levels within 3 weeks. Pyrosequencing of partial 16S rRNA genes did not indicate any significant changes in the relative abundance of any bacterial taxa with nano-TiO2 treatment, indicating that nano-TiO2 was toxic to a broad range of bacterial taxa and that recovery of the bacterial communities was not driven by changes in community composition. Addition of nano-TiO2 also resulted in short-term increases in respiration rates and denitrification enzyme activity, with both returning to control levels within 3 weeks. The results of this study demonstrate that single-pulse additions of nano-TiO2 to aquatic habitats have the potential to significantly affect the abundance and activity of benthic microbial communities and suggest that interactions of TiO2 nanoparticles with environmental matrices may limit the duration of their toxicity.

Dissimilatory nitrogen reduction in intertidal sediments of a temperate estuary: small scale heterogeneity and novel nitrate-to-ammonium reducers

The estuarine nitrogen cycle can be substantially altered due to anthropogenic activities resulting in increased amounts of inorganic nitrogen (mainly nitrate). In the past, denitrification was considered to be the main ecosystem process removing reactive nitrogen from the estuarine ecosystem. However, recent reports on the contribution of dissimilatory nitrate reduction to ammonium (DNRA) to nitrogen removal in these systems indicated a similar or higher importance, although the ratio between both processes remains ambiguous. Compared to denitrification, DNRA has been underexplored for the last decades and the key organisms carrying out the process in marine environments are largely unknown. Hence, as a first step to better understand the interplay between denitrification, DNRA and reduction of nitrate to nitrite in estuarine sediments, nitrogen reduction potentials were determined in sediments of the Paulina polder mudflat (Westerschelde estuary). We observed high variability in dominant nitrogen removing processes over a short distance (1.6 m), with nitrous oxide, ammonium and nitrite production rates differing significantly between all sampling sites. Denitrification occurred at all sites, DNRA was either the dominant process (two out of five sites) or absent, while nitrate reduction to nitrite was observed in most sites but never dominant. In addition, novel nitrate-to-ammonium reducers assigned to Thalassospira, Celeribacter, and Halomonas, for which DNRA was thus far unreported, were isolated, with DNRA phenotype reconfirmed through nrfA gene amplification. This study demonstrates high small scale heterogeneity among dissimilatory nitrate reduction processes in estuarine sediments and provides novel marine DNRA organisms that represent valuable alternatives to the current model organisms.

Effect of eastern oysters (Crassostrea virginica) on sediment carbon and nitrogen dynamics in an urban estuary

Oyster reefs have declined globally. Interest in their restoration has motivated research into oyster-mediated ecosystem services including effects on biodiversity, filtration, and nitrogen (N) cycling. Recent evidence suggests oysters may promote denitrification, or anaerobic respiration of nitrate (NO3-) into di-nitrogen gas, via benthic deposition of carbon (C) and N-rich biodeposits. However, the mechanisms whereby biodeposits promote N transformations prerequisite to denitrification (e.g., mineralization and nitrification) are unclear. Previous research has also not measured oysters' influence on N cycling in urbanized areas. In May 2010 we deployed eastern oysters (Crassostrea virginica) in mesh cages above sand-filled boxes at four sites across a nutrient gradient in Jamaica Bay, New York City (New York, USA). Oysters were arranged at four densities: 0, 40, 85, and 150 oysters/m2. For 17 months we measured water-column nutrients and chlorophyll a, every two weeks to monthly. Every two months we measured sediment ash-free dry mass (AFDM), exchangeable ammonium (NH4+), ammonification, nitrification, denitrification potential (DNP), and NO3- and C limitation of DNP. Oysters increased sediment AFDM at three of four sites, with the greatest increase at high density. Oysters did not affect any N pools or transformations. However, variation among sites and dates illustrated environmental drivers of C and N biogeochemistry in this urban estuary. Overall, nitrification was positively related to net ammonification, water column NH4+, and sediment NH4+, but was not correlated with DNP. Denitrification was consistently and strongly NO3- limited, while C was not limiting or secondarily limiting. Therefore, the oyster-mediated increase in AFDM did not affect DNP because C was not its primary driver. Also, because DNP was unrelated to nitrification, it is unlikely that biodeposit N was converted to NO3- for use as a denitrification substrate. Predicting times or sites where denitrification is driven by the C and N species originating from oyster biodeposits remains a challenge under eutrophic conditions. Towards this goal, we synthesized our conclusions with literature predictions in a conceptual model for pathways whereby oysters might influence C and N dynamics differently in oligotrophic relative to eutrophic ecosystems.

Assessment of denitrification process in lower Ishikari river system, Japan

Sediment denitrification rate and its role in removal of dissolved nitrate load in lower Ishikari river system were examined. Denitrification rate were measured using acetylene inhibition technique on the sediment samples collected during August 2009-July 2010. The denitrification rate varied from 0.001 to 1.9 μg Ng(-1) DM h(-1) with an average value of 0.21 μg Ng(-1) DM h(-1) in lower Ishikari river system. Denitrification rate showed positive correlation with dissolved nitrate concentration in the river basin, indicating overlying water column supplied nitrate for the sediment denitrification processes. Nutrient enrichment experiments result showed that denitrification rate increased significantly with addition of nitrate in case of samples collected from Barato Lake however no such increase was observed in the samples collected from Ishikari river main channel and its major tributaries indicating that factors other than substrate concentration such as population of denitrifier and hydrological properties of stream channel including channel depth and flow velocity may affects the denitrification rate in lower Ishikari river system. Denitrification rate showed no significant increase with the addition of labile carbon (glucose), indicating that sediment samples had sufficient organic matter to sustain denitrification activity. The result of nutrient spiraling model indicates that in- stream denitrification process removes on an average 5%d(-1) of dissolve nitrate load in Ishikari river. This study was carried out to fill the gap present in the availability of riverine denitrification rate measurement and its role in nitrogen budget from Japanese rivers characterize by small river length and high flow rate.

The effect of environmental and therapeutic concentrations of antibiotics on nitrate reduction rates in river sediment

The use of antibiotics in both human and veterinary medicine has led to increased presence of these compounds and antibiotic resistance in the environment. In this study, the effect of low, environmentally relevant (<mg L(-1)) and high therapeutic (>mg L(-1)) concentrations of vancomycin (VA), flumequine (FLU), and sulfamethoxazole (SMX) on nitrate reduction rates was studied in river sediments. Nitrate reduction rates were determined by supplying intact sediments for several weeks with both nitrate and antibiotics (ng L(-1), μg L(-1), and mg L(-1) concentrations), including a non-amended control. Furthermore the concentrations of the three investigated antibiotics were measured in the initial (natural) sediments and the sediments supplied with the antibiotics. The antibiotic concentrations in the sediments decreased (on average 62% for FLU and 93% for SMX) during the experiments, indicating loss of antibiotics due to sorption or (bio) degradation. Nitrate reduction rates were not affected by environmental concentrations of VA, FLU and SMX. FLU and SMX only partially inhibited nitrate reduction rates at high, therapeutic concentrations by 41 and 39% respectively. The three tested antibiotics significantly enhanced the production of nitrite, an intermediate in dissimilatory nitrate reduction. Nitrite production increased 1.9 and 1.4 fold for environmental VA concentrations (107 and 187 μg L(-1) respectively), application of 58 mg L(-1) SMX resulted in a 7.5 fold increase and augmented 16 and 8.5 fold in the presence of respectively 13 μg L(-1) and 52 mg L(-1) FLU. Even though inhibition of nitrate reduction rates was observed at therapeutic antibiotic concentrations, nitrate reduction proceeded under all experimental conditions, indicating the presence of resistance toward these antibiotics among the nitrate reducing bacteria. The accumulation of nitrite suggests that the nitrite reduction step was more affected than the overall nitrate reduction process.

Physiological and transcriptional studies of Cr(VI) reduction under aerobic and denitrifying conditions by an aquifer-derived pseudomonad

Cr(VI) is a widespread groundwater contaminant that is a potent toxin, mutagen, and carcinogen. In situ reductive immobilization is a favored approach for Cr(VI) bioremediation, and Cr(VI) reduction has been reported in a variety of aerobic, facultative, and anaerobic bacteria, including a number of pseudomonads. However, studies comparing Cr(VI) reduction under aerobic and denitrifying conditions in the same organism are not available. We have conducted studies with strain RCH2, a bacterium similar to Pseudomonas stutzeri that we isolated from a Cr-contaminated aquifer. Cell suspension studies with lactate demonstrated that Cr(VI) reduction could occur under either denitrifying or aerobic conditions (at comparable specific rates) and that reduction was at least 20-fold more rapid when the terminal electron acceptor (i.e., nitrate or O(2)) was present. Our results suggest that Cr(VI) reduction by strain RCH2 under either aerobic or denitrifying conditions is primarily cometabolic in the sense that the physiological electron acceptor (oxygen or nitrate) appears to be required. Under both aerobic and denitrifying conditions, the gene(s) associated with chromate reduction are not inducible by Cr. Continuous culture (chemostat) studies showed strong correlations (r(2) values >0.93) between nitrate reduction rate and the transcript copy number of either nirS (cytochrome cd(1) nitrite reductase) or narG (nitrate reductase α subunit). As our studies indicate that anaerobic Cr(VI) reduction by this pseudomonad requires active denitrification and that denitrification and chromate reduction rates are highly correlated (r(2) > 0.99), monitoring expression of such denitrification genes in biostimulated aquifers could provide valuable proxy information for in situ chromate reduction by similar bacteria even if the specific genes involved in chromate reduction have not been identified. We also report incomplete removal of reduced Cr from solution and on artifacts in the widely used diphenylcarbazide assay for Cr(VI), most notably, its complete inactivation in the presence of millimolar nitrite.

An updated process model for carbon oxidation, nitrification, and denitrification

The currently available comprehensive activated sludge models, ASM#1 (Grady et al., 1986) and its successor ASM#3 (Gujer et al., 1999), do not adequately describe nitrification and denitrification, with respect to ammonia oxidation inhibition, nitrite accumulation, or emissions of nitric oxide and nitrous oxide. A new comprehensive activated sludge process model, the Activated Sludge Model for Nitrogen (ASMN), is presented. The ASMN incorporates two nitrifying populations-ammonia-oxidizing bacteria and nitrite-oxidizing bacteria-using free ammonia and free nitrous acid, respectively, as their true substrates. The ASMN incorporates four-step denitrification (sequential reduction of nitrate to nitrogen gas via nitrite, nitric oxide, and nitrous oxide) using individual, reaction-specific parameters. Simulation results for ammonia, nitrate, soluble substrate, and biomass concentrations determined by using ASMN for three activated sludge process configurations under steady-state and dynamic municipal-type influent conditions are shown to be comparable with ASM#1 results.

Potential rates and pathways of microbial nitrate reduction in coastal sediments

Nitrate reduction plays a key role in the biogeochemical dynamics and microbial ecology of coastal sediments. Potential rates of nitrate reduction were measured on undisturbed sediment slices from two eutrophic coastal environments using flow-through reactors (FTR). Maximum potential nitrate reduction rates ranged over an order of magnitude, with values of up to 933 nmol cm(-3) h(-1), whereas affinity constants for NO(3) (-) fell mostly between 200 and 600 microM. Homogenized sediment slurries systematically yielded higher rates of nitrate reduction than the FTR experiments. Dentrification was the major nitrate removal pathway in the sediments, although excess ammonium production indicated a contribution of dissimilatory nitrate reduction to ammonium under nitrate-limiting conditions.

Microbiological degradation of pentane by immobilized cells of Arthrobacter sp

The increasing production of several plastics such as expanded polystyrene, widely used as packaging and building materials, has caused the release of considerable amounts of pentane employed as an expanding agent. Today many microorganisms are used to degrade hydrocarbons in order to minimize contamination caused by several industrial activities. The aim of our work was to identify a suitable microorganism to degrade pentane. We focused our attention on a strain of Arthrobacter sp. which in a shake-flask culture produced 95% degradation of a 10% mixture of pentane in a minimal medium after 42 days of incubation at 20 degrees C. Arthrobacter sp. cells were immobilized on a macroporous polystyrene particle matrix that provides a promising novel support for cell immobilization. The method involved culturing cells with the expanded polystyrene in shake-flasks, followed by in situ growth within the column. Scanning electron microscopy analysis showed extensive growth of Arthrobacter sp. on the polymeric surface. The immobilized microorganism was able to actively degrade a 10% mixture of pentane, allowing us to obtain a bioconversion yield of 90% after 36 h. Moreover, in repeated-batch operations, immobilized Arthrobacter sp. cells were able to maintain 85-95% pentane degradation during a 2 month period. Our results suggest that this type of bioreactor could be used in pentane environmental decontamination.