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Half-saturation Constant (half-saturation + constant)
Selected AbstractsComparison of biodegradation kinetic parameters for naphthalene in batch and sand column systems by pseudomonas putidaENVIRONMENTAL PROGRESS & SUSTAINABLE ENERGY, Issue 2 2001Jeong-Hun Park Kinetic parameters for the degradation of naphthalene by Pseudomonas putida ( ATCC 17484) were estimated in both batch and column assays, in order to evaluate the role of flow and cell attachment on biodegradation rates. Suspended cells and cells attached to Ottawa sand were used under a variety of biomass levels, column flow-rates, and substrate concentrations. In batch systems, degradation followed zero order kinetics across the entire concentration range, while the columns exhibited decreased rates at concentrations less than 100 (,g/L), describable by Michaelis-Menten kinetics. This is reflected in elevated values of the half-saturation constant, Ks, in columns. We offer the explanation that this may have resulted from reactive heterogeneity within the porous media, imposing a distribution of length-scales for transfer of substrate to the cell surfaces. Well-mixed batch systems are expected to have both shorter and more uniform transfer distances. When kinetic parameters obtained in batch system are used for prediction of degradation in columns, at least two factors,exposed reduction of exposed cell surface are a and heterogeneity of cell distribution,will likely reduce overall column degradation rates. [source] On the variability of respiration in terrestrial ecosystems: moving beyond Q10GLOBAL CHANGE BIOLOGY, Issue 2 2006ERIC A. DAVIDSON Abstract Respiration, which is the second most important carbon flux in ecosystems following gross primary productivity, is typically represented in biogeochemical models by simple temperature dependence equations. These equations were established in the 19th century and have been modified very little since then. Recent applications of these equations to data on soil respiration have produced highly variable apparent temperature sensitivities. This paper searches for reasons for this variability, ranging from biochemical reactions to ecosystem-scale substrate supply. For a simple membrane-bound enzymatic system that follows Michaelis,Menten kinetics, the temperature sensitivities of maximum enzyme activity (Vmax) and the half-saturation constant that reflects the affinity of the enzyme for the substrate (Km) can cancel each other to produce no net temperature dependence of the enzyme. Alternatively, when diffusion of substrates covaries with temperature, then the combined temperature sensitivity can be higher than that of each individual process. We also present examples to show that soluble carbon substrate supply is likely to be important at scales ranging from transport across membranes, diffusion through soil water films, allocation to aboveground and belowground plant tissues, phenological patterns of carbon allocation and growth, and intersite differences in productivity. Robust models of soil respiration will require that the direct effects of substrate supply, temperature, and desiccation stress be separated from the indirect effects of temperature and soil water content on substrate diffusion and availability. We speculate that apparent Q10 values of respiration that are significantly above about 2.5 probably indicate that some unidentified process of substrate supply is confounded with observed temperature variation. [source] Biodegradation kinetics of benzene, methyl tert -butyl ether, and toluene as a substrate under various substrate concentrationsJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 1 2007Chi-Wen Lin Abstract Owing to the complexity of conventional methods and shortcomings in determining kinetic parameters, a convenient approach using the nonlinear regression analysis of Monod or Haldane type nonlinear equations is presented. This method has been proven to provide accurate estimates of kinetic parameters. The major work in this study consisted of the testing of aromatic compound-degrading cultures in batch experiments for the biodegradation of benzene, methyl tert -butyl ether (MTBE), and toluene. Additionally, batch growth data of three pure cultures (i.e., Pseudomonas aeruginosa YAMT421, Ralstonia sp. YABE411 and Pseudomonas sp. YATO411) isolated from an industrial petrochemical wastewater treatment plant under aerobic conditions were assessed with the nonlinear regression technique and with a trial-and-error procedure to determine the kinetic parameters. The growth rates of MTBE-, benzene-, and toluene-degrading cultures on MTBE, benzene, and toluene were significant. Monod's model was a good fit for MTBE, benzene and toluene at low substrate concentrations. In contrast, Haldane's equation fitted well in substrate inhibition concentration. Monod and Haldane's expressions were found to describe the results of these experiments well, with fitting values higher than 98%. The kinetic parameters, including a maximum specific growth rate (µm), a half-saturation constant (Ks), and an inhibition constant (Ki), were given. Copyright © 2007 Society of Chemical Industry [source] Development and simulation studies of an unsteady state biofilter model for the treatment of cyclic air emissions of an ,-pinene gas streamJOURNAL OF CHEMICAL TECHNOLOGY & BIOTECHNOLOGY, Issue 7 2005Christina Dirk-Faitakis Abstract This paper describes the development and simulation of an unsteady state biofilter model used to predict dynamic behaviour of cyclically-operated biofilters and compares it with experimental results obtained from three, parallel, bench-scale biofilters treating both periodically fluctuating concentrations and constant concentrations of an ,-pinene-laden gas stream. The dynamic model, using kinetic parameters estimated from the constant concentration biofilter, was able to predict the performance of cyclic biofilters operating at short cycle periods (ie, in the order of minutes and hours). Steady state kinetic data from a constant concentration biofilter can be used to predict unsteady state biofilter operation. At a 24 h cycle period, the dynamic model compared well with experimental results. For long cycle periods (ie, hours and days), removal efficiency decreased after periods of non-loading: the longer the period of non-loading, the poorer the biofilter's performance at the re-commencement of pollutant loading. At longer time scales the model did not effectively predict transient behaviour, as adsorption and changes in kinetic parameters were not accounted for. Modelling results showed that similar biofiltration performance for the cyclic and constant concentration biofiltration of ,-pinene is expected for biofilters operating solely in the first order kinetics regime. Poorer performance for cyclic biofilters following Monod kinetics spanning the entire kinetics range is expected as the cycle amplitude increases. The most important parameters affecting the performance of a cyclically-operated biofilter with short cycle periods are: amplitude of cyclic fluctuations, Cg, max/Cg, relative value of the half-saturation constant in the Monod expression, Ks, and effective diffusivity of ,-pinene in the biofilm, De. Copyright © 2005 Society of Chemical Industry [source] Respiration of nitrous oxide in suboxic soilEUROPEAN JOURNAL OF SOIL SCIENCE, Issue 3 2009B. Vieten Summary Reduction of nitrous oxide (N2O) is an autonomous respiratory pathway. Nitrous oxide is an alternative electron acceptor to O2 when intensive biological activity and reduced diffusivity result in an O2 deficit. Hypoxic or anoxic micro sites may form even in well-aerated soils, and provide a sink for N2O diffusing through the gas-filled pore space. We reproduced similar in vitro conditions in suboxic (0.15% O2) flow-through incubation experiments with samples from a Stagnosol and from a Histosol. Apparent half-saturation constants (km) for N2O reduction were similar for both soils and were, on average, 3.8 ,mol mol,1 at 5°C, 5.1 ,mol mol,1 at 10°C, and 6.9 ,mol mol,1 at 20°C. Respiration of N2O was estimated to contribute a maximum proportion of 1.7% to total respiration in the Stagnosol (pH 7.0) and 0.9% in the Histosol (pH 2.9). [source] Phosphorus-limited growth dynamics in two Baltic Sea cyanobacteria, Nodularia sp. and Aphanizomenon sp.FEMS MICROBIOLOGY ECOLOGY, Issue 3 2006Jenny Degerholm Abstract Rates of carbon (C) specific growth and nitrogen (N2) fixation were monitored in cultures of Baltic Sea Nodularia and Aphanizomenon exposed to gradual limitation by inorganic phosphorus (P). Both cyanobacteria responded by decreased cellular P content followed by lowered rates of growth and N2 fixation. C-specific growth and cellular N content changed faster in Aphanizomenon both when inorganic P was lowered as well as during reintroduction of P. Aphanizomenon also showed a more rapid increase in N-specific N2 fixation associated with increased C-specific growth. When ambient concentrations of inorganic P declined, both cyanobacteria displayed higher rates of alkaline phosphatase (APase) activity. Lower substrate half-saturation constants (KM) and higher Vmax : KM ratio of the APase enzyme associated with Nodularia suggest a higher affinity for dissolved organic P (DOP) substrate than Aphanizomenon. Aphanizomenon, which appears more sensitive to changes in ambient dissolved inorganic P, may be adapted to environments with elevated concentrations of P or repeated intrusions of nutrient-rich water. Nodularia on the other hand, with its higher tolerance to increased P starvation may have an ecological advantage in stratified surface waters of the Baltic Sea during periods of low P availability. [source] Benthic organic carbon influences denitrification in streams with high nitrate concentrationFRESHWATER BIOLOGY, Issue 7 2007CLAY P. ARANGO Summary 1. Anthropogenic activities have increased reactive nitrogen availability, and now many streams carry large nitrate loads to coastal ecosystems. Denitrification is potentially an important nitrogen sink, but few studies have investigated the influence of benthic organic carbon on denitrification in nitrate-rich streams. 2. Using the acetylene-block assay, we measured denitrification rates associated with benthic substrata having different proportions of organic matter in agricultural streams in two states in the mid-west of the U.S.A., Illinois and Michigan. 3. In Illinois, benthic organic matter varied little between seasons (5.9,7.0% of stream sediment), but nitrate concentrations were high in summer (>10 mg N L,1) and low (<0.5 mg N L,1) in autumn. Across all seasons and streams, the rate of denitrification ranged from 0.01 to 4.77 ,g N g,1 DM h,1 and was positively related to stream-water nitrate concentration. Within each stream, denitrification was positively related to benthic organic matter only when nitrate concentration exceeded published half-saturation constants. 4. In Michigan, streams had high nitrate concentrations and diverse benthic substrata which varied from 0.7 to 72.7% organic matter. Denitrification rate ranged from 0.12 to 11.06 ,g N g,1 DM h,1 and was positively related to the proportion of organic matter in each substratum. 5. Taken together, these results indicate that benthic organic carbon may play an important role in stream nitrogen cycling by stimulating denitrification when nitrate concentrations are high. [source] | ||||||||||