共查询到20条相似文献,搜索用时 31 毫秒
1.
《Enzyme and microbial technology》2005,36(2-3):350-356
The enzyme reaction mechanism and kinetics for biosyntheses of deoxycytidine triphosphate (dCTP) and deoxythymidine triphosphate (dTTP) from the corresponding deoxycytidine diphosphate (dCDP) and deoxythymidine diphosphate (dTDP) catalyzed by pyruvate kinase were studied. The kinetic model for the two synthetic reactions was found to follow the Bi–Bi random rapid equilibrium mechanism similar to that of the biosynthesis of deoxyadenosine triphosphate (dATP) and deoxyguanosine triphosphate (dGTP) from the corresponding deoxyadenosine diphosphate (dADP) and deoxyguanosine diphosphate (dGDP). Kinetic constants involved in the reactions including the maximum reaction velocity, the Michaelis–Menten constants, and the inhibition constants for dCTP and dTTP biosyntheses were experimentally determined. This enzyme reaction requires Mg2+ ion and the optimal Mg2+ concentration was also determined. The experimental results showed a good agreement with the simulation results obtained from the kinetic model developed. The kinetics of the four biosynthetic reactions for deoxynucleoside triphosphates (dNTP) including dATP, dGTP, dCTP, and dTTP from the corresponding deoxynucleoside diphosphates (dNDP) including dADP, dGDP, dCDP, and dTDP were analyzed. The results suggest that the binding kinetics of phosphoenolpyruvate (PEP) and pyruvate are similar for all four biosynthetic reactions. The affinity of the dNDP substrates to enzyme is of the same order of magnitude as the corresponding dNTP as inhibitors. The order of reactivity and substrate specificity for dNDP is dADP > dGDP > dCDP > dTDP in the pyruvate kinase (PK) reactions. The results obtained from this study can be applied to bioreactor design and production of dCTP and dTTP for biosynthesis of DNA at a significantly lower cost compared to the currently available chemical method. 相似文献
2.
Choi JY Chowdhury G Zang H Angel KC Vu CC Peterson LA Guengerich FP 《The Journal of biological chemistry》2006,281(50):38244-38256
Previous studies have shown that replicative bacterial and viral DNA polymerases are able to bypass the mutagenic lesions O(6)-methyl and -benzyl (Bz) G. Recombinant human polymerase (pol) delta also copied past these two lesions but was totally blocked by O(6)-[4-oxo-4-(3-pyridyl)butyl] (Pob)G, an important mutagenic lesion formed following metabolic activation of the tobacco-specific carcinogen 4-(methylnitrosamino)-1-(3-pyridyl)-1-butanone. The human translesion pols iota and kappa produced mainly only 1-base incorporation opposite O(6)-MeG and O(6)-BzG and had very low activity in copying O(6)-PobG. Human pol eta copied past all three adducts. Steady-state kinetic analysis showed similar efficiencies of insertion opposite the O(6)-alkylG adducts for dCTP and dTTP with pol eta and kappa; pol iota showed a strong preference for dTTP. pol eta, iota, and kappa showed pre-steady-state kinetic bursts for dCTP incorporation opposite G and O(6)-MeG but little, if any, for O(6)-BzG or O(6)-PobG. Analysis of the pol eta O(6)-PobG products indicated that the insertion of G was opposite the base (C) 5' of the adduct, but this product was not extended. Mass spectrometry analysis of all of the pol eta primer extension products indicated multiple components, mainly with C or T inserted opposite O(6)-alkylG but with no deletions in the cases of O(6)-MeG and O(6)-PobG. With pol eta and O(6)-BzG, products were also obtained with -1 and -2 deletions and also with A inserted (opposite O(6)-BzG). The results with pol eta may be relevant to some mutations previously reported with O(6)-alkylG adducts in mammalian cells. 相似文献
3.
《DNA Repair》2017
Human PrimPol is a recently discovered bifunctional enzyme that displays DNA template-directed primase and polymerase activities. PrimPol has been implicated in nuclear and mitochondrial DNA replication fork progression and restart as well as DNA lesion bypass. Published evidence suggests that PrimPol is a Mn2+-dependent enzyme as it shows significantly improved primase and polymerase activities when binding Mn2+, rather than Mg2+, as a divalent metal ion cofactor. Consistently, our fluorescence anisotropy assays determined that PrimPol binds to a primer/template DNA substrate with affinities of 29 and 979 nM in the presence of Mn2+ and Mg2+, respectively. Our pre-steady-state kinetic analysis revealed that PrimPol incorporates correct dNTPs with 100-fold higher efficiency with Mn2+ than with Mg2+. Notably, the substitution fidelity of PrimPol in the presence of Mn2+ was determined to be in the range of 3.4 × 10−2 to 3.8 × 10−1, indicating that PrimPol is an error-prone polymerase. Furthermore, we kinetically determined the sugar selectivity of PrimPol to be 57–1800 with Mn2+ and 150–4500 with Mg2+, and found that PrimPol was able to incorporate the triphosphates of two anticancer drugs (cytarabine and gemcitabine), but not two antiviral drugs (emtricitabine and lamivudine). 相似文献
4.
《Aquatic Botany》2005,81(2):157-173
The main photosynthesis and respiration parameters (dark respiration rate, light saturated production rate, saturation irradiance, photosynthetic efficiency) were measured on a total of 23 macrophytes of the Thau lagoon (2 Phanerogams, 5 Chlorophyceae, 10 Rhodophyceae and 6 Phaeophyceae). Those measurements were performed in vitro under controlled conditions, close to the natural ones, and at several seasons. Concomitantly, measurements of pigment concentrations, carbon, phosphorous and nitrogen contents in tissues were performed. Seasonal intra-specific variability of photosynthetic parameters was found very high, enlightening an important acclimatation capacity. The highest photosynthetic capacities were found for Chlorophyceae (e.g. Monostroma obscurum thalli at 17 °C, 982 μmol O2 g−1 dw h−1 and 9.1 μmol O2 g−1 dw h−1/μmol photons m−2 s−1, respectively for light saturated net production rate and photosynthetic efficiency) and Phanerogams (e.g. Nanozostera noltii leaves at 25 °C, 583 μmol O2 g−1 dw h−1 and 2.6 μmol O2 g−1 dw h−1/μmol photons m−2 s−1 respectively for light saturated net production rate and photosynthetic efficiency). As expected, species with a high surface/volume ratio were found to be more productive than coarsely branched thalli and thick blades shaped species. Contrary to Rd (ranging 6.7–794 μmol O2 g−1 dw h−1, respectively for Rytiphlaea tinctoria at 7 °C and for Dasya sessilis at 25 °C) for which a positive relationship with water temperature was found whatever the species studied, the evolution of P/I curves with temperature exhibited different responses amongst the species. The results allowed to show summer nitrogen limitation for some species (Gracilaria bursa-pastoris and Ulva spp.) and to propose temperature preferences based on the photosynthetic parameters for some others (N. noltii, Zostera marina, Chaetomorpha linum). 相似文献
5.
Cumulative ozone uptake (COU, mmol m−2) and O3 flux (FO3, nmol m−2 s−1) were related to physiological, morphological and biochemical characteristics of field-grown mature evergreen Norway spruce [Picea abies (L.) Karst.], Cembran pine [Pinus cembra L.], and deciduous European larch [Larix decidua Mill.] trees at treeline. The threshold COU causing a statistically significant decline in photosynthetic capacity (Amax) ranged between 19.6 mmol m−2 in current-year needles of evergreen conifers and 22.0 6 mmol m−2 in short-shoot needles of deciduous L. decidua subjected to exposure periods of ≥84 and ≥43 days, respectively. The higher O3 sensitivity of deciduous L. decidua than of evergreen P abies and P. cembra was associated with differences in FO3 and specific leaf area (SLA), both being significantly higher in L. decidua. FO3 was 5.9 nmol m−2 s−1 in L. decidua and 2.7 nmol m−2 s−1 in evergreen conifers. Species-dependent differences were also related to detoxification capacity expressed through total surface area based concentrations of reduced ascorbate and α-tocopherol that both increased with SLA. Findings suggest that differences in O3 sensitivity between evergreen and deciduous conifers can be attributed to foliage type specific differences in SLA, the latter determining physiological and biochemical characteristics of the treeline conifers. 相似文献
6.
7.
《Aquatic Botany》2005,82(3):181-192
The influence of nitrogen and phosphorus pulses on Chaetomorpha linum (Muller) Kutzing growth and photosynthesis was studied in laboratory experiments. Photosynthesis and growth of C. linum from Tancada lagoon seems limited by both nitrogen and phosphorus, as indicated by the high rate (4.7–11.6 mg O2 g−1 dry weight h−1) of light-saturated photosynthesis (Pm) and growth rates observed under nitrogen plus phosphorus enrichment in relation to enrichment by nitrogen alone (2.9–7.6 mg O2 g−1 dry weight h−1). Significant increase in nitrogen and phosphorus content as percentage of dry weight was observed in C. linum fertilized with a single nutrient or with nitrogen plus phosphorus. In Tancada lagoon, when availability of nitrogen to primary producers is by pulses, an increase of nitrate concentration in the water column (from 6 to 100 μM) has a greater effect on growth of C. linum (growth rate: 0.13 day−1) than an increase in ammonium concentration (from 20 to 100 μM and growth rate: 0.11 day−1). For a given thallus nitrogen content (0.6–1.4% N), both Pm and the photosynthetic efficiency (α) normalized to dry weight were correlated (r2 = 0.73, p < 0.005) indicating that variations in electron transport were coupled to variations in C-fixation capacity. Optimizing both α and Pm may be a general characteristic of thin-structured opportunistic algae in more variable estuarine environments. 相似文献
8.
O6-methylguanine (O6-MeG) is a miscoding DNA lesion arising from the alkylation of guanine. This report uses the bacteriophage T4 DNA polymerase as a model to probe the roles of hydrogen-bonding interactions, shape/size, and nucleobase desolvation during the replication of this miscoding lesion. This was accomplished by using transient kinetic techniques to monitor the kinetic parameters for incorporating and extending natural and nonnatural nucleotides. In general, the efficiency of nucleotide incorporation does not depend on the hydrogen-bonding potential of the incoming nucleotide. Instead, nucleobase hydrophobicity and shape complementarity appear to be the preeminent factors controlling nucleotide incorporation. In addition, shape complementarity plays a large role in controlling the extension of various mispairs containing O6-MeG. This is evident as the rate constants for extension correlate with proper interglycosyl distances and symmetry between the base angles of the formed mispair. Base pairs not conforming to an acceptable geometry within the polymerase's active site are refractory to elongation and are processed via exonuclease proofreading. The collective data set encompassing nucleotide incorporation, extension, and excision is used to generate a model accounting for the mutagenic potential of O6-MeG observed in vivo. In addition, kinetic studies monitoring the incorporation and extension of nonnatural nucleotides identified an analog that displays high selectivity for incorporation opposite O6-MeG compared to unmodified purines. The unusual selectivity of this analog for replicating damaged DNA provides a novel biochemical tool to study translesion DNA synthesis. 相似文献
9.
《Aquatic Botany》2007,87(2):104-110
A large-scale mesocosm (sixteen 500 L tanks) experiment was conducted to investigate the effects of hypersalinity (45–65 psu), porewater sulfide (2–6 mM) and nighttime water column hypoxia (5–3 mg L−1) on the tropical seagrass Thalassia testudinum Banks ex König. We examined stressor effects on growth, shoot survival, tissue sulfur (S0, TS, δ34S) and leaf quantum efficiencies, as well as, porewater sulfides (∑TSpw) and mesocosm water column O2 dynamics. Sulfide was injected into intact seagrass cores of T. testudinum exposing below-ground tissues to 2, 4, and 6 mM S2−, but rapid oxidation resulted in ∑TSpw < 1.5 mM. Hypersalinity at 65 psu lowered sulfide oxidation and significantly affected plant growth rates and quantum efficiencies (Fv/Fm < 0.70). The most depleted rhizome δ34S signatures were also observed at 65 psu, suggesting increased sulfide exposure. Hypoxia did not influence ∑TSpw and plant growth, but strengthened the hypersalinity response and decreased rhizome S0, indicating less efficient oxidation of ∑TSpw. Following nighttime hypoxia treatments, ecosystem level metabolism responded to salinity treatments. When O2 levels were reduced to 5 and 4 mg L−1, daytime O2 levels recovered to approximately 6 mg L−1; however, this recovery was more limited when O2 levels were lowered to 3 mg L−1. Subsequent to O2 reductions to 3 mg O2 L−1, nighttime O2 levels rose in the 35 and 45 psu tanks, stayed the same in the 55 psu tanks, and declined in the 65 psu tanks. Thus, hypersalinity at 65 psu affects T. testudinum's oxidizing capacity and places subtle demands on the positive O2 balance at an ecosystem level. This O2 demand may influence T. testudinum die-off events, particularly after periods of high temperature and salinity. We hypothesize that the interaction between hypersalinity and sulfide toxicity in T. testudinum is their synergistic effect on the critical O2 balance of the plant. 相似文献
10.
We previously reported that Neq A523R DNA polymerase is more efficient in PCR than wild-type Neq DNA polymerase, and amplifies products more rapidly. Neq A523R DNA polymerase also amplifies templates more rapidly than Pfu DNA polymerase, but has a lower fidelity than Pfu DNA polymerase. To improve product yield and the fidelity of amplification simultaneously, we constructed and characterized the double mutant Neq A523R/N540R. The yield of PCR products was greater for Neq A523R/N540R DNA polymerase than wild-type and other mutant DNA polymerases, and the Neq double mutant catalyzed amplification of a 12-kb PCR product from a lambda template with an extension time of 3 min. The PCR error rate of Neq A523R/N540R DNA polymerase (6.3 × 10−5) was roughly similar to that of Pfu DNA polymerase (4.8 × 10−5), but much lower than those of wild-type Neq DNA polymerase (57.2 × 10−5), Neq A523R DNA polymerase (13.1 × 10−5), and Neq N540R DNA polymerase (37.7 × 10−5). These results indicated that A523R and N540R mutations of Neq DNA polymerase had synergistic effects on its fidelity. 相似文献
11.
《Aquatic Botany》2002,72(3-4):219-233
We studied the potential role of dissolved inorganic carbon (DIC) in determining vegetation dominance of Potamogeton pectinatus L. and Chara aspera Deth. ex Willd. by monitoring the seasonal dynamics of DIC in a shallow lake and comparing the use of DIC of the two species. The HCO3−-concentration in summer dropped from 2.5 to <0.5 mM with seasonally increasing Chara biomass, whereas outside the vegetation concentrations remained at 2.5 mM. Inside Potamogeton spp. vegetation DIC decreased from 2.5 to ca. 0.75 mM HCO3−. A growth experiment showed ash-free biomass for P. pectinatus was nearly two times as high as for C. aspera at 3 mM HCO3−, but almost two times lower at 0.5 mM than at 3.0. In a separate experiment, P. pectinatus precultured at a relatively low HCO3−-level had a lower net photosynthetic rate (Pmax, 0.1 mmol O2 g−1 DW h−1) than C. aspera (Pmax, 0.1 mmol O2 g−1 DW h−1) over the range of HCO3−-concentrations tested (Pmax, 0.14 mmol O2 g−1 DW h−1). In response to CO2 no significant differences between the compensation points (P. pectinatus, 28 mM; C. aspera 66 mM), were observed, but the photosynthetic rate increased faster than for C. aspera than for P. pectinatus. Under field conditions, the use of CO2 is not important since inside vegetation CO2-concentrations were below 10 μM, and thus, not available for photosynthesis of either species during the main part of the growth season. It is suggested that C. aspera may be a better competitor for HCO3− than P. pectinatus in conditions with a low HCO3− supply. As HCO3− is a strong limiting factor for growth inside the vegetation and probably the only carbon source available, the superior ability of C. aspera to use HCO3− may be an important factor explaining its present dominance in Veluwemeer. 相似文献
12.
《Aquatic Botany》2005,83(3):187-192
We investigated the effect of intraspecific competition on growth parameters and photosynthesis of the salt marsh species Atriplex prostrata Boucher in order to distinguish the effects of density-dependent growth inhibition from salt stress. High plant density caused a reduction of 30% in height, 82% in stem dry mass, 80% in leaf dry mass, and 95% in root dry mass. High density also induced a pronounced 72% reduction in leaf area, 29% decrease in length of mature internodes and 50% decline in net photosynthetic rate. The alteration of net photosynthesis paralleled growth inhibition, decreasing from 7.6 ± 0.9 μmol CO2 m−2 s−1 at low density to 3.5 ± 0.4 μmol CO2 m−2 s−1 at high density, indicating growth inhibition caused by intraspecific competition is mainly due to a decline in net photosynthesis rate. Plants grown at high density also exhibited a reduction in stomatal conductance from 0.7 ± 0.1 mol H2O m−2 s−1 at low density to 0.3 ± 0.1 mol H2O m−2 s−1 at high density and a reduction in transpiration rate from 6.0 ± 0.3 mmol H2O m−2 s−1 at low density to 4.3 ± 0.3 mmol H2O m−2 s−1 at high density. Biomass production was inhibited by an increase in plant density, which reduced the rate of photosynthesis, stomatal conductance and leaf area of plants. 相似文献
13.
《DNA Repair》2016
Apurinic/apyrimidinic (AP) endonucleases are important DNA repair enzymes involved in two overlapping pathways: DNA glycosylase-initiated base excision (BER) and AP endonuclease-initiated nucleotide incision repair (NIR). In the BER pathway, AP endonucleases cleave DNA at AP sites and 3'-blocking moieties generated by DNA glycosylases, whereas in NIR, the same AP endonucleases incise DNA 5' to a wide variety of oxidized bases. The flowering plant Arabidopsis thaliana contains three genes encoding homologues of major human AP endonuclease 1 (APE1): Arp, Ape1L and Ape2. It has been shown that all three proteins contain AP site cleavage and 3'-repair phosphodiesterase activities; however, it was not known whether the plant AP endonucleases contain the NIR activity. Here, we report that ARP proteins from Arabidopsis and common wheat (Triticum aestivum) contain NIR and 3' → 5' exonuclease activities in addition to their AP endonuclease and 3'-repair phosphodiesterase functions. The steady-state kinetic parameters of reactions indicate that Arabidopsis ARP cleaves oligonucleotide duplexes containing α-anomeric 2'-deoxyadenosine (αdA) and 5,6-dihydrouridine (DHU) with efficiencies (kcat/KM = 134 and 7.3 μM−1·min−1, respectively) comparable to those of the human counterpart. However, the ARP-catalyzed 3'-repair phosphodiesterase and 3' → 5' exonuclease activities (kcat/KM = 314 and 34 μM−1·min−1, respectively) were about 10-fold less efficient as compared to those of APE1. Interestingly, homozygous A. thaliana arp–/– mutant exhibited high sensitivity to methyl methanesulfonate and tert-butyl hydroperoxide, but not to H2O2, suggesting that ARP is a major plant AP endonuclease that removes abasic sites and specific types of oxidative DNA base damage. Taken together, these data establish the presence of the NIR pathway in plants and suggest its possible role in the repair of DNA damage generated by oxidative stress. 相似文献
14.
《Enzyme and microbial technology》2005,36(2-3):307-313
Aerobic granulation is a process in which suspended biomass aggregate and form discrete well-defined granules in aerobic systems. To investigate the properties and kinetics of aerobic granular sludge, aerobic granules were cultivated with glucose synthetic wastewater in a series of sequencing batch reactors (SBR). The spherical shaped granules were observed on 8th day with the mean diameter of 0.1 mm. With the organic loading rate (OLR) being increased to 4.0 g COD L−1 d−1, aerobic granules grew matured with spherical shape. The size of granules ranged from 1.2 to 1.8 mm, and the corresponding settling velocity of individual granule was 24.2–36.4 m h−1. The oxygen utilization rate (OUR) of mature granules was 41.90 g O2 kg MLSS−1 h−1, which was two times higher than that of activated sludge (18.32 g O2 kg MLSS−1 h−1). The experimental data indicated that the substrate utilization and biomass growth kinetics generally followed Monod's kinetics model. The corresponding kinetic coefficients of k (maximum specific substrate utilization rate), Ks (half velocity coefficient), Y (growth yield coefficient) and Kd (decay coefficient) were determined as follows, kc = 23.65 d−1, Kc = 3367.05 mg L−1, KN = 0.038 d−1, KN = 29.65 mg L−1, Y = 0.1927–0.2022 mg MMLS (mg COD)−1 and Kd = 0.00845–0.0135 d−1, respectively. Those properties of aerobic granules made aerobic granules system had a short setup period, high substrate utilization rate and low sludge production. 相似文献
15.
《Harmful algae》2014
Benthic dinoflagellates of the genus Ostreopsis are found all over the world in temperate, subtropical, and tropical coastal regions. Our recent studies revealed that a putative “cryptic” species of Ostreopsis ovata is present widely along Japanese coasts. This organism, Ostreopsis sp. 1, possesses palytoxin analogs and thus its toxic blooms may be responsible for potential toxification of marine organisms. To evaluate the bloom dynamics of Ostreopsis sp. 1, the present study examined the growth responses of Ostreopsis sp. 1 strain s0716 to various light intensities (photon flux densities: μmol photons m−2 s−1) using a newly devised photoirradiation-culture system. This novel system has white light-emitting diodes (LEDs) capable of more closely simulating the wavelength spectrum of light entering the oceanic water column than do fluorescent tubes and halogen lamps. In this system, the light intensity of the white LEDs was reduced through two polarizing filters by varying the rotation angles of the filters. Thereby, the new system was capable of culturing microalgae under well-controlled light intensity conditions. Ostreopsis sp. 1 grew proportionally when light intensity was increased from 49.5 to 199 μmol photons m−2 s−1, but its growth appeared to be inhibited slightly at ≥263 μmol photons m−2 s−1. The relationship between observed growth rates and light intensity was calculated at R > 0.99 (P < 0.01) using a regression analysis with a modified equation of the photosynthesis-light intensity (P-L) model. The equation determined the critical light intensities for growth of Ostreopsis sp. 1 and the organism's growth potential as follows: (1) the threshold light intensity for growth: 29.8 μmol photons m−2 s−1; (2) the optimum light intensity (Lm) giving the maximum growth rate (μmax = 0.659 divisions day−1): 196 μmol photons m−2 s−1; (3) the optimum light intensity range (Lopt) giving ≥95% μmax: 130–330 μmol photons m−2 s−1; (4) the semi-optimum range (Lsopt) giving ≥80% μmax: 90 to over 460 μmol photons m−2 s−1. The Lsopt represents 4.5–23% ambient light intensity present in surface waters off of a temperate region of the Japanese coast, Tosa Bay; putatively, this semi-optimum range of light intensity appears at depth of 12.9–27.8 m. Considering these issues, our data indicate that Ostreopsis sp. 1 in coastal environments may form blooms at ca. ∼28 m depth in regions along Japanese coasts. 相似文献
16.
Chlorella vulgaris was cultivated in two different 2.0 L-helicoidal and horizontal photobioreactors at 5 klux using the bicarbonate contained in the medium and ambient air as the main CO2 sources. The influence of bicarbonate concentration on biomass growth as well as lipid content and profile was first investigated in shake flasks, where the stationary phase was achieved in about one half the time required by the control. The best NaHCO3 concentration (0.2 g L−1) was then used in both photobioreactors. While the fed-batch run performed in the helicoidal photobioreactor provided the best result in terms of biomass productivity, which was (84.8 mg L−1 d−1) about 2.5-fold that of the batch run, the horizontal configuration ensured the highest lipid productivity (10.3 mg L−1 d−1) because of a higher lipid content of biomass (22.8%). These preliminary results suggest that the photobioreactor configuration is a key factor either for the growth or the composition of this microalga. The lipid quality of C. vulgaris biomass grown in both photobioreactors is expected to meet the standards for biodiesel, especially in the case of the helicoidal configuration, provided that further efforts will be made to optimize the conditions for its production as a biodiesel source. 相似文献
17.
Functional indicators are being increasingly used to assess waterway health but their responses to pressure in non-wadeable rivers have not been widely documented or applied in modern survey designs that provide unbiased estimates of extent. This study tests the response of river metabolism and loss in cotton strip tensile strength across a land use pressure gradient in non-wadeable rivers of northern New Zealand, and reports extent estimates for river metabolism and decomposition rates. Following adjustment for probability of selection, ecosystem respiration (ER) and gross primary production (GPP) for the target population of order 5–7 non-wadeable rivers averaged −7.3 and 4.8 g O2 m−2 d−1, respectively, with average P/R < 1 indicating dominance by heterotrophic processes. Ecosystem respiration was <−3.3 g O2 m−2 d−1 for 75% of non-wadeable river length with around 20% of length between −10 and −20 g O2 m−2 d−1. Cumulative distribution functions of cotton strength loss estimates indicated a more-or-less linear relationship with river km reflecting an even spread of decay rates (range in k 0.0007–0.2875 d−1) across non-wadeable rivers regionally. A non-linear relationship with land cover was detected for GPP which was typically <5 g O2 m−2 d−1 where natural vegetation cover was below 20% and greater than 80% of upstream catchment area. For cotton strength loss, the relationship with land cover was wedge-shaped such that sites with >60% natural cover had low decay rates (<0.02 d−1) with variability below this increasing as natural cover declined. Using published criteria for assessing waterway health based on ER and GPP, 232–298 km (20–29%) of non-wadeable river length was considered to have severely impaired ecosystem functioning, and 436–530 km (42–50%) had no evidence of impact on river metabolism. 相似文献
18.
《Animal Feed Science and Technology》1998,75(2):145-155
Buffered propionic acid (BPA) was evaluated as a potential treatment for the elimination of Salmonella spp. in poultry mash. A primary poultry isolate marker strain of Salmonella typhimurium was added as either a broth or in a dry chalk carrier form to poultry mash containing soybean meal as a protein supplement. The mash was supplemented with buffered propionic acid at 2, 4, 6, 8, 10, 20, 30, 50 and 100 g kg−1 diet and samples were enumerated for indigenous aerobic bacteria, fungi and the S. typhimurium marker strain. Total indigenous aerobic bacteria and fungal populations were generally decreased by addition of more than 20 g BPA kg−1, but an addition of 100 g BPA kg−1 mash was usually required to achieve reductions of approximately 90% of indigenous aerobic bacteria and 99% of indigenous fungi. After 7 days of storage, 8 g BPA kg−1 mash also reduced S. typhimurium populations by more than 90% in mash inoculated via chalk, while at least 50 g BPA kg−1 mash was required to provide the same level of reduction in mash inoculated with a liquid culture of S. typhimurium. Although BPA does not appear to be an overly effective antimicrobial agent with respect to indigenous aerobic bacterial populations in animal feed, higher concentrations may have the potential for reducing fungal and Salmonella spp. contamination in poultry mash. 相似文献
19.
《Bioorganic & medicinal chemistry letters》2014,24(1):173-176
l-Arabinitol 4-dehydrogenase (LAD) from Hypocrea jecorina (HjLAD) was cloned and overexpressed in Escherichia coli BL21 (DE3). The kinetics of l-arabinitol oxidation by NAD+, catalyzed by HjLAD, was studied within the pH range of 7.0–9.5 at 25 °C. The turnover number (kcat) and the catalytic efficiency (kcat/Km) were 4200 min−1 and 290 mM−1 min−1, respectively. HjLAD showed the highest turnover number and catalytic efficiency among all previously characterized LADs. In further application of HjLAD, rare l-sugar l-xylulose was produced by the enzymatic oxidation of arabinitol to give a yield of approximately 86%. 相似文献
20.
《Aquatic Botany》2005,81(4):353-366
Carbon fixation and allocation were studied using 13C incubation and leaf marking techniques in mature monospecific stands of Enhalus acoroides L.f. Royle in August 1998 and January 1999 in Banten Bay, Indonesia. The highest rate of 13C uptake (>0.008 g 13C g C−1 d−1) was found in the middle to distal parts of leaves of E. acoroides. Young and senescing leaves numbers had lower 13C incorporation compared to mature leaves. The incorporation of 13C by epiphytes on the leaves was higher than that of the leaves themselves (>0.01 g 13C g C−1 d−1). The results showed that turbidity of the water influenced the leaf growth, productivity and Relative Growth Rate of E. acoroides, which were lower at Kepuh Island, the more turbid site. However, at Kepuh Island, where the water column was turbid, the plant could still harvest sufficient light for an uptake rate of 13C, higher than the uptake rates at Kubur and Panjang Islands, stations with a much more transparent water column (on average 0.0047 g 13C g C−1 d−1 at Kepuh Island, versus 0.0045 g 13C g C−1 d−1 at Panjang Island and 0.0034 g 13C g C−1 d−1 at Kubur Island). There was evidence that 13C was exported from the incubated shoots to the roots and rhizomes and to neighboring shoots of E. acoroides in clear water, but not in turbid water. We suggest that single shoots of E. acoroides are able to grow in turbid water under low light conditions. They assimilate sufficient carbon for their own maintenance but are not able to export to neighboring plant parts. 相似文献