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1.
Although previous research has demonstrated that NO 3− inhibits microbial Fe(III) reduction in laboratory cultures and natural sediments, the mechanisms of this inhibition have not been fully studied in an environmentally relevant medium that utilizes solid-phase, iron oxide minerals as a Fe(III) source. To study the dynamics of Fe and NO 3− biogeochemistry when ferric (hydr)oxides are used as the Fe(III) source, Shewanella putrefaciens 200 was incubated under anoxic conditions in a low-ionic-strength, artificial groundwater medium with various amounts of NO 3− and synthetic, high-surface-area goethite. Results showed that the presence of NO 3− inhibited microbial goethite reduction more severely than it inhibited microbial reduction of the aqueous or microcrystalline sources of Fe(III) used in other studies. More interestingly, the presence of goethite also resulted in a twofold decrease in the rate of NO 3− reduction, a 10-fold decrease in the rate of NO 2− reduction, and a 20-fold increase in the amounts of N 2O produced. Nitrogen stable isotope experiments that utilized δ 15N values of N 2O to distinguish between chemical and biological reduction of NO 2− revealed that the N 2O produced during NO 2− or NO 3− reduction in the presence of goethite was primarily of abiotic origin. These results indicate that concomitant microbial Fe(III) and NO 3− reduction produces NO 2− and Fe(II), which then abiotically react to reduce NO 2− to N 2O with the subsequent oxidation of Fe(II) to Fe(III). 相似文献
2.
A lithotrophic freshwater Beggiatoa strain was enriched in O 2-H 2S gradient tubes to investigate its ability to oxidize sulfide with NO 3− as an alternative electron acceptor. The gradient tubes contained different NO 3− concentrations, and the chemotactic response of the Beggiatoa mats was observed. The effects of the Beggiatoa sp. on vertical gradients of O 2, H 2S, pH, and NO 3− were determined with microsensors. The more NO 3− that was added to the agar, the deeper the Beggiatoa filaments glided into anoxic agar layers, suggesting that the Beggiatoa sp. used NO 3− to oxidize sulfide at depths below the depth that O 2 penetrated. In the presence of NO 3− Beggiatoa formed thick mats (>8 mm), compared to the thin mats (ca. 0.4 mm) that were formed when no NO 3− was added. These thick mats spatially separated O 2 and sulfide but not NO 3− and sulfide, and therefore NO 3− must have served as the electron acceptor for sulfide oxidation. This interpretation is consistent with a fourfold-lower O 2 flux and a twofold-higher sulfide flux into the NO 3−-exposed mats compared to the fluxes for controls without NO 3−. Additionally, a pronounced pH maximum was observed within the Beggiatoa mat; such a pH maximum is known to occur when sulfide is oxidized to S 0 with NO 3− as the electron acceptor. 相似文献
3.
Cystathionine β-synthase (CBS) is a pyridoxal phosphate-dependent enzyme that catalyzes the condensation of homocysteine with serine or with cysteine to form cystathionine and either water or hydrogen sulfide, respectively. Human CBS possesses a noncatalytic heme cofactor with cysteine and histidine as ligands, which in its oxidized state is relatively unreactive. Ferric CBS (Fe(III)-CBS) can be reduced by strong chemical and biochemical reductants to Fe(II)-CBS, which can bind carbon monoxide (CO) or nitric oxide (NO •), leading to inactive enzyme. Alternatively, Fe(II)-CBS can be reoxidized by O 2 to Fe(III)-CBS, forming superoxide radical anion (O 2˙̄). In this study, we describe the kinetics of nitrite (NO 2−) reduction by Fe(II)-CBS to form Fe(II)NO •-CBS. The second order rate constant for the reaction of Fe(II)-CBS with nitrite was obtained at low dithionite concentrations. Reoxidation of Fe(II)NO •-CBS by O 2 showed complex kinetic behavior and led to peroxynitrite (ONOO −) formation, which was detected using the fluorescent probe, coumarin boronic acid. Thus, in addition to being a potential source of superoxide radical, CBS constitutes a previously unrecognized source of NO • and peroxynitrite. 相似文献
4.
Experiments demonstrated that Beggiatoa could induce a H 2S-depleted suboxic zone of more than 10 mm in marine sediments and cause a divergence in sediment NO 3− reduction from denitrification to dissimilatory NO 3− reduction to ammonium. pH, O 2, and H 2S profiles indicated that the bacteria oxidized H 2S with NO 3− and transported S 0 to the sediment surface for aerobic oxidation. 相似文献
5.
BioDeNOx is an integrated physicochemical and biological process for the removal of nitrogen oxides (NOx) from flue gases. In this process, the flue gas is purged through a scrubber containing a solution of Fe(II)EDTA 2−, which binds the NOx to form an Fe(II)EDTA·NO 2− complex. Subsequently, this complex is reduced in the bioreactor to dinitrogen by microbial denitrification. Fe(II)EDTA 2−, which is oxidized to Fe(III)EDTA − by oxygen in the flue gas, is regenerated by microbial iron reduction. In this study, the microbial communities of both lab- and pilot-scale reactors were studied using culture-dependent and -independent approaches. A pure bacterial strain, KT-1, closely affiliated by 16S rRNA analysis to the gram-positive denitrifying bacterium Bacillus azotoformans, was obtained. DNA-DNA homology of the isolate with the type strain was 89%, indicating that strain KT-1 belongs to the species B. azotoformans. Strain KT-1 reduces Fe(II)EDTA·NO 2− complex to N 2 using ethanol, acetate, and Fe(II)EDTA 2− as electron donors. It does not reduce Fe(III)EDTA −. Denaturing gradient gel electrophoresis analysis of PCR-amplified 16S rRNA gene fragments showed the presence of bacteria closely affiliated with members of the phylum Deferribacteres, an Fe(III)-reducing group of bacteria. Fluorescent in situ hybridization with oligonucleotide probes designed for strain KT-1 and members of the phylum Deferribacteres showed that the latter were more dominant in both reactors. 相似文献
6.
We screened soybean rhizobia originating from three germplasm collections for the ability to grow anaerobically in the presence of NO 3− and for differences in final product formation from anaerobic NO 3− metabolism. Denitrification abilities of selected strains as free-living bacteria and as bacteroids were compared. Anaerobic growth in the presence of NO 3− was observed in 270 of 321 strains of soybean rhizobia. All strains belonging to the 135 serogroup did not grow anaerobically in the presence of NO 3−. An investigation with several strains indicated that bacteria not growing anaerobically in the presence of NO 3− also did not utilize NO 3− as the sole N source aerobically. An exception was strain USDA 33, which grew on NO 3− but failed to denitrify. Dissimilation of NO 3− by the free-living cultures proceeded without the significant release of intermediate products. Nitrous oxide reductase was inhibited by C 2H 2, but preceding steps of denitrification were not affected. Final products of denitrification were NO 2−, N 2O, or N 2; serogroups 31, 46, 76, and 94 predominantly liberated NO 2−, whereas evolution of N 2 was prevalent in serogroups 110 and 122, and all three were formed as final products by strains belonging to serogroups 6 and 123. Anaerobic metabolism of NO 3− by bacteroid preparations of Bradyrhizobium japonicum proceeded without delay and was evident by NO 2− accumulation irrespective of which final product was formed by the strain as free-living bacteria. Anaerobic C 2H 2 reduction in the presence of NO 3− was observed in bacteroid preparations capable of NO 3− respiration but was absent in bacteria that were determined to be deficient in dissimilatory nitrate reductase. 相似文献
7.
A recent study (D. C. Cooper, F. W. Picardal, A. Schimmelmann, and A. J. Coby, Appl. Environ. Microbiol. 69:3517-3525, 2003) has shown that NO 3− and NO 2− (NO x−) reduction by Shewanella putrefaciens 200 is inhibited in the presence of goethite. The hypothetical mechanism offered to explain this finding involved the formation of a Fe(III) (hydr)oxide coating on the cell via the surface-catalyzed, abiotic reaction between Fe 2+ and NO 2−. This coating could then inhibit reduction of NO x− by physically blocking transport into the cell. Although the data in the previous study were consistent with such an explanation, the hypothesis was largely speculative. In the current work, this hypothesis was tested and its environmental significance explored through a number of experiments. The inhibition of ~3 mM NO 3− reduction was observed during reduction of a variety of Fe(III) (hydr)oxides, including goethite, hematite, and an iron-bearing, natural sediment. Inhibition of oxygen and fumarate reduction was observed following treatment of cells with Fe 2+ and NO 2−, demonstrating that utilization of other soluble electron acceptors could also be inhibited. Previous adsorption of Fe 2+ onto Paracoccus denitrificans inhibited NO x− reduction, showing that Fe(II) can reduce rates of soluble electron acceptor utilization by non-iron-reducing bacteria. NO 2− was chemically reduced to N 2O by goethite or cell-sorbed Fe 2+, but not at appreciable rates by aqueous Fe 2+. Transmission and scanning electron microscopy showed an electron-dense, Fe-enriched coating on cells treated with Fe 2+ and NO 2−. The formation and effects of such coatings underscore the complexity of the biogeochemical reactions that occur in the subsurface. 相似文献
8.
Microbial communities have the potential to control the biogeochemical fate of some radionuclides in contaminated land scenarios or in the vicinity of a geological repository for radioactive waste. However, there have been few studies of ionizing radiation effects on microbial communities in sediment systems. Here, acetate and lactate amended sediment microcosms irradiated with gamma radiation at 0.5 or 30 Gy h −1 for 8 weeks all displayed NO 3− and Fe(III) reduction, although the rate of Fe(III) reduction was decreased in 30-Gy h −1 treatments. These systems were dominated by fermentation processes. Pyrosequencing indicated that the 30-Gy h −1 treatment resulted in a community dominated by two Clostridial species. In systems containing no added electron donor, irradiation at either dose rate did not restrict NO 3−, Fe(III), or SO 42− reduction. Rather, Fe(III) reduction was stimulated in the 0.5-Gy h −1-treated systems. In irradiated systems, there was a relative increase in the proportion of bacteria capable of Fe(III) reduction, with Geothrix fermentans and Geobacter sp. identified in the 0.5-Gy h −1 and 30-Gy h −1 treatments, respectively. These results indicate that biogeochemical processes will likely not be restricted by dose rates in such environments, and electron accepting processes may even be stimulated by radiation. 相似文献
9.
The contribution of the biochemical pathways nitrification, denitrification, and dissimilatory NO 3− reduction to NH 4+ (DNRA) to the accumulation of NO 2− in freshwaters is governed by the species compositions of the bacterial populations resident in the sediments, available carbon (C) and nitrogen (N) substrates, and environmental conditions. Recent studies of major rivers in Northern Ireland have shown that high NO 2− concentrations found in summer, under warm, slow-flowing conditions, arise from anaerobic NO 3− reduction. Locally, agricultural pollutants entering rivers are important C and N sources, providing ideal substrates for the aquatic bacteria involved in cycling of N. In this study a range of organic C compounds commonly found in agricultural pollutants were provided as energy sources in 48-h incubation experiments to investigate if the chemical compositions of the pollutants affected which NO 3− reduction pathway was followed and influenced subsequent NO 2− accumulation. Carbon stored within the sediments was sufficient to support DNRA and denitrifier populations, and the resulting NO 2− peak (80 μg of N liter −1 [approximate]) observed at 24 h was indicative of the simultaneous activities of both bacterial groups. The value of glycine as an energy source for denitrification or DNRA appeared to be limited, but glycine was an important source of additional N. Glucose was an efficient substrate for both the denitrification and DNRA pathways, with a NO 2− peak of 160 μg of N liter −1 noted at 24 h. Addition of formate and acetate stimulated continuous NO 2− production throughout the 48-h period, caused by partial inhibition of the denitrification pathway. The formate treatment resulted in a high NO 2− accumulation (1,300 μg of N liter −1 [approximate]), and acetate treatment resulted in a low NO 2− concentration (<100 μg of N liter −1). 相似文献
10.
A modification of the Adler capillary assay was used to evaluate the chemotactic responses of several denitrifiers to nitrate and nitrite. Strong positive chemotaxis was observed to NO 3− and NO 2− by soil isolates of Pseudomonas aeruginosa, Pseudomonas fluorescens, and Pseudomonas stutzeri, with the peak response occurring at 10 −3 M for both attractants. In addition, a strong chemoattraction to serine (peak response at 10 −2 M), tryptone, and a soil extract, but not to NH 4+, was observed for all denitrifiers tested. Chemotaxis was not dependent on a previous growth on NO 3−, NO 2−, or a soil extract, and the chemoattraction to NO 3− occurred when the bacteria were grown aerobically or anaerobically. However, the best response to NO 3− was usually observed when the cells were grown aerobically with 10 mM NO 3− in the growth medium. Capillary tubes containing 10 −3 M NO 3− submerged into soil-water mixtures elicited a significant chemotactic response to NO 3− by the indigenous soil microflora, the majority of which were Pseudomonas spp. A chemotactic strain of P. fluorescens also was shown to survive significantly better in aerobic and anaerobic soils than was a nonmotile strain of the same species. Both strains had equal growth rates in liquid cultures. Thus, chemotaxis may be one mechanism by which denitrifiers successfully compete for available NO 3− and NO 2−, and which may facilitate the survival of naturally occurring populations of some denitrifiers. 相似文献
11.
The role of NO 3− and NO 2− in the induction of nitrite reductase (NiR) activity in detached leaves of 8-day-old barley ( Hordeum vulgare L.) seedlings was investigated. Barley leaves contained 6 to 8 micromoles NO 2−/gram fresh weight × hour of endogenous NiR activity when grown in N-free solutions. Supply of both NO 2− and NO 3− induced the enzyme activity above the endogenous levels (5 and 10 times, respectively at 10 millimolar NO 2− and NO 3− over a 24 hour period). In NO 3−-supplied leaves, NiR induction occurred at an ambient NO 3− concentration of as low as 0.05 millimolar; however, no NiR induction was found in leaves supplied with NO 2− until the ambient NO 2− concentration was 0.5 millimolar. Nitrate accumulated in NO 2−-fed leaves. The amount of NO 3− accumulating in NO 2−-fed leaves induced similar levels of NiR as did equivalent amounts of NO 3− accumulating in NO 3−-fed leaves. Induction of NiR in NO 2−-fed leaves was not seen until NO 3− was detectable (30 nanomoles/gram fresh weight) in the leaves. The internal concentrations of NO 3−, irrespective of N source, were highly correlated with the levels of NiR induced. When the reduction of NO 3− to NO 2− was inhibited by WO 42−, the induction of NiR was inhibited only partially. The results indicate that in barley leaves NiR is induced by NO 3− directly, i.e. without being reduced to NO 2−, and that absorbed NO 2− induces the enzyme activity indirectly after being oxidized to NO 3− within the leaf. 相似文献
12.
Studies were carried out to elucidate the nature and importance of Fe 3+ reduction in anaerobic slurries of marine surface sediment. A constant accumulation of Fe 2+ took place immediately after the endogenous NO 3− was depleted. Pasteurized controls showed no activity of Fe 3+ reduction. Additions of 0.2 mM NO 3− and NO 2− to the active slurries arrested the Fe 3+ reduction, and the process was resumed only after a depletion of the added compounds. Extended, initial aeration of the sediment did not affect the capacity for reduction of NO 3− and Fe 3+, but the treatments with NO 3− increased the capacity for Fe 3+ reduction. Addition of 20 mM MoO 42− completely inhibited the SO 42− reduction, but did not affect the reduction of Fe 3+. The process of Fe 3+ reduction was most likely associated with the activity of facultative anaerobic, NO 3−-reducing bacteria. In surface sediment, the bulk of the Fe 3+ reduction may be microbial, and the process may be important for mineralization in situ if the availability of NO 3− is low. 相似文献
13.
Mining-impacted sediments of Lake Coeur d'Alene, Idaho, contain more than 10% metals on a dry weight basis, approximately 80% of which is iron. Since iron (hydr)oxides adsorb toxic, ore-associated elements, such as arsenic, iron (hydr)oxide reduction may in part control the mobility and bioavailability of these elements. Geochemical and microbiological data were collected to examine the ecological role of dissimilatory Fe(III)-reducing bacteria in this habitat. The concentration of mild-acid-extractable Fe(II) increased with sediment depth up to 50 g kg −1, suggesting that iron reduction has occurred recently. The maximum concentrations of dissolved Fe(II) in interstitial water (41 mg liter −1) occurred 10 to 15 cm beneath the sediment-water interface, suggesting that sulfidogenesis may not be the predominant terminal electron-accepting process in this environment and that dissolved Fe(II) arises from biological reductive dissolution of iron (hydr)oxides. The concentration of sedimentary magnetite (Fe 3O 4), a common product of bacterial Fe(III) hydroxide reduction, was as much as 15.5 g kg −1. Most-probable-number enrichment cultures revealed that the mean density of Fe(III)-reducing bacteria was 8.3 × 10 5 cells g (dry weight) of sediment −1. Two new strains of dissimilatory Fe(III)-reducing bacteria were isolated from surface sediments. Collectively, the results of this study support the hypothesis that dissimilatory reduction of iron has been and continues to be an important biogeochemical process in the environment examined. 相似文献
14.
We examined nitrate-dependent Fe 2+ oxidation mediated by anaerobic ammonium oxidation (anammox) bacteria. Enrichment cultures of “ Candidatus Brocadia sinica” anaerobically oxidized Fe 2+ and reduced NO 3− to nitrogen gas at rates of 3.7 ± 0.2 and 1.3 ± 0.1 (mean ± standard deviation [SD]) nmol mg protein −1 min −1, respectively (37°C and pH 7.3). This nitrate reduction rate is an order of magnitude lower than the anammox activity of “ Ca. Brocadia sinica” (10 to 75 nmol NH 4+ mg protein −1 min −1). A 15N tracer experiment demonstrated that coupling of nitrate-dependent Fe 2+ oxidation and the anammox reaction was responsible for producing nitrogen gas from NO 3− by “ Ca. Brocadia sinica.” The activities of nitrate-dependent Fe 2+ oxidation were dependent on temperature and pH, and the highest activities were seen at temperatures of 30 to 45°C and pHs ranging from 5.9 to 9.8. The mean half-saturation constant for NO 3− ± SD of “ Ca. Brocadia sinica” was determined to be 51 ± 21 μM. Nitrate-dependent Fe 2+ oxidation was further demonstrated by another anammox bacterium, “ Candidatus Scalindua sp.,” whose rates of Fe 2+ oxidation and NO 3− reduction were 4.7 ± 0.59 and 1.45 ± 0.05 nmol mg protein −1 min −1, respectively (20°C and pH 7.3). Co-occurrence of nitrate-dependent Fe 2+ oxidation and the anammox reaction decreased the molar ratios of consumed NO 2− to consumed NH 4+ (ΔNO 2−/ΔNH 4+) and produced NO 3− to consumed NH 4+ (ΔNO 3−/ΔNH 4+). These reactions are preferable to the application of anammox processes for wastewater treatment. 相似文献
15.
The effects of several photosynthetic inhibitors and uncouplers of oxidative phosphorylation on NO 3− and NO 2− assimilation were studied using detached barley ( Hordeum vulgare L. cv Numar) leaves in which only endogenous NO 3− or NO 2− were available for reduction. Uncouplers of oxidative phosphorylation greatly increased NO 3− reduction in both light and darkness, while photosynthetic inhibitors did not. The NO2− concentration in the control leaves was very low in both light and darkness; 98% or more of the NO2− formed from NO3− was further assimilated in control leaves. More NO2− accumulated in the leaves in light and darkness in the presence of photosynthetic inhibitors. Of this NO2−, 94% or more was further assimilated. It appears that metabolites, either external or internal to the chloroplast, capable of reducing NADP (which, in turn, could reduce ferredoxin via NADP reductase) might support NO2− reduction in darkness and light when photosynthetic electron flow is inhibited by photosynthetic inhibitors. Nitrite assimilation was much more sensitive to uncouplers in darkness than in light: in darkness, 74% or more of NO2− formed from NO3− was further assimilated, whereas in light, 95% or more of the NO2− was further assimilated. 相似文献
16.
Nitrite oxidation is the second step of nitrification. It is the primary source of oceanic nitrate, the predominant form of bioavailable nitrogen in the ocean. Despite its obvious importance, nitrite oxidation has rarely been investigated in marine settings. We determined nitrite oxidation rates directly in 15N-incubation experiments and compared the rates with those of nitrate reduction to nitrite, ammonia oxidation, anammox, denitrification, as well as dissimilatory nitrate/nitrite reduction to ammonium in the Namibian oxygen minimum zone (OMZ). Nitrite oxidation (⩽372 n M NO 2− d −1) was detected throughout the OMZ even when in situ oxygen concentrations were low to non-detectable. Nitrite oxidation rates often exceeded ammonia oxidation rates, whereas nitrate reduction served as an alternative and significant source of nitrite. Nitrite oxidation and anammox co-occurred in these oxygen-deficient waters, suggesting that nitrite-oxidizing bacteria (NOB) likely compete with anammox bacteria for nitrite when substrate availability became low. Among all of the known NOB genera targeted via catalyzed reporter deposition fluorescence in situ hybridization, only Nitrospina and Nitrococcus were detectable in the Namibian OMZ samples investigated. These NOB were abundant throughout the OMZ and contributed up to ∼9% of total microbial community. Our combined results reveal that a considerable fraction of the recently recycled nitrogen or reduced NO 3− was re-oxidized back to NO 3− via nitrite oxidation, instead of being lost from the system through the anammox or denitrification pathways. 相似文献
17.
Steady state cultures of Anabaena flos-aquae were established over a wide range of phosphate-limited growth rates while N was supplied as either NH 3, NO 3−, or N 2 gas. At growth rates greater than 0.03 per hour, rates of gross and net carbon fixation were similar on all N sources. However, at lower growth rates (<0.03 per hour) in the NO 3− and N 2 cultures, gross photosynthesis greatly exceeded net photosynthesis. The increase in photosynthetic O 2 evolution with growth rate was greatest when N requirements were met by NO 3− and least when met by NH 3. These results were combined with previously reported measurements of cellular chemical composition, N assimilation, and acetylene reduction (Layzell, Turpin, Elrifi 1985 Plant Physiol 78: 739-745) to construct empirical models of carbon and energy flow for cultures grown at 30, 60, and 100% of their maximal growth rate on all N sources. The models suggested that over this growth range, 89 to 100% of photodriven electrons were allocated to biomass production in the NH 3 cells, whereas only 49 to 74% and 54 to 90% were partitioned to biomass in the NO 3−-and N 2-grown cells, respectively. The models were used to estimate the relative contribution of active, maintenance, and establishment costs associated with NO 3− and N 2 assimilation over the entire range of growth rates. The models showed that the relative contribution of the component costs of N assimilation were growth rate dependent. At higher growth rates, the major costs for NO 3− assimilation were the active costs, while in N 2-fixing cultures the major energetic requirements were those associated with heterocyst establishment and maintenance. It was concluded that compared with NO 3− assimilation, N 2 fixation was energetically unfavorable due to the costs of heterocyst establishment and maintenance, rather than the active costs of N 2 assimilation. 相似文献
18.
Filamentous sulfur bacteria of the genus Thioploca occur as dense mats on the continental shelf off the coast of Chile and Peru. Since little is known about their nitrogen, sulfur, and carbon metabolism, this study was undertaken to investigate their (eco)physiology. Thioploca is able to store internally high concentrations of sulfur globules and nitrate. It has been previously hypothesized that these large vacuolated bacteria can oxidize sulfide by reducing their internally stored nitrate. We examined this nitrate reduction by incubation experiments of washed Thioploca sheaths with trichomes in combination with 15N compounds and mass spectrometry and found that these Thioploca samples produce ammonium at a rate of 1 nmol min −1 mg of protein −1. Controls showed no significant activity. Sulfate was shown to be the end product of sulfide oxidation and was observed at a rate of 2 to 3 nmol min −1 mg of protein −1. The ammonium and sulfate production rates were not influenced by the addition of sulfide, suggesting that sulfide is first oxidized to elemental sulfur, and in a second independent step elemental sulfur is oxidized to sulfate. The average sulfide oxidation rate measured was 5 nmol min −1 mg of protein −1 and could be increased to 10.7 nmol min −1 mg of protein −1 after the trichomes were starved for 45 h. Incorporation of 14CO 2 was at a rate of 0.4 to 0.8 nmol min −1 mg of protein −1, which is half the rate calculated from sulfide oxidation. [2- 14C]acetate incorporation was 0.4 nmol min −1 mg of protein −1, which is equal to the CO 2 fixation rate, and no 14CO 2 production was detected. These results suggest that Thioploca species are facultative chemolithoautotrophs capable of mixotrophic growth. Microautoradiography confirmed that Thioploca cells assimilated the majority of the radiocarbon from [2- 14C]acetate, with only a minor contribution by epibiontic bacteria present in the samples. 相似文献
19.
Phosphate-limited chemostat cultures were used to study cell growth and N assimilation in Anabaena flos-aquae under various N sources to determine the relative energetic costs associated with the assimilation of NH 3, NO 3−, or N 2. Expressed as a function of relative growth rate, steady state cellular P contents and PO 4 assimilation rates did not vary with N-source. However, N-source did alter the maximal PO 4-limited growth rate achieved by the cultures: the NO 3− and N 2 cultures attained only 97 and 80%, respectively, of the maximal growth rate of the NH 3 grown cells. Cellular biomass and C contents did not vary with growth rate, but changed with N source. The NO 3−-grown cells were the smallest (627 ± 34 micromoles C · 10 −9 cells), while NH 3-grown cells were largest (900 ± 44 micromoles C · 10 −9 cells) and N 2-fixing cells were intermediate (726 ± 48 micromoles C · 10 −9 cells) in size. In the NO 3−-and N 2-grown cultures, N content per cell was only 57 and 63%, respectively, of that in the NH 3-grown cells. Heterocysts were absent in NH 3-grown cultures but were present in both the N 2 and NO 3− cultures. In the NO 3−-grown cultures C 2H 2 reduction was detected only at high growth rates, where it was estimated to account for a maximum of 6% of the N assimilated. In the N 2-fixing cultures the acetylene:N 2 ratio varied from 3.4:1 at lower growth rates to 3.0:1 at growth rates approaching maximal. Compared with NH3, the assimilation of NO3− and N2 resulted either in a decrease in cellular C (NO3− and N2 cultures) or in a lower maximal growth rate (N2 culture only). The observed changes in cell C content were used to calculate the net cost (in electron pair equivalents) associated with growth on NO3− or N2 compared with NH3. 相似文献
20.
Nitrogen (N) is an essential nutrient in the sea and its distribution is controlled by microorganisms. Within the N cycle, nitrite (NO 2−) has a central role because its intermediate redox state allows both oxidation and reduction, and so it may be used by several coupled and/or competing microbial processes. In the upper water column and oxygen minimum zone (OMZ) of the eastern tropical North Pacific Ocean (ETNP), we investigated aerobic NO 2− oxidation, and its relationship to ammonia (NH 3) oxidation, using rate measurements, quantification of NO 2−-oxidizing bacteria via quantitative PCR (QPCR), and pyrosequencing. 15NO 2− oxidation rates typically exhibited two subsurface maxima at six stations sampled: one located below the euphotic zone and beneath NH 3 oxidation rate maxima, and another within the OMZ. 15NO 2− oxidation rates were highest where dissolved oxygen concentrations were <5 μ M, where NO 2− accumulated, and when nitrate (NO 3−) reductase genes were expressed; they are likely sustained by NO 3− reduction at these depths. QPCR and pyrosequencing data were strongly correlated ( r2=0.79), and indicated that Nitrospina bacteria numbered up to 9.25% of bacterial communities. Different Nitrospina groups were distributed across different depth ranges, suggesting significant ecological diversity within Nitrospina as a whole. Across the data set, 15NO 2− oxidation rates were decoupled from 15NH 4+ oxidation rates, but correlated with Nitrospina ( r2=0.246, P<0.05) and NO 2− concentrations ( r2=0.276, P<0.05). Our findings suggest that Nitrospina have a quantitatively important role in NO 2− oxidation and N cycling in the ETNP, and provide new insight into their ecology and interactions with other N-cycling processes in this biogeochemically important region of the ocean. 相似文献
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