54 research outputs found
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Effect of moisture, temperature, and sulfur dioxide on color of dried apples
When sulfur dioxide is used in commercial drying of fruits in
combination with low storage temperatures, browning and microbial spoilage
can be inhibited. However, the relationship between moisture level,
storage temperature, and sulfur dioxide content on the color of dried
fruit is not known; therefore the purpose of this research was to
determine the influence of temperature, sulfur dioxide content, and
moisture level on the color of dried apples during storage.
Dices (3/4" x 1/2" x 1/4") of Golden Delicious apples were dried
after a 90 sec dip in aqueous solutions of 2500, 5000, or 7500 ppm of
sulfur dioxide obtained from sodium bisulfite. The apples were dried to
five different levels of moisture (13, 18, 22, 24, and 26% wet basis)
and stored in controlled temperature rooms at 1°, 21°, and 38°C.
Periodically the samples were analyzed for color (color index (CI)
defined as Hunter L x a [subscript L] x b [subscript L]), total and free sulfur dioxide, moisture
and water activity (a [subscript w]), to appreciate the changes of quality during 385
days of storage.
Sulfur dioxide level was directly influenced by storage temperature.
As temperature increased, the sulfur dioxide level in the dried
apples decreased following a negative exponential curve. At 1°C nearly
no variation in sulfur dioxide level was observed during the experiment.
Loss of free sulfur dioxide followed the same pattern as total sulfur
dioxide. The concentration of free sulfur dioxide was a larger proportion
of the total as the concentration of total sulfur dioxide was
increased.
Moisture content of the dried apples decreased during storage at
38°C, but at 21°C moisture content decreased in the first 40 days
reaching a level that remained constant until the end of the experiment.
The constant level was approximately 85% of the initial moisture level.
No appreciable change of water content occurred at 1°C. Water activities
of the samples ranged between 0.40 and 0.85 and the optimal
levels for color retention at the lower concentrations of total sulfur
dioxide were approximately 0.75 a [subscript w]. This corresponded to a moisture of
approximately 20%.
Total sulfur dioxide, moisture and CI analysis of the dried apples
were used to derive three equations, one for each temperature of
storage. From these equations, the following relationships were evident.
CI described the changes in color during the 385 days of the
experiment. At 1°C sulfur dioxide and moisture influenced the changes
of color approximately the same extent. Moisture content of 20% at all
levels of sulfur dioxide at 1°C was optimal for maximum color
preservation. Samples stored at 1°C retained an acceptable color longer
than those stored at 21°C or 38°C. The approximation of the equation
derived from regression analysis of the data of changes in color at 1°C
was 63%.
Changes in color at 21°C occurred faster than at 1°C. Acceptable
colors were found until the 188 day samples. The water content of the
samples had more influence on the color changes than the total sulfur
dioxide content as determined by the regression equation. The regression
equation describes 88% of the variations in color. Concentrations
higher than 1500 ppm of total sulfur dioxide were necessary to maintain
samples of acceptable colors for periods up to 188 days with moisture
levels close to 20%. Storage temperature of 21°C was satisfactory for
samples that do not require storage periods longer than 200 days and
contain 1500 ppm of total sulfur dioxide and 20% moisture.
Very rapid changes in the parameters studied were observed at 38°C.
The samples were very dark after 40 days and none had an acceptable
color after 101 days of storage. The regression equation derived from
the data described 87% of the variations of color. The temperature of
38°C is not recommended under any condition for storage of dried apples.
It was concluded that temperatures lower than 21°C and concentrations
of approximately 1500 ppm of sulfur dioxide in the fruit tissue
could preserve acceptable colors in samples of dried apples for periods
of 200 days. Longer storage periods would be possible as temperature
approaches 1°C.
Using the equations obtained in this experiment, an estimation can
be made of the storage life of dried apples. The response surface
diagrams obtained are useful for visual comprehension of the influence
of temperature, sulfur dioxide and moisture on color throughout the
time of storage
Role of a Fur homolog in iron metabolism in Nitrosomonas europaea
<p>Abstract</p> <p>Background</p> <p>In response to environmental iron concentrations, many bacteria coordinately regulate transcription of genes involved in iron acquisition via the ferric uptake regulation (Fur) system. The genome of <it>Nitrosomonas europaea</it>, an ammonia-oxidizing bacterium, carries three genes (NE0616, NE0730 and NE1722) encoding proteins belonging to Fur family.</p> <p>Results</p> <p>Of the three <it>N. europaea fur </it>homologs, only the Fur homolog encoded by gene NE0616 complemented the <it>Escherichia coli </it>H1780 <it>fur </it>mutant. A <it>N. europaea fur:kanP </it>mutant strain was created by insertion of kanamycin-resistance cassette in the promoter region of NE0616 <it>fur </it>homolog. The total cellular iron contents of the <it>fur:kanP </it>mutant strain increased by 1.5-fold compared to wild type when grown in Fe-replete media. Relative to the wild type, the <it>fur:kanP </it>mutant exhibited increased sensitivity to iron at or above 500 μM concentrations. Unlike the wild type, the <it>fur:kanP </it>mutant was capable of utilizing iron-bound ferrioxamine without any lag phase and showed over expression of several outer membrane TonB-dependent receptor proteins irrespective of Fe availability.</p> <p>Conclusions</p> <p>Our studies have clearly indicated a role in Fe regulation by the Fur protein encoded by <it>N. europaea </it>NE0616 gene. Additional studies are required to fully delineate role of this <it>fur </it>homolog.</p
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Steady-State Growth under Inorganic Carbon Limitation Conditions Increases Energy Consumption for Maintenance and Enhances Nitrous Oxide Production in Nitrosomonas europaea
Nitrosomonas europaea is a chemolithoautotrophic bacterium that oxidizes ammonia (NH₃) to obtain energy for growth on carbon dioxide (CO₂) and can also produce nitrous oxide (N₂O), a greenhouse gas. We interrogated the growth, physiological, and transcriptome responses of N. europaea to conditions of replete (>5.2 mM) and limited inorganic carbon (IC) provided by either 1.0 mM or 0.2 mM sodium carbonate (Na₂CO₃) supplemented with atmospheric CO₂. IC-limited cultures oxidized 25 to 58% of available NH₃ to nitrite, depending on the dilution rate and Na₂CO₃ concentration. IC limitation resulted in a 2.3-fold increase in cellular maintenance energy requirements compared to those for NH₃-limited cultures. Rates of N₂O production increased 2.5- and 6.3-fold under the two IC-limited conditions, increasing the percentage of oxidized NH₃-N that was transformed to N₂O-N from 0.5% (replete) up to 4.4% (0.2 mM Na₂CO₃). Transcriptome analysis showed differential expression (P ≤ 0.05) of 488 genes (20% of inventory) between replete and IC-limited conditions, but few differences were detected between the two IC-limiting treatments. IC-limited conditions resulted in a decreased expression of ammonium/ammonia transporter and ammonia monooxygenase subunits and increased the expression of genes involved in C₁ metabolism, including the genes for RuBisCO (cbb gene cluster), carbonic anhydrase, folate-linked metabolism of C₁ moieties, and putative C salvage due to oxygenase activity of RuBisCO. Increased expression of nitrite reductase (gene cluster NE0924 to NE0927) correlated with increased production of N₂O. Together, these data suggest that N. europaea adapts physiologically during IC-limited steady-state growth, which leads to the uncoupling of NH₃ oxidation from growth and increased N₂O production.
IMPORTANCE: Nitrification, the aerobic oxidation of ammonia to nitrate via nitrite, is an important process in the global nitrogen cycle. This process is generally dependent on ammonia-oxidizing microorganisms and nitrite-oxidizing bacteria. Most nitrifiers are chemolithoautotrophs that fix inorganic carbon (CO₂) for growth. Here, we investigate how inorganic carbon limitation modifies the physiology and transcriptome of Nitrosomonas europaea, a model ammonia-oxidizing bacterium, and report on increased production of N₂O, a potent greenhouse gas. This study, along with previous work, suggests that inorganic carbon limitation may be an important factor in controlling N₂O emissions from nitrification in soils and wastewater treatment
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Kinetic characterization of the soluble butane monooxygenase from Thauera butanivorans, formerly ‘Pseudomonas butanovora’
Soluble butane monooxygenase (sBMO), a three-component di-iron monooxygenase complex
expressed by the C2–C9 alkane-utilizing bacterium Thauera butanivorans, was kinetically
characterized by measuring substrate specificities for C1–C5 alkanes and product inhibition
profiles. sBMO has high sequence homology with soluble methane monooxygenase (sMMO) and
shares a similar substrate range, including gaseous and liquid alkanes, aromatics, alkenes and
halogenated xenobiotics. Results indicated that butane was the preferred substrate (defined by
kcat : Km ratios). Relative rates of oxidation for C1–C5 alkanes differed minimally, implying that
substrate specificity is heavily influenced by differences in substrate Km values. The low
micromolar Km for linear C2–C5 alkanes and the millimolar Km for methane demonstrate that
sBMO is two to three orders of magnitude more specific for physiologically relevant substrates of
T. butanivorans. Methanol, the product of methane oxidation and also a substrate itself, was found
to have similar Km and kcat values to those of methane. This inability to kinetically discriminate
between the C1 alkane and C1 alcohol is observed as a steady-state concentration of methanol
during the two-step oxidation of methane to formaldehyde by sBMO. Unlike methanol, alcohols
with chain length C2–C5 do not compete effectively with their respective alkane substrates.
Results from product inhibition experiments suggest that the geometry of the active site is
optimized for linear molecules four to five carbons in length and is influenced by the regulatory
protein component B (butane monooxygenase regulatory component; BMOB). The data suggest
that alkane oxidation by sBMO is highly specialized for the turnover of C3–C5 alkanes and the
release of their respective alcohol products. Additionally, sBMO is particularly efficient at
preventing methane oxidation during growth on linear alkanes ≥C2, despite its high sequence
homology with sMMO. These results represent, to the best of our knowledge, the first kinetic in
vitro characterization of the closest known homologue of sMM
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Identifying Potential Mechanisms Enabling Acidophily in the Ammonia-Oxidizing Archaeon "Candidatus Nitrosotalea devanaterra"
Ammonia oxidation is the first and rate-limiting step in nitrification and is dominated by two distinct groups of microorganisms in soil: ammonia-oxidizing archaea (AOA) and ammonia-oxidizing bacteria (AOB). AOA are often more abundant than AOB and dominate activity in acid soils. The mechanism of ammonia oxidation under acidic conditions has been a long-standing paradox. While high rates of ammonia oxidation are frequently measured in acid soils, cultivated ammonia oxidizers grew only at near-neutral pH when grown in standard laboratory culture. Although a number of mechanisms have been demonstrated to enable neutrophilic AOB growth at low pH in the laboratory, these have not been demonstrated in soil, and the recent cultivation of the obligately acidophilic ammonia oxidizer “Candidatus Nitrosotalea devanaterra” provides a more parsimonious explanation for the observed high rates of activity. Analysis of the sequenced genome, transcriptional activity, and lipid content of “Ca. Nitrosotalea devanaterra” reveals that previously proposed mechanisms used by AOB for growth at low pH are not essential for archaeal ammonia oxidation in acidic environments. Instead, the genome indicates that “Ca. Nitrosotalea devanaterra” contains genes encoding both a predicted high-affinity substrate acquisition system and potential pH homeostasis mechanisms absent in neutrophilic AOA. Analysis of mRNA revealed that candidate genes encoding the proposed homeostasis mechanisms were all expressed during acidophilic growth, and lipid profiling by high-performance liquid chromatography–mass spectrometry (HPLC-MS) demonstrated that the membrane lipids of “Ca. Nitrosotalea devanaterra” were not dominated by crenarchaeol, as found in neutrophilic AOA. This study for the first time describes a genome of an obligately acidophilic ammonia oxidizer and identifies potential mechanisms enabling this unique phenotype for future biochemical characterization
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Use of aliphatic n-alkynes to discriminate soil nitrification activities of ammonia-oxidizing thaumarchaea and bacteria
Ammonia (NH₃)-oxidizing bacteria (AOB) and thaumarchaea (AOA) co-occupy most soils, yet no short-term growth-independent method exists to determine their relative contributions to nitrification in situ. Microbial monooxygenases differ in their vulnerability to inactivation by aliphatic n-alkynes, and we found that NH₃ oxidation by the marine thaumarchaeon Nitrosopumilus maritimus was unaffected during a 24-h exposure to ≤20 μM concentrations of 1-alkynes C₈ and C₉. In contrast, NH₃ oxidation by two AOB (Nitrosomonas europaea and Nitrosospira multiformis) was quickly and irreversibly inactivated by 1 μM C₈ (octyne). Evidence that nitrification carried out by soilborne AOA was also insensitive to octyne was obtained. In incubations (21 or 28 days) of two different whole soils, both acetylene and octyne effectively prevented NH₄⁺-stimulated increases in AOB population densities, but octyne did not prevent increases in AOA population densities that were prevented by acetylene. Furthermore, octyne-resistant, NH₄⁺-stimulated net nitrification rates of 2 and 7 μg N/g soil/day persisted throughout the incubation of the two soils. Other evidence that octyne-resistant nitrification was due to AOA included (i) a positive correlation of octyne-resistant nitrification in soil slurries of cropped and noncropped soils with allylthiourea-resistant activity (100 μM) and (ii) the finding that the fraction of octyne-resistant nitrification in soil slurries correlated with the fraction of nitrification that recovered from irreversible acetylene inactivation in the presence of bacterial protein synthesis inhibitors and with the octyne-resistant fraction of NH₄⁺-saturated net nitrification measured in whole soils. Octyne can be useful in short-term assays to discriminate AOA and AOB contributions to soil nitrification.This is an author's peer-reviewed final manuscript, as accepted by the publisher. The published article is copyrighted by the American Society for Microbiology and can be found at: http://aem.asm.org/
Ca. Nitrososphaera and Bradyrhizobium are inversely correlated and related to agricultural practices in long-term field experiments
Agricultural land management, such as fertilization, liming, and tillage affects soil properties, including pH, organic matter content, nitrification rates, and the microbial community. Three different study sites were used to identify microorganisms that correlate with agricultural land use and to determine which factors regulate the relative abundance of the microbial signatures of the agricultural land-use. The three sites included in this study are the Broadbalk Experiment at Rothamsted Research, UK, the Everglades Agricultural Area, Florida, USA, and the Kellogg Biological Station, Michigan, USA. The effects of agricultural management on the abundance and diversity of bacteria and archaea were determined using high throughput, barcoded 16S rRNA sequencing. In addition, the relative abundance of these organisms was correlated with soil features. Two groups of microorganisms involved in nitrogen cycle were highly correlated with land use at all three sites. The ammonia oxidizing-archaea, dominated by Ca. Nitrososphaera, were positively correlated with agriculture while a ubiquitous group of soil bacteria closely related to the diazotrophic symbiont, Bradyrhizobium, was negatively correlated with agricultural management. Analysis of successional plots showed that the abundance of ammonia oxidizing-archaea declined and the abundance of bradyrhizobia increased with time away from agriculture. This observation suggests that the effect of agriculture on the relative abundance of these genera is reversible. Soil pH and NH(3) concentrations were positively correlated with archaeal abundance but negatively correlated with the abundance of Bradyrhizobium. The high correlations of Ca. Nitrososphaera and Bradyrhizobium abundances with agricultural management at three long-term experiments with different edaphoclimatic conditions allowed us to suggest these two genera as signature microorganisms for agricultural land use
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Complete genome sequence of Nitrosomonas sp. Is79, an ammonia oxidizing bacterium adapted to low ammonium concentrations
Nitrosomonas sp. Is79 is a chemolithoautotrophic ammonia-oxidizing bacterium that belongs to the family Nitrosomonadaceae within the phylum Proteobacteria. Ammonia oxidation is the first step of nitrification, an important process in the global nitrogen cycle ultimately resulting in the production of nitrate. Nitrosomonas sp. Is79 is an ammonia oxidizer of high interest because it is adapted to low ammonium and can be found in freshwater environments around the world. The 3,783,444-bp chromosome with a total of 3,553 protein coding genes and 44 RNA genes was sequenced by the DOE-Joint Genome Institute Program CSP 2006.Keywords: nitrification,
Ammonia-oxidizing bacteria,
Ammonia oxidation,
Nitrosomonas,
nitrogen cycle,
oligotrophic,
freshwate
Alkane Utilization by Rhodococcus Strain NTU-1 Alone and in Its Natural Association with Bacillus fusiformis L-l and Ochrobactrum sp.
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Obligate autotrophy in the ammonia oxidizing bacterium Nitrosomonas europaea.
Closing report for project DOE-FG02-03ER15436. The project studied obligate autotrophy in the ammonia oxidizing bacterium Nitrosomonas europaea. Nitrosomonas europaea can obtain all of its energy and reductant for growth from the oxidation of ammonia to nitrite and is, therefore, classified as a chemolithotroph. This bacterium is also an autotroph, which can derive all cellular carbon from carbon dioxide. N. europaea seems incapable of growth with other carbon or energy sources. This restricted capability is surprising given that ammonia is a poor energy source. The main goal of the project was to examine the basis of autotrophy in N. europaea or, thought of another way, to determine the barriers to heterotrophy. The approach was enabled by the N. europaea genome sequence, stimulating new ways of thinking about this physiological paradox—an insistence on a single, albeit poor, energy source. Objective 1 was to examine the expression and regulation of the genes coding for alpha-ketoglutarate dehydrogenase, determine if the enzyme’s activity is present, and determine whether alteration of the expression levels influences autotrophic growth. Although Nitrosomonas europaea lacks measurable alpha-ketoglutarate dehydrogenase activity, the genome sequence revealed the presence of the genes encoding the enzyme. A knockout mutation was created in the sucA gene encoding the E1 subunit. Compared to wild-type cells, the mutant strain showed an accelerated loss of ammonia monooxygenase and hydroxylamine oxidoreductase activities upon entering stationary phase. In addition, unlike wild-type cells, the mutant strain showed a marked lag in the ability to resume growth in response to pH adjustments in late stationary phase. The results were published in Hommes N.G., Kurth E. G., Sayavedra-Soto L.A., and Arp D.J. (2006) Disruption of sucA, which encodes a subunit of alpha-ketoglutarate dehydrogenase, affects the survival of Nitrosomonas europaea in stationary phase. Journal of Bacteriology 188:343-347. Objective 2 was to determine the basis of fructose stimulation of growth on ammonia, examine fructose metabolism, and determine the impact of other compounds on growth on ammonia. Previous studies showed that N. europaea can utilize limited amounts of certain organic compounds, including amino acids, pyruvate, and acetate, although no organic compound has been reported to support the growth of N. europaea. The genomic sequence of N. europaea revealed a potential permease for fructose. N. europaea utilized fructose and other compounds as carbon sources to support growth. Cultures were incubated in the presence of fructose or other organic compounds in sealed bottles purged of CO(2). In these cultures, addition of either fructose or pyruvate as the sole carbon source resulted in a two- to threefold increase in optical density and protein content in 3 to 4 days. Studies with [(14)C]fructose showed that >90% of the carbon incorporated by the cells during growth was derived from fructose. Cultures containing mannose, glucose, glycerol, mannitol, citrate, or acetate showed little or no growth. N. europaea was not able to grow with fructose as an energy source, although the presence of fructose did provide an energy benefit to the cells. These results show that N. europaea can be grown in carbon dioxide free medium by using fructose and pyruvate as carbon sources and may now be considered a facultative chemolithoorganotroph. The results were published in Hommes N.G., Sayavedra-Soto L.A. and Arp. D.J. (2003). Chemolithotrophic growth of Nitrosomonas europaea on fructose. Journal of Bacteriology. 185:6809-2773. Objective 3 attempted to grow N. europaea heterotrophically through pathways predicted by the genome. Experiments with mutant strains and complementation studies were performed to test whether N. europaea can utilize other carbon sources. N. europaea was not able to grow heterotrophically in the conditions tested in this objective
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