Effect of Iron Bioavailability on Dissolved Hydrogen Concentrations During Microbial Iron Reduction

Dissolved hydrogen (H2) concentrations have been shown to correlate with specific terminal electron accepting processes (TEAPs) in aquifers. The research presented herein examined the effect of iron bioavailability on H2 concentrations during iron reduction in flow-through column experiments filled with soil obtained from the uncontaminated background area of the Field Research Center (FRC), Oak Ridge, TN and amended with acetate as the electron donor. The first column experiment measured H2 concentrations over 500 days of column operation that fluctuated within a substantial range around an average of 3.9 nM. Iron reduction was determined to be the dominant electron accepting process. AQDS (9,10-anthraquinone-2,6-disulfonic acid) was then used to determine if H2 concentrations during iron reduction were related to iron bioavailability. For this purpose, a 100-day flow-through column experiment was conducted that compared the effect of AQDS on iron reduction and subsequent H2 concentrations using two columns in parallel. Both columns were packed with FRC soil and inoculated with Geobacter sulfurreducens but only one was supplied with AQDS. The addition of AQDS increased the rate of iron reduction in the flow-through column and slightly decreased the steady-state H2 concentrations from an average of 4.0 nM for the column without AQDS to 2.0 nM for the column with AQDS. The results of this study therefore show that H2 can be used as an indicator to monitor rate and bioavailability changes during microbial iron reduction.     

Bioavailability of selenium from meat and broccoli as determined by retention and distribution of 75Se

The concentration of selenium (Se), an essential nutrient, is variable in foods, depending, in part, on how and where foods are produced; some foods accumulate substantial amounts of Se when produced on high-Se soils. The chemical form of Se also differs among foods. Broccoli is a Se-accumulating plant that contains many methylated forms of Se, and Se bioavailability from broccoli has been reported to be low. Red meats such as pork or beef could accumulate Se when the animal is fed high-Se diets, and Se from such meats has been reported to be highly bioavailable for selenoprotein synthesis. In a further attempt to characterize the utilization of Se from broccoli and meats such as pork or beef, we have fed rats diets adequate (0.1 μg Se/g diet) in Se or high in Se (1.5 μg S/g diet), with the Se source being either high-Se broccoli or beef. Rats were then given test meals of broccoli or pork intrinsically labeled with ⁷⁵Se. When dietary Se was nutritionally adequate (0.1 μg/g diet), more ⁷⁵Se from pork than broccoli was retained in tissues; however, there were no significant differences in whole-body retention when dietary Se was high (1.5 μg/g diet). A significantly greater percentage of ⁷⁵Se from broccoli than pork was excreted in the urine and dietary Se did not affect urinary excretion of broccoli ⁷⁵Se, but the amount excreted from pork varied directly with dietary Se intake. Radiolabeled ⁷⁵Se derived from pork effectively labeled selenoproteins in all tissues examined, but ⁷⁵Se from broccoli was undetectable in selenoproteins. These differences in retention and distribution of Se from broccoli or pork are consistent with reported differences in bioavailability of Se from beef and broccoli. They also suggest that there are fewer differences in bioavailability when Se is consumed in supranutritional amounts.     

Effects of carbon substrate enrichment and DOC concentration on biodegradation of PAHs in soil

AIMS: Two common reasons to explain slow environmental biodegradation of polycyclic aromatic hydrocarbons (PAHs), namely lack of appropriate carbon sources for microbial growth and limited bioavailability of PAHs, were tested in a laboratory bioassay using a creosote-contaminated soil. METHODS AND RESULTS: The soil, containing a total of 8 mg g-1 of 16 PAHs, was sieved and incubated in bottles for 45 days. The first explanation was tested by enrichment with the analogue anthracene and the non-analogue myristic acid, and both failed to stimulate degradation of all PAHs except anthracene. The second explanation was tested by addition of different concentrations of dissolved organic carbon (DOC), with effects depending on the DOC concentration and the molecular size of the PAH. The degradation was enhanced from 10 to 35% for 12 PAHs when the soil was saturated. The degraded amounts of individual PAHs were proportional to their concentration in the soil. CONCLUSIONS: The slow in situ degradation of PAHs was enhanced by more than three times by adding water as a solvent. Addition of DOC facilitated the degradation of four- to six-ring PAHs. SIGNIFICANCE AND IMPACT OF STUDY: Bioremediation of PAH-contaminated sites may be facilitated by creating water-saturated conditions but retarded by addition of other carbon substrates, such as analogue compounds.     

Biology of platelet-derived growth factors in development

Platelet-derived growth factor (PDGF) was one of the first growth factors to be characterized, and the PDGF family of ligand and receptors has remained an archetype system for studies of the mechanisms of action of growth factors and receptor tyrosine kinases for more than two decades. The small size of the family has also facilitated genetic studies and, in particular, manipulations of the mouse PDGF and PDGF receptor genes have given important insights into the role of this family during mammalian development. These studies have shown that discrete populations of mesenchymal and neuroectodermal progenitor cells depend on PDGF signaling for their growth and distribution within developing organs. Other studies suggest that the same, or similar, cells may be targeted by exaggerated PDGF signaling in a number of pathological processes, including different types of cancer. The present review summarizes current views on the roles of PDGFs in developmental processes, and discusses the critical importance of the amount, spatial distribution, and bioavailability of the PDGF proteins for acquisition of the correct number and location of target cells.     

Equivalent uptake of organic and inorganic zinc by monkey kidney fibroblasts, human intestinal epithelial cells, or perfused mouse intestine

Zinc (Zn) is recognized as an essential nutrient, and is added as a supplement to animal and human diets. There are claims that zinc methionine (ZnMet) forms a stable complex that is preferentially transported into tissues, and this has contributed to uncertainty about conflicting reports on the bioavailability of various Zn compounds. This study evaluated the cellular and intestinal uptake of inorganic and organic forms of Zn. Steady-state uptake of⁶⁵Zn by human intestine epithelial cells, and monkey kidney fibroblasts was not significantly different with zinc chloride (ZnCl₂), ZnMet, or zinc propionate (ZnProp) (P > 0.05). Uptake of⁶⁵Zn from zinc chelated with EDTA was significantly lower (P < 0.01). In live mice,⁶⁵Zn uptake by perfused intestine and deposition in intestine and liver showed no significant difference between ZnCl₂ and ZnMet. Equimolar [⁶⁵Zn]methionine and zinc[³⁵S]methionine were prepared according to a patented method that yields “ complexed” Zn. Cellular uptake of the radiolabeled methionine was <0.1% of the radiolabeled Zn from these complexes, indicating separate uptake of the Zn and methionine. Gel filtration did not distinguish between⁶⁵Zn in ZnCl₂, ZnProp, or reagent ZnMet, though feed-grade ZnMet containing >10% protein did give a higher-mol-wt form of⁶⁵Zn. Results of this study show equivalent uptake of Zn from inorganic and organic compounds, and support recent feed trials on Zn bioavailability.     

Bioavailability of enterai yeast—selenium in preterm infants

There is no data or literature on the effects of supplementing infants with yeast selenium, although its intestinal absorption and bioavailability are higher in adults compared with other selenium compounds. The aim of the present investigation was to study the impact of selenium enriched yeast on the serum selenium concentration of preterm infants living in a low selenium area (Hungary). Twenty-eight preterm infants with mean ± SD birth weight of 962 ± 129 g and gestational age 27 ± 1 wk were randomized into two groups at birth with respect to selenium supplementation. In the supplemented group (n = 14) infants received 4.8 mg yeast selenium containing 5 μg selenium daily via nasogastric drip during the first 14 postnatal days. The nonsupplemented infants were used as a reference group. In the supplemented group, the serum selenium concentration increased from 32.1 ± 8.5 μg/L to 41.5 ± 6.5 μg/L and in the nonsupplemented group it decreased from 25.9 ± 6.8 μg/L to 18.2 ± 6.4 μg/L from birth in two weeks time. Compared with previous studies, our results suggest that the bioavailability of selenium in the form of yeast selenium is higher than that of other selenium compounds used for preterm infants. We did not observe any complications or side-effects owing to enterai yeast selenium supplementation. We conclude that selenium enriched yeast is a safe and an effective form of short-term enterai selenium supplementation for infants.     

UPTAKE OF CADMIUM BY FRESHWATER GREEN ALGAE: EFFECTS OF PH AND AQUATIC HUMIC SUBSTANCES1

The effects of humic substances and low pH on short‐term Cd uptake by Pseudokirchneriella subcapitata (Korshikov) Hindak and Chlamydomonas reinhardtii Dang were investigated under defined exposure conditions. The uptake experiments were run in the presence of either a synthetic organic ligand (nitrilotriacetate) or natural organic ligands (Suwannee River fulvic or humic acid). An ion‐exchange method was used to measure the free Cd²⁺ concentrations in the exposure solutions. At pH 5, measured free Cd²⁺ concentrations agreed with estimations made using the geochemical equilibrium model WHAM, but at pH 7 the model overestimated complexation by both Suwannee River fulvic and humic acids compared with the ion‐exchange measurements. Consistent with the metal internalization step being rate limiting for overall short‐term uptake, intracellular Cd uptake was linear for exposure times less than 20 min at pH 5 or pH 7 for both algal species. After taking into account complexation of Cd in solution, Suwannee River humic substances had no additional effects on cadmium uptake at pH 7, as would be predicted by the free ion model. This absence of effects other than complexation persisted at pH 5, where the tendency of humic substances to adsorb to the algal cell surface is favored. Changes in pH strongly influenced Cd uptake, with the intracellular flux of Cd being at least 20 times lower at pH 5 than at pH 7 for P. subcapitata. Our results support models such as the free ion model or the biotic ligand model, in which humic substances act indirectly on Cd uptake by reducing the bioavailability of Cd by complexation in solution.     

Assessment of Bioavailability after In Vitro Digestion and First Pass Metabolism of Bioactive Peptides from Collagen Hydrolysates

Collagen hydrolysates (CHs) are composed of bioactive peptides (BAPs), which possess health enhancing properties. There is a knowledge gap regarding the bioavailability of these BAPs that involves intestinal transport and hepatic first pass effects. A simulated gastrointestinal model was used to generate digesta from two CHs (CH-GL and CH-OPT), which were applied to a novel transwell co-culture of human intestinal epithelium cell line-6 (HIEC-6) and hepatic (HepG2) cells to simulate in vivo conditions of absorption and first pass metabolism. Peptide transport, hepatic first pass effects, and bioavailability were determined by measuring BAPs (Gly-Pro, Hyp-Gly, Ala-Hyp, Pro-Hyp, Gly-Pro-Hyp) using an innovative capillary electrophoresis method. All peptides were transported across the intestinal cell layer to varying degrees with both CHs; however, Gly-Pro-Hyp was transported only with CH-GL, but not CH-OPT. Notable hepatic production was observed for Ala-Hyp with both CH treatments, and for Pro-Hyp and Gly-Pro with CH-GL only. All peptides were bioavailable (>10%), except for Gly-Pro-Hyp after CH-OPT. Overall, a high degree of transport and hepatic first pass effects on CH-derived BAPs were observed. Further research is needed to explore the hepatic mechanisms related to the production of BAPs and the bifunctional effects of the bioavailable BAPs noted in this study.     

Rhamnolipid biosurfactants decrease the toxicity of chlorinated phenols to Pseudomonas putida DOT-T1E

Aims:  To investigate the effect of a mixture of rhamnolipid R1 and R2 biosurfactants produced by a Pseudomonas aeruginosa strain on the toxicity of phenol and chlorophenols to Pseudomonas putida DOT-T1E.
Methods and Results:  Toxicity was quantified by the effective concentration 50% (EC50), that is the concentration that causes a 50% inhibition of bacterial growth. The presence of 300 mg l⁻¹ rhamnolipids, that is at about twice their critical micelle concentration (CMC), increased the EC50 of phenol, 4-chlorophenol, 2,4-dichlorophenol and 2,4,5-trichlorophenol by about 12, 19, 32 and 40%, respectively, and consequently reduced the bioavailability and the freely dissolved concentration of the toxic phenolic compounds. The reduction was related to the phenols' octanol–water partition coefficients ( K _ow).
Conclusions:  The reduction in toxicity of the phenols can be explained by a combination of toxin accumulation in biosurfactant micelles and hydrophobic interactions of the phenols with rhamnolipid-based dissolved organic carbon.
Significance and Impact of the Study:  Results provide evidence that next to the effect of the micelle formation also hydrophobic interactions with rhamnolipid-based dissolved organic carbon affects the bioavailability of the phenols. Quantifying the effect of biosurfactants on the toxicity of hydrophobic compounds such as phenols thus appears to be a useful approach to assess their bioavailable equilibrium concentration.     

NITRATE UTILIZATION BY PHYTOPLANKTON IN LAKE SUPERIOR IS IMPAIRED BY LOW NUTRIENT (P,Fe) AVAILABILITY AND SEASONAL LIGHT LIMITATION — A CYANOBACTERIAL BIOREPORTER STUDY1

We previously developed a luminescent Synechococystis sp. strain PCC 6803 cyanobacterial bioreporter that is used as a real‐time whole‐cell sensor to assess nitrate assimilatory capacity in freshwaters. Applying the bioreporter assay to Lake Superior, a system whose nitrate levels have increased 6‐fold since 1900, we investigated factors that constrain nitrate utilization in this oligotrophic system. Clean sampling methods were used to collect water from Lake Superior during spring and summer 2004, and nitrate utilization was measured by monitoring bioreporter luminescence. Bioreporter response was monitored during experiments in which the lake water was amended with nutrients and incubated under light regimes simulating integrated spring and summer mixing depths. These studies demonstrated that nitrate utilization was enhanced at most stations following addition of phosphorus (P). Moreover, at many stations, addition of iron (Fe) enhanced the P effect. Strength‐of‐effect statistical analysis provided the individual contribution of P and Fe toward stimulating bioreporter response. In general, distance from shore and season were not good predictors of nitrate assimilatory capacity. Manipulation of light flux during bioreporter experiments also showed that light intensities experienced during spring mixing are likely insufficient to saturate the rate of nitrate utilization. Overall, these data suggest that P‐limited algae are deficient in their ability to assimilate nitrate in Lake Superior. Furthermore, we suggest that a secondary limitation for Fe may occur that further constrains nitrate drawdown. Lastly, during spring, light fluxes are sufficiently low to prevent maximal nitrate utilization, even in the absence of nutrient limitation.