Stable isotope fractionation during bacterial sulfate reduction is controlled by reoxidation of intermediates     

Muna Mangalo 《Geochimica et cosmochimica acta》2007,71(17):4161-4171

Bacterial sulfate reduction is one of the most important respiration processes in anoxic habitats and is often assessed by analyzing the results of stable isotope fractionation. However, stable isotope fractionation is supposed to be influenced by the reduction rate and other parameters, such as temperature. We studied here the mechanistic basics of observed differences in stable isotope fractionation during bacterial sulfate reduction. Batch experiments with four sulfate-reducing strains (Desulfovibrio desulfuricans, Desulfobacca acetoxidans, Desulfonatronovibrio hydrogenovorans, and strain TRM1) were performed. These microorganisms metabolize different carbon sources (lactate, acetate, formate, and toluene) and showed broad variations in their sulfur isotope enrichment factors. We performed a series of experiments on isotope exchange of ¹⁸O between residual sulfate and ambient water. Batch experiments were conducted with ¹⁸O-enriched (δ¹⁸O_water = +700‰) and depleted water (δ¹⁸O_water = −40‰), respectively, and the stable ¹⁸O isotope shift in the residual sulfate was followed. For Desulfovibrio desulfuricans and Desulfonatronovibrio hydrogenovorans, which are both characterized by low sulfur isotope fractionation (ε_S > −13.2‰), δ¹⁸O values in the remaining sulfate increased by only 50‰ during growth when ¹⁸O-enriched water was used for the growth medium. In contrast, with Desulfobacca acetoxidans and strain TRM1 (ε_S < −22.7‰) the residual sulfate showed an increase of the sulfate δ¹⁸O close to the values of the enriched water of +700‰. In the experiments with δ¹⁸O-depleted water, the oxygen isotope values in the residual sulfate stayed fairly constant for strains Desulfovibrio desulfuricans, Desulfobacca acetoxidans and Desulfonatronovibrio hydrogenovorans. However, strain TRM1, which exhibits the lowest sulfur isotope fractionation factor (ε_S < −38.7‰) showed slightly decreasing δ¹⁸O values.Our results give strong evidence that the oxygen atoms of sulfate exchange with water during sulfate reduction. However, this neither takes place in the sulfate itself nor during formation of APS (adenosine-5′-phosphosulfate), but rather in intermediates of the sulfate reduction pathway. These may in turn be partially reoxidized to form sulfate. This reoxidation leads to an incorporation of oxygen from water into the “recycled” sulfate changing the overall ¹⁸O isotopic composition of the remaining sulfate fraction. Our study shows that such incorporation of ¹⁸O is correlated with the stable isotope enrichment factor for sulfur measured during sulfate reduction. The reoxidation of intermediates of the sulfate reduction pathway does also strongly influence the sulfur stable isotope enrichment factor. This aforesaid reoxidation is probably dependent on the metabolic conversion of the substrate and therefore also influences the stable isotope fractionation factor indirectly in a rate dependent manner. However, this effect is only indirect. The sulfur isotope enrichment factors for the kinetic reactions themselves are probably not rate dependent.  相似文献   

Effect of hot dry summer tropical climate on forage intake and milk yield in Holstein–Friesian and indigenous zebu cows in Sudan     

Muna Mahjoub M Ahmed  Adam Ibrahim El Amin 《Journal of Arid Environments》1997,35(4):737-746

The effects of dry summer ambient temperature on some physiological and productive responses were studied in an exotic European breed (Holstein–Friesian) and a local zebu breed (Butana ecotype) of cattle in Sudan. Rectal temperature and respiration rate were positively correlated, and dry matter intake and milk yield were negatively correlated with variation in ambient temperature. However, significant adverse effects on high ambient temperature were only observed in the exotic breed.  相似文献   

利用欧拉反褶积法估计二度磁性体深度与位置   总被引：7，自引：0，他引：7  

史辉  刘天佑 《物探与化探》2005,29(3):230-233

为了利用欧拉反褶积进行快速计算,提出了多个大小不同的滑动窗口进行多次覆盖的算法,对高精度磁测剖面逐点反复计算,并根据统计学原理从大量计算结果中剔除坏解,保留合理的解;还根据2-D模型讨论了结构指数与欧拉反褶积的结果的关系。将该方法应用于我国西北某地区的高精度航磁资料处理,获得较好的地质效果。  相似文献   

Influence of the enzyme dissimilatory sulfite reductase on stable isotope fractionation during sulfate reduction     

Muna Mangalo  Rainer U. Meckenstock  Willibald Stichler 《Geochimica et cosmochimica acta》2008,72(6):1513-1520

The stable isotopes of sulfate are often used as a tool to assess bacterial sulfate reduction on the macro scale. However, the mechanisms of stable isotope fractionation of sulfur and oxygen at the enzymatic level are not yet fully understood. In batch experiments with water enriched in ¹⁸O we investigated the effect of different nitrite concentrations on sulfur isotope fractionation by Desulfovibrio desulfuricans.With increasing nitrite concentrations, we found sulfur isotope enrichment factors ranging from −11.2 ± 1.8‰ to −22.5 ± 3.2‰. Furthermore, the δ¹⁸O values in the remaining sulfate increased from approximately 50-120‰ when ¹⁸O-enriched water was supplied. Since ¹⁸O-exchange with ambient water does not take place in sulfate, but rather in intermediates of the sulfate reduction pathway (e.g. ), we suggest that nitrite affects the steady-state concentration and the extent of reoxidation of the metabolic intermediate sulfite to sulfate during sulfate reduction. Given that nitrite is known to inhibit the production of the enzyme dissimilatory sulfite reductase, our results suggest that the activity of the dissimilatory sulfite reductase regulates the kinetic isotope fractionation of sulfur and oxygen during bacterial sulfate reduction. Our novel results also imply that isotope fractionation during bacterial sulfate reduction strongly depends on the cell internal enzymatic regulation rather than on the physico-chemical features of the individual enzymes.  相似文献