趋磁细菌磁小体的研究进展

趋磁细菌是一类能够沿着磁场方向运动的革兰氏阴性细菌的总称,其最显著的共同特征是胞内能够合成链状排列、具有外膜包被的Fe3O4或Fe3S4磁性颗粒—磁小体。概要介绍了趋磁细菌的发现、生态分布、一般特性和分离纯化存在的问题,重点讨论了磁小体合成的主要过程,并对将来磁小体的研究进行了展望。     

South-seeking magnetotactic bacteria in the Northern Hemisphere

Magnetotactic bacteria contain membrane-bound intracellular iron crystals (magnetosomes) and respond to magnetic fields. Polar magnetotactic bacteria in vertical chemical gradients are thought to respond to high oxygen levels by swimming downward into areas with low or no oxygen (toward geomagnetic north in the Northern Hemisphere and geomagnetic south in the Southern Hemisphere). We identified populations of polar magnetotactic bacteria in the Northern Hemisphere that respond to high oxygen levels by swimming toward geomagnetic south, the opposite of all previously reported magnetotactic behavior. The percentage of magnetotactic bacteria with south polarity in the environment is positively correlated with higher redox potential. The coexistence of magnetotactic bacteria with opposing polarities in the same redox environment conflicts with current models of the adaptive value of magnetotaxis.     

Magnetic microstructure of magnetotactic bacteria by electron holography

Off-axis electron holography in the transmission electron microscope was used to correlate the physical and magnetic microstructure of magnetite nanocrystals in magnetotactic bacteria. The magnetite crystals were all single magnetic domains, and the magnetization directions of small superparamagnetic crystals were constrained by magnetic interactions with larger crystals in the chains. Shape anisotropy was found to dominate magnetocrystalline anisotropy in elongated crystals. A coercive field between 300 and 450 oersted was determined for one chain.     

A cultured greigite-producing magnetotactic bacterium in a novel group of sulfate-reducing bacteria

Magnetotactic bacteria contain magnetosomes--intracellular, membrane-bounded, magnetic nanocrystals of magnetite (Fe(3)O(4)) or greigite (Fe(3)S(4))--that cause the bacteria to swim along geomagnetic field lines. We isolated a greigite-producing magnetotactic bacterium from a brackish spring in Death Valley National Park, California, USA, strain BW-1, that is able to biomineralize greigite and magnetite depending on culture conditions. A phylogenetic comparison of BW-1 and similar uncultured greigite- and/or magnetite-producing magnetotactic bacteria from freshwater to hypersaline habitats shows that these organisms represent a previously unknown group of sulfate-reducing bacteria in the Deltaproteobacteria. Genomic analysis of BW-1 reveals the presence of two different magnetosome gene clusters, suggesting that one may be responsible for greigite biomineralization and the other for magnetite.     

趋磁性木材的制备与多功能化修饰

受生物体磁场感应特征的启发而被开发利用的趋磁性木材是一种新型的功能性材料,当前对它的研究仍在起步阶段。文中系统地探讨了水热工艺参数,如反应的温度、时间和反应物的浓度对趋磁性木材结构和性能的影响,并通过扫描电子显微镜(SEM)、X射线衍射(XRD)仪和磁学测量系统对趋磁性木材的表面形貌、晶体类型和磁性能进行了分析。结果发现,高的反应温度有利于在木材表面生成粒径更大的磁性粒子,从而得到具有更高饱和磁化强度的磁性木材。反应时间对趋磁性木材形成初期的形貌和磁性有重要影响,但当反应时间达到8 h时,木材上磁性粒子数量达到饱和状态。反应物浓度对生长在木材表面磁性粒子的纯度具有很重要的影响,合适的反应物浓度能够确保得到高纯度的磁性粒子。此外,在优化工艺的基础上对趋磁性木材进行疏水改性,可以得到同时具有抗紫外老化性能、超疏水性和磁性的多功能性木材,拓展了趋磁性木材的应用领域。     

Magnetite: Behavior near the Single-Domain Threshold

Maximum values for the single-domain threshold d(0) and superpara-magnetic threshold d(8) in pure magnetite are found to be 570+/-50 and 350+/-50 angstroms, respectively. Particles larger than do but smaller than about 0.25 micron have size-dependent saturation remanences and coercive forces like those of multidomain particles, but intense and stable thermoremanent magnetization like that of single-domain particles. The presence of magnetite grains in this size range could account for the essentially single-domain character of stable natural remanence in many volcanic and intrusive rocks.     

Attenuation of low-frequency sound in freshwater

Lake Superior was chosen as an experimental site to compare sound absorption of freshwater with the results of absorption measurements in seawater. The relaxation-like absorption at 1 kilohertz occurring in seawater is also present in freshwater. A relaxation related to the structural characteristics rather than to the salt content of water may be responsible for the anomalous absorption.     

Chemotaxis in bacteria

Motile Escherichia coli placed at one end of a capillary tube containing an energy source and oxygen migrate out into the tube in one or two bands, which are clearly visible to the naked eye and can also be demonstrated by photography, microscopy, and densitometry and by assaying for bacteria throughout the tube. The formation of two bands is not due to heterogeneity among the bacteria, since the bacteria in each band, when reused, will form two more bands. If an anaerobically utilizable energy source such as galactose is present in excess over the oxygen, the first band consumes all the oxygen and a part of the sugar and the second band uses the residual sugar anaerobically. On the other hand, if oxygen is present in excess over the sugar, the first band oxidizes all the sugar and leaves behind unused oxygen, and the second band uses up the residual oxygen to oxidize an endogenous energy source. The essence of the matter is that the bacteria create a gradient of oxygen or of an energy source, and then they move preferentially in the direction of the higher concentration of the chemical. As a consequence, bands of bacteria (or rings of bacteria in the case of agar plates) form and move out. These results show that E. coli is chemotactic toward oxygen and energy sources such as galactose, glucose, aspartic acid, threonine, or serine. The full repertoire of chemotactic responses by E. coli is no doubt greater than this, and a more complete list remains to be compiled. The studies reported here demonstrate that chemotaxis allows bacteria to find that environment which provides them with the greatest supply of energy. It is clearly an advantage for bacteria to be able to carry out chemotaxis, since by this means they can avoid unfavorable conditions and seek optimum surroundings. Finally, it is necessary to acknowledge the pioneering work of Englemann, Pfeffer, and the other late-19thcentury biologists who discovered chemotaxis in bacteria, and to point out that the studies reported here fully confirm the earlier reports of Beijerinck (4) and Sherris and his collaborators (5,6) on a band of bacteria chemotactic toward oxygen. By using a chemically defined medium instead of a complex broth, it has been possible to study this band more closely and to demonstrate in addition the occurrence of a second band of bacteria chemotactic toward an energy source. Beijerinck (4) did, in fact, sometimes observe a second band, but he did not offer an explanation for it.     

Predator-induced life-history shifts in a freshwater snail

The snail Physella virgata virgata, a widely distributed freshwater pulmonate, was observed to change its life-history characteristics in the presence of the crayfish Orconectes virilis in spring-fed Oklahoma streams. These changes were apparently initiated by a water-borne cue released when crayfish fed on conspecific snails. In the presence of the cue, snails exhibited rapid growth rates and little reproduction until they reached a size of about 10 mm after 8 months. In the absence of the cue, snails typically grew to about 4 mm (3.5 months) and then began reproduction. The chemically inducible shift indicates that the life histories of these snails are phenotypically plastic. By increasing the variance associated with size and age of maturity, prey may increase the likelihood of coexisting with seasonal predators.     

Sperm in "parhenogenetic" freshwater gastrotrichs

Freshwater members of the phylum Gastrotricha have been considered obligate parthenogens. In Lepidodermelia squammata, the species for which there is most evidence for parthenogenesis, sperm have been discovered. This finding will necessitate reexamination of the nature of sexuality and life cycles and of the concept of "species" in freshwater gastrotrichs.     

High-purity calcium carbonate in freshwater clam shell

The calcium carbonate in freshwater clam shells is simlilar in purity to that designated reagent grade. A simple reprecipitation of the shell extract results in a product having less Sr and Mg than reagent grade CaCO(3). Clams are harvested commercially, and discarded shells are a high-quality raw material for the production of CaCO(3).     

攀枝花钒钛磁铁矿区土壤重金属污染特征及评价

以攀枝花钒钛磁铁矿区土壤为对象,研究土壤中重金属污染的特征,并采用Hakanson提出的潜在生态危害指数法对土壤中重金属的潜在生态危害进行了评价。结果表明,攀枝花矿区排土场和西区堆渣场表层土壤中重金属含量较高,矿区表层土壤中Cu、Pb、Cd发生了中度和重度污染,从潜在生态危害指数(RI)来看,有5个采样点为强生态危险,总体上生态危害程度不高。污染主要集中在矿区排土场和堆渣场,尾矿库坝土壤生态危害较小,以轻微生态危害为主。     

Magnetite in CI Carbonaceous Meteorites: Origin by Aqueous Activity on a Planetesimal Surface

The composition and morphology of magnetite in CI carbonaceous meteorites appear incompatible with a nebular origin. Mineralization on the meteorite parent body is a more plausible mode of formation. The iodine-xenon age of this material therefore dates an episode of secondary mineralization on a planetesimal rather than the epoch of condensation in the primitive solar nebula.     

日本淡水渔业发展战略

为借鉴日本淡水渔业的先进发展经验,对日本淡水渔业制度、相关法律以及战略(内向型、外向型)进行了研究.结果表明,日本渔业发展战略向名优水产养殖品种(鳗鱼)和贸易品种(锦鲤)转移,并取得良好效果.进一步厘清了养鳗是维持日本淡水养殖产业的关键,同时通过日本生态修复措施保住了部分国内鳗鱼供给.日本政企联合实施锦鲤出口型战略,中...     

我国淡水鱼类寄生虫的研究概述

鱼类寄生虫病是目前水产养殖中发病率最高的疾病,因此深入研究鱼类寄生虫的种类及其及所致危害等对于预防和控制鱼类寄生虫病具有重要意义.文章对我国淡水鱼类寄生虫病的种类、防治方法进行了简单的归纳总结,又从肉眼观察、显微镜观察和分子免疫学等3个方面对鱼类寄生虫病的相关研究进展进行了介绍,并对该领域未来前景进行了展望.     

蜜蜂体内磁铁矿(Fe3O4)来源问题的研究

以蜂蜜和花粉为实验材料 ,应用X射线衍射仪来检测花粉和蜂蜜中的Fe3 O4。结果表明 :在花粉和蜂蜜中没有检测出Fe3 O4,由此我们推断蜜蜂可以合成Fe3 O4。     

中温条件下添加磁铁粉对麦秆厌氧发酵特性的影响

为探究不同添加量磁铁粉在不同温度条件下对厌氧发酵体系的影响,在发酵底物含固率为8%,麦秆为发酵原料,不同温度(20℃、25℃、30℃、35℃)下,向发酵瓶中分别添加不同剂量的磁铁粉(0、2、4、6、8 g/L),分析厌氧发酵过程中pH、碱度、甲烷产量、脱氢酶活性和纤维素酶活性的动态变化。结果表明,适量磁铁粉添加可以促进发酵系统的启动、增强系统稳定性,促进发酵产气。冗余分析和相关性分析表明温度为该试验最关键影响因子。动力学参数比较得出,麦秆在20℃、25℃、30℃和35℃进行厌氧发酵时,磁铁粉添加量分别为8、6、4、2 g/L可以使体系获得最好的发酵效率。