《科学》（20190614出版）一周论文导读

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《科学》（20190614出版）一周论文导读

2019 年 6 月 16 日 科学网

翻译 | 徐徐

Science, 14 JUNE 2019, VOL 364, ISSUE 6445

《科学》2019年6月14日，第6445期364卷

天文学Astronomy

Close Cassini flybys of Saturn’s ring moons Pan, Daphnis, Atlas, Pandora, and Epimetheus

卡西尼号近距离飞越土星环卫星——潘、达夫涅斯、阿特拉斯、潘多拉和伊比米修斯

▲ 作者：B. J. Buratti, P. C. Thomas, E. Roussos, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/eaat2349

▲ 摘要：

土星的主光环系统与一组小卫星有关。这些小卫星要么嵌入土星光环中，要么与光环相互作用，改变光环的形状和组成。

2016年12月~2017年4月，在卡西尼号执行“掠环轨道”任务期间，5次近距离飞越了潘、达夫涅斯、阿特拉斯、潘多拉和伊比米修斯卫星。

探测器返回了关于这些卫星形态、结构、粒子环境和成分的数据，以及紫外和热红外图像。

我们发现，这些卫星表面的光学特性由两个相互竞争的过程决定：一是土星主环系统中形成的红色物质的污染，二是源自土卫二的火山羽状物中明亮的冰粒或水蒸气的累积。

▲ Abstract

Saturn’s main ring system is associated with a set of small moons that either are embedded within it or interact with the rings to alter their shape and composition. Five close flybys of the moons Pan, Daphnis, Atlas, Pandora, and Epimetheus were performed between December 2016 and April 2017 during the ring-grazing orbits of the Cassini mission. Dataon the moons’ morphology, structure, particle environment, and composition were returned, along with images in the ultra violet and thermal infrared. We find that the optical properties of the moons’ surfaces are determined by two competing processes: contamination by a red material formed in Saturn’s mainring system and accretion of bright icy particles or water vapor from volcanicplumes originating on the moon Enceladus.

Measurement and implications of Saturn’s gravity field and ring mass

土星重力场和环质量的测量及其意义

▲ 作者：L. Iess, B. Militzer, Y. Kaspi, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/eaat2965

▲ 摘要：

土星的内部结构、风的深度、光环的质量和年龄限制了它的形成和演化。

在卡西尼号任务的最后阶段，这艘宇宙飞船在这颗行星和它最内层的光环之间俯冲，高度在云层上方2600~3900公里。

在其中6次穿越中，与地球的无线电联系得以监控，从而确定该行星的引力场及其光环的质量。

我们发现，土星的重力偏离了理论预期，需要大气的差速旋转延伸到至少9000公里的深度。光环的总质量为（1.54±0.49）× 10¹⁹千克（0.41±0.13倍于土卫一），表明光环可能形成于10⁷ ~ 10⁸年前。

▲ Abstract

The interior structure of Saturn, the depth of its winds, and the mass and age of its rings constrain its formation and evolution. In the final phase of the Cassini mission, the spacecraft dived between the planet and its innermost ring, at altitudes of 2600 to 3900 kilometers above the cloud tops. During six of these crossings, a radio link with Earth was monitored to determine the gravitational field of the planet and the mass of its rings. We find that Saturn’s gravity deviates from the oretical expectations and requires differential rotation of the atmosphere extending to a depth of at least 9000 kilometers. The total mass of the rings is (1.54 ±0.49) × 10¹⁹kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10⁷ to 10⁸ years ago.

生物学Biology

Discovery and inhibition of an interspecies gut bacterial pathway for Levodopa metabolism

发现和抑制左旋多巴代谢的种间肠道细菌路径

▲ 作者：Vayu Maini Rekdal, Elizabeth N. Bess, Jordan E. Bisanz, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/eaau6323

▲ 摘要：

人体肠道微生物群代谢帕金森氏症药物——左旋多巴（L-dopa），可能降低药物的可用性并引起副作用。

然而，在病人体内导致这种活性的生物体、基因和酶及其对宿主靶向药物抑制的敏感性尚不清楚。

这里，我们描述了肠道细菌L-dopa代谢的种间通路。来自粪肠球菌依赖于吡哆醛磷酸盐的酪氨酸脱羧酶将L-dopa转化为多巴胺，然后由来自迟缓埃格特菌依赖于钼的脱羟酶将多巴胺转化为间酪氨酸。

这些酶可以预测复杂人体肠道微生物群的药物代谢。

尽管一种靶向宿主芳香氨基酸脱羧酶的药物并不能阻止肠道微生物L-dopa脱羧，但我们发现了一种可抑制帕金森氏症患者微生物群活性的化合物，并且提高了小鼠体内L-dopa的生物利用度。

▲ Abstract

The human gut microbiota metabolizes the Parkinson’s disease medication Levodopa (L-dopa), potentially reducing drug availability and causing side effects. However, the organisms, genes, and enzymes responsible for this activity in patients and their susceptibility to inhibition by host-targeted drugs are unknown. Here, we describe an interspecies pathway for gut bacterial L-dopa metabolism. Conversion of L-dopato dopamine by a pyridoxal phosphate-dependent tyrosine decarboxylase from Enterococcus faecalis is followed by transformation of dopamine to m-tyramineby a molybdenum-dependent dehydroxylase from Eggerthella lenta. These enzymes predict drug metabolism in complex human gut microbiotas. Although a drug that targets host aromatic amino acid decarboxylase does not prevent gut microbialL-dopa decarboxylation, we identified a compound that inhibits this activity in Parkinson’s patient microbiotas and increases L-dopa bioavailability in mice.

A microbial factory for defensive kahalalides in a tripartite marine symbiosis

一种用于三方海洋共生的微生物工厂

▲ 作者：Jindong Zan, Zhiyuan Li, Ma. Diarey Tianero, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/eaaw6732

▲ 摘要：

在自然生态系统中，针对捕食者的化学防御广泛存在。有时，分类上相距遥远的生物体共享相同的防御化学物质。

这里，我们描述了一种不寻常的三方海洋共生，其中细胞内的细菌共生体（Candidatus Endobryopsis kahalalidefaciens）利用多种生物合成酶，将简单的底物转化为复杂分子库（kahalalides），用于对宿主的化学防御，以抵御捕食。

kahalalides随后被第三个伙伴——草食性软体动物Elysia rufescens劫持，并被用于类似防御。Ca. E. kahalalidefaciens失去了自由生活的许多基本特征，并成为生产kahalalide的工厂。

细菌、藻类和动物之间的这种相互作用强调了化学防御在复杂共生生物进化中的重要性。

▲ Abstract

Chemical defense against predators is widespread in natural ecosystems. Occasionally, taxonomically distant organisms share the same defense chemical. Here, we describe an unusual tripartite marine symbiosis, in which an intracellular bacterial symbiont (“Candidatus Endobryopsis kahalalidefaciens”) uses a diverse array of biosynthetic enzymes to convert simple substrates into a library of complex molecules (the kahalalides) for chemical defense of the host, the alga Bryopsis sp., against predation. The kahalalides are subsequently hijacked by a third partner, the herbivorous mollusk Elysia rufescens, and employed similarly for defense. “Ca.E. kahalalidefaciens” has lost many essential traits for free living and acts as a factory for kahalalide production. This interaction between a bacterium, an alga, and an animal highlights the importance of chemical defense in the evolution of complex symbioses.

物理学Physics

Femtosecond x-ray diffraction reveals aliquid–liquid phase transition in phase-change materials

飞秒X射线衍射揭示相变材料的液—液相变

▲ 作者：Peter Zalden, Florian Quirin, Mathias Schumacher, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/1062

▲ 摘要：

在相变存储器中，材料在玻璃态和晶体态之间循环。其结晶过程的高度依赖于温度的动力学，使其能应用于存储技术，但这种转变尚未在原子尺度上得到解决。

利用飞秒X射线衍射和从头计算法的计算机模拟，我们确定了相变材料在熔体淬火和结晶过程中随时间变化的对相关函数。

在660和610开氏温度下，我们发现了相变材料Ag₄In₃Sb₆₇Te₂₆和Ge₁₅Sb₈₅的液—液相变。这种转变主要是由派尔斯畸变的开始引起的，其振幅与扩散率表观活化能的增加有关。

这揭示了原子结构和动力学之间的关系，使系统优化存储开关动力学成为可能。

▲ Abstract

In phase-change memory devices, a materialis cycled between glassy and crystalline states. The highly temperature-dependent kinetics of its crystallization process enables application in memory technology, but the transition has not been resolved on an atomic scale. Using femtosecond x-ray diffraction and ab initio computer simulations, we determined the time-dependent pair-correlation function of phase-change materials through out the melt-quenching and crystallization process. We found a liquid–liquid phase transition in the phase-change materials Ag₄In₃Sb₆₇Te₂₆ and Ge₁₅Sb₈₅at 660 and 610 kelvin, respectively. The transition is predominantly caused bythe onset of Peierls distortions, the amplitude of which correlates with an increase of the apparent activation energy of diffusivity. This reveals are lationship between atomic structure and kinetics, enabling a systematic optimization of the memory-switching kinetics.

Phase-only transmissive spatial light modulator based on tunable dielectric metasurface

基于可调谐介电质超表面的纯相位透射空间光调制器

▲ 作者：Shi-Qiang Li, Xuewu Xu, Rasna Maruthiyodan Veetil, et al

▲ 链接：

https://science.sciencemag.org/content/364/6445/1087

▲ 摘要：

增强现实、固态光探测与测距（LIDAR）以及全息显示技术的快速发展，需要高分辨率、高视角的空间光调制器（SLMs）来满足日益增长的客户需求。目前可用的SLMs的性能受到几个微米级像素大小的限制。

这里，我们提出了可调谐介电质超表面的概念。其受到液晶调制，可提供突变相位，从而实现像素大小的小型化。

我们提出了一种基于超表面的透射SLM。其被配置成产生效能大于35%、大波束偏转角为11°的主动波束转向。

获得的高分辨率和转向角为发展下一代激光雷达和显示技术提供了机会。

▲ Abstract

Rapidly developing augmented reality, solid-state light detection and ranging (LIDAR), and holographic display technologies require spatial light modulators (SLMs) with high resolution and viewing angle to satisfy increasing customer demands. Performance of currently available SLMs is limited by their large pixel sizes on the order of several micrometers. Here, we propose a concept of tunable dielectric metasurfaces modulated by liquid crystal, which can provide abrupt phase change, thus enabling pixel-size miniaturization. We present a metasurface-based transmissive SLM, configured to generate active beam steering with >35% efficiency and a large beam deflection angle of 11°. The high resolution and steering angle obtained provide opportunities to develop the next generation of LIDAR and display technologies.