《科學》（20190614出版）一周論文導讀

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Science, 14 JUNE 2019, VOL 364, ISSUE 6445

《科學》2019年6月14日，第6445期364卷

天文學Astronomy

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

卡西尼號近距離飛越土星環衛星——潘、達夫涅斯、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

鏈接：

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

摘要：

土星的主光環系統與一組小衛星有關。這些小衛星要麼嵌入土星光環中，要麼與光環相互作用，改變光環的形狀和組成。

2016年12月~2017年4月，在卡西尼號執行「掠環軌道」任務期間，5次近距離飛越了潘、達夫涅斯、阿特拉斯、潘多拉和伊比米修斯衛星。

探測器返回了關於這些衛星形態、結構、粒子環境和成分的數據，以及紫外和熱紅外圖像。

我們發現，這些衛星表面的光學特性由兩個相互競爭的過程決定：一是土星主環系統中形成的紅色物質的污染，二是源自土衛二的火山羽狀物中明亮的冰粒或水蒸氣的累積。

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

土星重力場和環質量的測量及其意義

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

鏈接：

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

摘要：

土星的內部結構、風的深度、光環的質量和年齡限制了它的形成和演化。

在卡西尼號任務的最後階段，這艘宇宙飛船在這顆行星和它最內層的光環之間俯衝，高度在雲層上方2600~3900公里。

在其中6次穿越中，與地球的無線電聯繫得以監控，從而確定該行星的引力場及其光環的質量。

我們發現，土星的重力偏離了理論預期，需要大氣的差速旋轉延伸到至少9000公里的深度。光環的總質量為（1.54±0.49）× 10¹⁹千克（0.41±0.13倍於土衛一），表明光環可能形成於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) × 10¹⁹ kilograms (0.41 ± 0.13 times that of the moon Mimas), indicating that the rings may have formed 10⁷to 10⁸years ago.

生物學Biology

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

發現和抑制左旋多巴代謝的種間腸道細菌路徑

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

鏈接：

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

摘要：

人體腸道微生物群代謝帕金森氏症藥物——左旋多巴（L-dopa），可能降低藥物的可用性並引起副作用。

然而，在病人體內導致這種活性的生物體、基因和酶及其對宿主靶向藥物抑制的敏感性尚不清楚。

這裡，我們描述了腸道細菌L-dopa代謝的種間通路。來自糞腸球菌依賴於吡哆醛磷酸鹽的酪氨酸脫羧酶將L-dopa轉化為多巴胺，然後由來自遲緩埃格特菌依賴於鉬的脫羥酶將多巴胺轉化為間酪氨酸。

這些酶可以預測複雜人體腸道微生物群的藥物代謝。

儘管一種靶向宿主芳香氨基酸脫羧酶的藥物並不能阻止腸道微生物L-dopa脫羧，但我們發現了一種可抑制帕金森氏症患者微生物群活性的化合物，並且提高了小鼠體內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

一種用於三方海洋共生的微生物工廠

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

鏈接：

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

摘要：

在自然生態系統中，針對捕食者的化學防禦廣泛存在。有時，分類上相距遙遠的生物體共享相同的防禦化學物質。

這裡，我們描述了一種不尋常的三方海洋共生，其中細胞內的細菌共生體（Candidatus Endobryopsis kahalalidefaciens）利用多種生物合成酶，將簡單的底物轉化為複雜分子庫（kahalalides），用於對宿主的化學防禦，以抵禦捕食。

kahalalides隨後被第三個夥伴——草食性軟體動物Elysia rufescens劫持，並被用於類似防禦。Ca. E. kahalalidefaciens失去了自由生活的許多基本特徵，並成為生產kahalalide的工廠。

細菌、藻類和動物之間的這種相互作用強調了化學防禦在複雜共生生物進化中的重要性。

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.

物理學Physics

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

飛秒X射線衍射揭示相變材料的液—液相變

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

鏈接：

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

摘要：

在相變存儲器中，材料在玻璃態和晶體態之間循環。其結晶過程的高度依賴於溫度的動力學，使其能應用於存儲技術，但這種轉變尚未在原子尺度上得到解決。

利用飛秒X射線衍射和從頭計演算法的計算機模擬，我們確定了相變材料在熔體淬火和結晶過程中隨時間變化的對相關函數。

在660和610開氏溫度下，我們發現了相變材料Ag₄In₃Sb₆₇Te₂₆和Ge₁₅Sb₈₅的液—液相變。這種轉變主要是由派爾斯畸變的開始引起的，其振幅與擴散率表觀活化能的增加有關。

這揭示了原子結構和動力學之間的關係，使系統優化存儲開關動力學成為可能。

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 Ag₄In₃Sb₆₇Te₂₆and Ge₁₅Sb₈₅ at 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

基於可調諧介電質超表面的純相位透射空間光調製器

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

鏈接：

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

摘要：

增強現實、固態光探測與測距（LIDAR）以及全息顯示技術的快速發展，需要高解析度、高視角的空間光調製器（SLMs）來滿足日益增長的客戶需求。目前可用的SLMs的性能受到幾個微米級像素大小的限制。

這裡，我們提出了可調諧介電質超表面的概念。其受到液晶調製，可提供突變相位，從而實現像素大小的小型化。

我們提出了一種基於超表面的透射SLM。其被配置成產生效能大於35%、大波束偏轉角為11°的主動波束轉向。

獲得的高解析度和轉向角為發展下一代激光雷達和顯示技術提供了機會。

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.

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