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锌的生物利用度新数据挑战早期生命进化观点

锌的生物利用度新数据挑战早期生命进化观点

美国加州大学滨河分校的生物地质化学家们对十分流行的古海洋化学假设进行了验证,结果发现它是错误的。该研究发表在近期的《自然-地球科学》杂志上。

许多研究人员将原核生物的多样性与增殖的延迟归因于早期海水中金属锌的浓度过低。对于人类而言,锌对于基本细胞增殖多过程都是必备的。锌结合蛋白主要位于细胞核中,它在细胞基因转录调控过程中起到关键作用。在对北美、非洲、澳洲、亚洲以及欧洲的海相黑色页岩研究后,科学家发现这些页岩中含有高浓度的锌,25亿年至5.42亿年前的原生代海水中锌的浓度与现代海水中锌的浓度相近。研究人员表示,“海相黑色页岩的锌浓度与海水中锌的浓度有着直接关系,锌在这些贯穿整个地球历史的岩石中含量十分丰富。在海水中,没有发现锌的生物制约性的证据。”

先前的研究假设原生代的海洋含有大量的硫磺酸,基于此,斯科特解释了锌限制性与真核生物的进化之间的联系。在大量的硫磺酸的条件下,单质锌可能会变得稀少,因为它会迅速生成沉淀。在2011年一篇论文中,该研究小组证明了原生代海洋更可能含有大量铁。

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NATURE GEOSCIENCE: Bioavailability of zinc in marine systems through time 

doi:10.1038/ngeo1679

The redox state of the oceans strongly influences the concentration of dissolved trace metals in sea water. Changes in the redox state of the oceans are thought to have limited the availability of some trace metals in the past, particularly during the Proterozoic eon, 2,500 to 542 million years ago. Of these trace metals, zinc (Zn) is of particular importance to eukaryotic organisms, because it is essential for a wide range of basic cellular functions. It has been suggested that during the Proterozoic, marine environments were broadly euxinic—that is, anoxic and sulphidic—which would have resulted in low Zn availability. Low Zn bioavailability could therefore be responsible for an observed delay in eukaryote diversification. Here we present a compilation of Zn abundance data from black shales deposited under euxinic conditions from the Precambrian time to the present. We show that these values track first-order trends in seawater Zn availability. Contrary to previous estimates, we find that Zn concentrations during the Proterozoic were similar to modern concentrations, supporting recent studies that call for limited euxinia at this time. Instead, we propose that predominantly anoxic and iron-rich deep oceans, combined with large hydrothermal fluxes of Zn, maintained high levels of dissolved Zn throughout the oceans. We thus suggest that the protracted diversification of eukaryotic Zn-binding proteins was not a result of Znbiolimitation.

NATURE
: Widespread iron-rich conditions in the mid-Proterozoic ocean 

doi:10.1038/nature10327

The chemical composition of the ocean changed markedly with the oxidation of the Earth’s surface, and this process has profoundly influenced the evolutionary and ecological history of life. The early Earth was characterized by a reducing ocean–atmosphere system, whereas the Phanerozoic eon (less than 542 million years ago) is known for a stable and oxygenated biosphere conducive to the radiation of animals. The redox characteristics of surface environments during Earth’s middle age (1.8–1 billion years ago) are less well known, but it is generally assumed that the mid-Proterozoic was home to a globally sulphidic (euxinic) deep ocean. Here we present iron data from a suite of mid-Proterozoic marine mudstones. Contrary to the popular model, our results indicate that ferruginous (anoxic and Fe2+-rich) conditions were both spatially and temporally extensive across diverse palaeogeographic settings in the mid-Proterozoic ocean, inviting new models for the temporal distribution of iron formations and the availability of bioessential trace elements during a critical window for eukaryotic evolution.

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