Limits of life in MgCl2-containing environments:: chaotropicity defines the window
Limits of life in MgCl2-containing environments:: chaotropicity defines the window
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DOI：
10.1111/j.1462-2920.2006.01212.x
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发表时间：
2007-03-01
影响因子：
5.1
通讯作者：
McGenity, Terry J.
中科院分区：
文献类型：
--
作者：
Hallsworth, John E.;Yakimov, Michail M.;McGenity, Terry J.
地球生物圈的物理化学条件对生命过程和生物分子的结构和功能的维持来说过于苛刻或不一致。为了定义地球上的生命窗口（或许还能了解其他地方的生命所能容忍的极限），从而了解在这种极端条件下运行的一些最不寻常的生物活动，有必要了解这些窗口之外系统故障的原因和细胞基础。由于水在生物分子和生物过程中起着如此重要的作用，其可用性、性质和行为是限制生命的关键参数之一。咸水沃茨主宰着地球，海洋拥有地球96.5%的水。含盐地下水、内陆海或咸水湖还有1%，这一数量超过了世界上可用的淡水。大约四分之一的地球陆地被盐覆盖，通常超过100米厚。蒸发岩沉积物在其盐晶体内部和之间含有高盐水沃茨，甚至含有大型地下盐湖，因此代表了重要的微生物栖息地。盐对生物圈的性质和范围有重大影响，因为溶质从根本上影响水的可用性（水活性），并发挥其他活动，也影响生物系统（如离子，亲液，离液序列和那些影响细胞膨压），因此可能是细胞系统的主要压力源。尽管盐具有应激效应，但高盐环境中可能大量存在耐盐或依赖盐的微生物，即嗜盐菌。在高盐环境中最常见的盐是NaCl，但许多蒸发岩沉积物和盐水也富含其他盐，包括MgCl 2（数亿吨水氯镁石，MgCl 2.6H（2）O，仅在一个地层中出现）。镁（Mg）是溶解在海水中的第三大最丰富的元素，在地壳和整个太阳系中无处不在，它与各种阴离子一起存在。氯化镁极易溶于水，因此在盐水中可以达到高浓度（> 5 M）。然而，虽然NaCl占主导地位的高盐环境是多种盐适应微生物的栖息地，但在富含MgCl 2的环境中存在相互矛盾的生命迹象。在这项工作中，我们试图获得新的见解如何MgCl 2影响细胞系统，以评估MgCl 2是否可以确定生命的窗口，如果是这样，推导出这个窗口的值。我们已经剖析了MgCl 2溶液的两个相关细胞应激相关活性，即水活性降低和离液性，并分析了自然环境中不同MgCl 2浓度下的生命特征，即海水的0.05-5.05 M MgCl 2梯度：发现盆地的高盐卤水界面-位于地中海海底的一个大型稳定的卤水湖，几乎充满了MgCl 2。我们在这里记录的异常离液性氯化镁，并表明，这种属性，而不是水活度降低，抑制生命的变性生物大分子。在体外，浓度低于1 M的MgCl 2可完全抑制测试酶; MgCl 2浓度高于1.26 M的培养基可抑制从Discovery界面所有部分采集的样品中微生物的生长。（硫酸盐还原菌和产甲烷菌）沿海水的整个MgCl 2梯度沿着检测到：发现盐水界面，mRNA，活性微生物的高度不稳定的指示剂，仅在MgCl 2浓度小于2.3 M时从化跃层的上部回收。我们还表明，在高浓度的氯化镁的极端chaotropicity不仅变性大分子，而且还保留了更稳定的：这样的指示剂分子，迄今为止被视为生命的证据，因此可能是误导签名在离液环境。因此，MgCl 2的离液性似乎是一个决定生命的窗口参数，这里获得的结果表明，在没有补偿（例如，亲液性）溶质的情况下，生命的MgCl 2浓度上限约为2.3 M。
The biosphere of planet Earth is delineated by physico-chemical conditions that are too harsh for, or inconsistent with, life processes and maintenance of the structure and function of biomolecules. To define the window of life on Earth (and perhaps gain insights into the limits that life could tolerate elsewhere), and hence understand some of the most unusual biological activities that operate at such extremes, it is necessary to understand the causes and cellular basis of systems failure beyond these windows. Because water plays such a central role in biomolecules and bioprocesses, its availability, properties and behaviour are among the key life-limiting parameters. Saline waters dominate the Earth, with the oceans holding 96.5% of the planet's water. Saline groundwater, inland seas or saltwater lakes hold another 1%, a quantity that exceeds the world's available freshwater. About one quarter of Earth's land mass is underlain by salt, often more than 100 m thick. Evaporite deposits contain hypersaline waters within and between their salt crystals, and even contain large subterranean salt lakes, and therefore represent significant microbial habitats. Salts have a major impact on the nature and extent of the biosphere, because solutes radically influence water's availability (water activity) and exert other activities that also affect biological systems (e.g. ionic, kosmotropic, chaotropic and those that affect cell turgor), and as a consequence can be major stressors of cellular systems. Despite the stressor effects of salts, hypersaline environments can be heavily populated with salt-tolerant or -dependent microbes, the halophiles. The most common salt in hypersaline environments is NaCl, but many evaporite deposits and brines are also rich in other salts, including MgCl2 (several hundred million tonnes of bischofite, MgCl2.6H(2)O, occur in one formation alone). Magnesium (Mg) is the third most abundant element dissolved in seawater and is ubiquitous in the Earth's crust, and throughout the Solar System, where it exists in association with a variety of anions. Magnesium chloride is exceptionally soluble in water, so can achieve high concentrations (> 5 M) in brines. However, while NaCl-dominated hypersaline environments are habitats for a rich variety of salt-adapted microbes, there are contradictory indications of life in MgCl2-rich environments. In this work, we have sought to obtain new insights into how MgCl2 affects cellular systems, to assess whether MgCl2 can determine the window of life, and, if so, to derive a value for this window. We have dissected two relevant cellular stress-related activities of MgCl2 solutions, namely water activity reduction and chaotropicity, and analysed signatures of life at different concentrations of MgCl2 in a natural environment, namely the 0.05-5.05 M MgCl2 gradient of the seawater : hypersaline brine interface of Discovery Basin - a large, stable brine lake almost saturated with MgCl2, located on the Mediterranean Sea floor. We document here the exceptional chaotropicity of MgCl2, and show that this property, rather than water activity reduction, inhibits life by denaturing biological macromolecules. In vitro, a test enzyme was totally inhibited by MgCl2 at concentrations below 1 M; and culture medium with MgCl2 concentrations above 1.26 M inhibited the growth of microbes in samples taken from all parts of the Discovery interface.Although DNA and rNA from key microbial groups (sulfate reducers and methanogens) were detected along the entire MgCl2 gradient of the seawater : Discovery brine interface, mRNA, a highly labile indicator of active microbes, was recovered only from the upper part of the chemocline at MgCl2 concentrations of less than 2.3 M. We also show that the extreme chaotropicity of MgCl2 at high concentrations not only denatures macromolecules, but also preserves the more stable ones: such indicator molecules, hitherto regarded as evidence of life, may thus be misleading signatures in chaotropic environments. Thus, the chaotropicity of MgCl2 would appear to be a window-of-life-determining parameter, and the results obtained here suggest that the upper MgCl2 concentration for life, in the absence of compensating (e.g. kosmotropic) solutes, is about 2.3 M.