Nitrogen powering life in an active serpentinising system - an analogue to early life on Earth

地球上的生命是如何开始的？尽管分歧依然存在，但有一点是肯定的，那就是第一生命需要水、能源和非生物制造的有机化合物；而第一生命的最佳候选者是微生物。要在早期的地球上发现它们，最好的观察地点是水与来自地球内部未反应的岩石相遇的地方。地幔岩石被称为橄榄岩，通常存在于海底以下6公里或陆地表面以下40公里处，由于板块构造的原因，沿着板块边界的逆冲推覆可能会使其浮出水面。这些橄榄岩是还原金属组分的储存库，尤其是铁，暴露于水中时与水反应生成气态氢(H2)。这会引发一系列自发反应，释放能量，将二氧化碳(CO2)转化为重碳酸盐、甲烷(CH4)和其他简单的有机化合物。这些反应统称为“蛇纹化”，因此为生命的出现提供了理想的环境。今天，它们出现在海底的低温热液系统中，或者出现在蛇绿岩中，蛇绿岩是古代洋壳和上地幔，在陆地上被抬升，比如在阿曼苏丹国发现的那样。这些很可能是第一个生命摇篮最好的现代类比。到目前为止，已经进行了许多使用这些系统的研究，试图了解生物圈是如何在地球上或其他星球上进化的。然而，在所有这些研究中，缺少氮(N)的来源，氮是制造DNA、酶和蛋白质的关键元素。今天，许多生态系统中的生物生长受到氮有效性的限制。虽然自地球早期以来，大气中就有大量的氮以气态氮的形式存在，但生命要使用这种氮，必须打破氮气的强三键，这需要相当大的能量。氮也可能以亚硝酸盐(NO2)和硝酸盐(NO3)的形式出现，但这两种物质首先都是由大气中的氮气闪电产生的，然后在与暴露的地幔橄榄岩接触之前被降雨到海洋中。最近，岩石分析发现，铵(NH4+)有时会取代矿物中的某些金属(如钾)，因此固体地球的氮含量是大气的~7倍。因此，如果生命能够利用这个巨大的氮源，早期生物圈就不会受到N的限制。另一方面，N可以以多种形式的不同电化学势存在，因此它的许多变化可以自发地与其他化学物质一起发生，以产生支持生命的能量。最值得注意的是，当氧气耗尽时，NO3是呼吸(呼吸)的首选替代品，从而将食物(有机碳)燃烧成二氧化碳，以获得生命新陈代谢所需的能量。与此同时，一些微生物可能会利用NO2和NH4+或CH4之间的反应产生的能量来从二氧化碳中制造自己的食物，这类似于植物进行光合作用，但使用的是化学能而不是阳光。因此，随着现代地下蛇纹化系统中存在各种N-形式，可能会发生各种N-转换来为内部的微生物群提供动力。这些反应的活动及其对环境的影响从来没有被评估过，这是无伤大雅的。这个项目试图研究地下生物圈是如何获得N的，以及在蛇纹化系统中地下N循环是如何运作和与地下生物圈相互作用的。我们将使用罕见的重N-15N-来跟踪微生物对N的转化，并将岩石和流体中的15N-含量作为示踪剂，结合最先进的生物成像和基因表达，评估微生物如何获得其细胞N，以及N-转化在多大程度上为地下生命提供动力。我们将使用阿曼蛇绿岩，这是世界上最大、暴露最好的洋壳和上地幔地块，因为它很容易进入，而且阿曼钻井项目提供了新钻的深孔和钻芯，因此我们将使用它作为主动蛇纹岩系统的模型。

How did life begin on Earth? While disagreements remain, one thing for certain is that the first life needed water, a source of energy and non-biologically made organic compounds; and the best candidate for the first life was a microbe. To find these on early Earth, the best place to look would be where water met unreacted rocks from the Earth's interior. Mantle rocks called peridotites, normally residing >6 km below the seafloor or 40 km below land surface, could be brought to surface by overthrust along plate boundaries due to plate tectonics. These peridotites are a reservoir of reduced metallic components, especially iron, which react with water when exposed to form gaseous hydrogen (H2). This then triggers a series of spontaneous reactions that release energy and turn carbon dioxide (CO2) into bicarbonate and methane (CH4), and other simple organic compounds. These reactions, collectively known as 'serpentinisation', thus provide the ideal setting for the emergence of life. Today, these occur in low-temperature hydrothermal systems on the seafloor, or in 'ophiolites', ancient ocean crust and upper mantle that got uplifted on land such as that found in the Sultanate of Oman. These are likely the best modern analogues of the first cradle of life. Many studies have been conducted to date using these systems to try to understand how the biosphere has been evolving on Earth and perhaps on other planets.Missing in all these investigations, however, is the source of nitrogen (N), the key element used to make DNA, enzymes and proteins. Biological growth in many ecosystems today is limited by the availability of N. Although substantial amounts of N have been present in the atmosphere as gaseous N2 since early Earth, for life to use this N the strong triple bond of N2 has to be broken, and it takes considerable energy. N could also have come as nitrite (NO2) and nitrate (NO3), but both first had to be made by lightning from atmospheric N2, and then rained into the ocean before coming in contact with exposed mantle peridotites. Recently, rock analyses have found that ammonium (NH4+) sometimes replaces certain metals (e.g. potassium) in minerals such that the solid Earth holds ~7 times the N as the atmosphere. Hence, if life can tap into this immense N source, the early biosphere would not be N-limited.On the other hand, N can exist in several forms of varying electrochemical potentials, and so its many transformations can occur spontaneously with other chemicals to generate energy to support life. Most notably, NO3 is the first-choice alternative used for breathing (respiration) when oxygen runs out, thereby burning 'food' (organic carbon) into CO2 to obtain the necessary energy for life metabolisms. Meanwhile, some microbes may harness the energy from the reactions between NO2 and NH4+ or CH4 to make their own food from CO2, akin to plants performing photosynthesis but with chemical energy instead of sunlight. Therefore, as various N-forms are present in modern subsurface serpentinising systems, various N-transformations may occur to power the microbiome within. The activities of these reactions and their impacts on the environment have never been assessed, nonetheless.This project seeks to examine how subsurface biosphere acquires N, and how subsurface N-cycling operates and interacts with the subsurface biosphere in a serpentinising system. We will use the rare heavy form of N -15N- to track N-transformations by microbes, and 15N-content in rocks and fluids as tracers, combined with state-of-the-art bioimaging and gene expression, to assess how microbes obtain their cellular N, and to what extent N-transformations are 'actively' powering subsurface life. We will use the Oman ophiolite, the world's largest, best exposed block of oceanic crust and upper mantle as a model active serpentinising system, given its easy access and the newly drilled deep boreholes and drill cores made available by the Oman Drilling Project.