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？尽管仍存在分歧，但有一点是肯定的，即第一个生命需要水，一种能量来源和非生物合成的有机化合物;而第一个生命的最佳候选者是微生物。要在早期地球上找到这些，最好的地方是水与地球内部未反应的岩石相遇的地方。称为橄榄岩的地幔岩，通常位于海底以下6公里或陆地表面以下40公里处，由于板块构造，可以通过沿沿着板块边界的逆冲推覆作用带到地表。这些橄榄岩是还原金属成分的储集层，特别是铁，当暴露时与水反应形成气态氢（H2）。这会引发一系列自发反应，释放能量，将二氧化碳（CO2）转化为碳酸氢盐和甲烷（CH 4）以及其他简单的有机化合物。这些反应统称为“蛇形化”，为生命的出现提供了理想的环境。今天，这些都发生在海底的低温热液系统中，或者在“蛇绿岩”，古老的海洋地壳和上地幔中，这些地壳和上地幔在陆地上隆起，如在阿曼苏丹国发现的。这些可能是第一个生命摇篮的最佳现代模拟物。迄今为止，人们已经利用这些系统进行了许多研究，试图了解地球乃至其他星球上生物圈的演变过程。然而，所有这些研究都缺少氮（N）的来源，而氮是制造DNA、酶和蛋白质的关键元素。当今许多生态系统中的生物生长受到氮素可利用性的限制。虽然从地球早期开始，大气中就有大量的氮以气态N2的形式存在，但生命要使用这些氮，必须打破N2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，但这两种物质首先必须通过闪电从大气中的N2中产生，然后在与暴露的地幔橄榄岩接触之前下雨进入海洋。最近，岩石分析发现，铵（NH 4+）有时会取代矿物中的某些金属（例如钾），使得固体地球的N含量是大气的7倍。因此，如果生命能够利用这一巨大的氮源，那么早期生物圈就不会受到氮的限制。另一方面，氮可以以多种不同的电化学电位形式存在，因此它的许多转化可以与其他化学物质自发地发生，以产生支持生命的能量。最值得注意的是，当氧气耗尽时，NO3是用于呼吸（呼吸）的首选替代品，从而将“食物”（有机碳）燃烧成CO2，以获得生命代谢所需的能量。与此同时，一些微生物可以利用NO2和NH 4+或CH 4之间反应的能量，从CO2中制造自己的食物，类似于植物进行光合作用，但使用化学能而不是阳光。因此，由于各种N-形式存在于现代地下蛇形系统中，可能会发生各种N-转化，为内部的微生物组提供动力。然而，这些反应的活动及其对环境的影响从未被评估过。本项目旨在研究地下生物圈如何获得N，以及地下N循环如何在蛇形系统中与地下生物圈相互作用。我们将使用N-15 N-的罕见重形式来跟踪微生物的N-转化，并将岩石和流体中的15 N含量作为示踪剂，结合最先进的生物成像和基因表达，以评估微生物如何获得其细胞N，以及N-转化在多大程度上“积极”为地下生命提供动力。我们将使用阿曼蛇绿岩，世界上最大的，最好的暴露块体的海洋地壳和上地幔作为一个模型活跃的蛇纹岩系统，鉴于其易于访问和新钻的深钻孔和岩心提供的阿曼钻探项目。

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.