Isotope insight into microbial processes on the North Pond Leg, IODP expedition 336

微生物是一种单细胞生物，遍布地球表面，包括海洋和海洋中的沉积物。许多微生物像人类一样呼吸氧气，在这个过程中消耗有机碳。然而，在海洋沉积物中，通常没有足够的氧气——在一些地方根本没有氧气。在这些“缺氧”环境中，有微生物“呼吸”其他分子，如硫酸盐。了解这些生物在我们所谓的“深层生物圈”中的功能是很重要的，因为它们与地球上最早的一些生命相似。我们特别感兴趣的是这些生物如何呼吸硫酸盐，产生硫化物，以及其他生物如何利用这些硫化物——这被称为微生物硫循环。这一点尤其重要，因为这是海洋沉积物中主要的微生物过程，因此，深层生物圈中使用的硫酸盐的数量会影响沉积物中最终埋藏的有机碳的数量，而有机碳的数量又与大气中的氧气含量直接相关。对硫循环的正确理解可以帮助我们探索和理解氧气在大气中的进化时间和方式，以及这可能如何影响地球上生命的进化。此外，甲烷是一种非常强大的温室气体，迄今为止造成了20%的全球变暖，它在海洋沉积物中大量产生，但从未进入大气，因为硫酸盐还原消耗了它。了解硫酸盐还原如何与甲烷消耗耦合，对于确定海洋沉积物是否可能成为这种强效温室气体的来源非常重要。此外，呼吸硫化物的生物也可能生活在海底沉积物下面岩石的裂缝和孔隙中。当这些生物消耗硫化物时，它们会产生酸，酸会腐蚀（“风化”）周围的岩石。这导致钙、硅和其他重要化学物质的释放，最终进入海水。了解向海洋输送化学物质的数量和速率是很重要的，因为这与被带到海洋并储存在海洋中的无机碳的数量直接相关。向海洋输送碳的变化对数百万年来的气候有着重要的影响，因为海洋中的碳含量对大气中的二氧化碳水平有很大的影响，因此对全球温度也有很大的影响。P-I Turchyn之前的工作是利用硫酸盐中同位素的比例来了解硫酸盐还原和硫循环的过程。一种元素的同位素是该元素（在这种情况下是硫或氧）的一种形式，它的原子核中有额外的中子。这些额外的中子使含有较重同位素的分子在化学反应中表现不同。地球化学家对硫酸盐（SO4）有两种感兴趣的同位素比率——34S到32S和18O到16O。我们可以利用这些差异来确定生物在地下生活的位置，以及它们是如何处理硫的。在这个建议中，我们想进一步探讨这个问题。我们将在大西洋中脊的北池钻井点观察海底下的液体。我们将测量流体中硫酸盐中硫和氧的同位素比率，并将其与来自同一地点的各种沉积物的测量结果结合起来，以更好地了解硫在地下是如何变化的。这将使我们能够重建微生物呼吸过程中碳在哪里以及如何被氧化，以及下垫岩石中是否有足够的硫基微生物来显著影响风化过程。这项研究有可能极大地扩展我们对地下深层生物圈过程的认识。

Microbes are single celled organisms that live all over the surface of the Earth, including in the oceans and in the sediments within the oceans. Many microbes respire oxygen, like humans do, consuming organic carbon in the process. However, in marine sediments, there often isn't enough oxygen - in some places there is no oxygen at all. In these "anoxic" environments there are microbes that "respire" other molecules such as sulphate. Understanding the functioning of these organisms in what we call the "deep biosphere" is important because they are similar to some of the earliest life on Earth. We are particularly interested in how these organisms respire sulphate, produce sulphide, and how other organisms use this sulphide - this is called the microbial sulphur cycle. This is especially important because this is the dominant microbial process in marine sediments, therefore the amount of sulphate that is used in the deep biosphere influences how much organic carbon is ultimately buried in sediments, which links directly to the amount of oxygen in our atmosphere. A proper understanding of the sulphur cycle could help us explore and understand when and how oxygen evolved in the atmosphere and how this may have influenced the evolution of life on Earth. Also, methane, a very powerful greenhouse gas responsible for 20% of the global warming to date, is produced abundantly in marine sediments, but never makes it to the atmosphere because sulphate reduction consumes it. Understanding how sulphate reduction is coupled to methane consumption is important to determining if marine sediments could become a source of this potent greenhouse gas. Furthermore, it is possible that the organisms which respire sulphide may also live in cracks and pores in the rock beneath the sediment on the seafloor. As these organisms use up sulphides, they produce acids which would corrode ("weather") the surrounding rock. This results in the release of calcium, silicon, and other important chemicals, which ultimately end up in seawater. It is important to understand the amount and rates of delivery of chemicals to the ocean because this is directly linked to the amount of inorganic carbon that is brought to, and stored in the ocean. Changes in the delivery of carbon to the ocean have important implications for climate over millions of years because the amount of carbon in the ocean has a large impact on atmospheric carbon dioxide levels and therefore global temperature. P-I Turchyn's previous work has used ratios of isotopes in sulphate to understand the processes of sulphate reduction and sulphur cycling. An isotope of an element is a form of that element (sulphur or oxygen in this case) that has extra neutrons in its nucleus. These extra neutrons make molecules containing the heavier isotopes behave differently in chemical reactions. Sulphate (SO4) has two isotope ratios of interest to geochemists - 34S to 32S and 18O to 16O. We can use these differences to pinpoint where organisms are living in the subsurface, and how they are processing sulphur. In this proposal we would like to explore this further. We will look at fluids from under the ocean floor at the North Pond drilling sites on the Mid-Atlantic Ridge. We will measure the isotope ratios for sulphur and oxygen in sulphate in the fluids, and combine this with measurements on various sediments from these same sites to better understand how sulphur is modified in the subsurface. This will allow us to reconstruct where and how the carbon is oxidized during microbial respiration, and whether there is sufficient sulphur-based microbiology in the underlying rock to significantly affect weathering processes. This research has the possibility to greatly expand our knowledge of processes in the subsurface deep biosphere.