Mass Independent Fractionation of Magnesium Isotopes by Bacteria; A New Tool for Searching for Life on Earth and Beyond

细菌代表着一些最早的生命形式，在地球上已经存在了35亿年。然而，它们很少被保存在岩石记录中，它们的存在通常是从叠层石灰岩中的层状结构推断出来的，这些石灰岩是已知的由现代叠层岩中的细菌垫形成的。因此，拥有其他细菌活动的示踪剂将是有用的，这些示踪剂不一定依赖于保存在岩石记录中的细菌。镁是地壳中第八丰富的元素，也是海水中第四丰富的元素。因此，它是生命的重要组成部分，在细胞能量的产生以及植物叶绿素和人类饮食功能中发挥着关键作用。镁有三种同位素(原子质量分别为24、25和26)，自然界中发生的无机过程会产生镁同位素比率的差异，这只是与它们的相对原子质量有关，因此，当26毫克/24毫克的比率与25毫克/24毫克的比率相对应时，所有陆地物质基本上是一条直线。这条线被称为陆地质量分馏线。然而，最近的实验发现，一些细菌更喜欢使用25毫克而不是24毫克和26毫克2-3次来在他们的细胞中制造ATP能量。这样做的净效果是，这些细菌变得富含25毫克，应该位于陆地质量分馏之上。这种从镁的其他同位素中分离出的25毫克的分馏称为质量无关分馏。关键是，这种25毫克的浓缩物是过去细菌活动的潜在确凿证据，如果它能被保存在地质记录中的话。细菌通常在诱导碳酸盐矿物沉淀方面起作用。皮尔斯(co-i)最近的实验工作证明，细菌产生的一些碳酸盐确实有25 mg的浓缩，这是第一个显示镁的质量独立分馏的陆地物质。然而，在最初的实验中分析的材料很可能是蜂窝和碳酸盐沉淀物的混合物。这项研究的目的是用细菌沉淀具有广泛范围的镁/钙比的碳酸盐(与地球上观察到的类似)，并使用开放大学开发的一项新的实验技术来测量细菌和碳酸盐材料的镁同位素组成。这将使我们能够评估细菌摄取25毫克的程度，以及这个信号是如何传递到碳酸盐的。同时，我们将研究一些古老的微生物碳酸盐，在这种情况下，保存完好的苏格兰叠层岩，看看它们是否确实在其镁同位素组成中记录了细菌活动的证据。最后，我们将分析一些来自火星陨石的碳酸盐，看看它们是否记录了火星上细菌活动的迹象。

Bacteria represent some of the earliest forms of life and have been present on the Earth for 3.5 billion years. However, they are rarely preserved in the rock record and their presence is often inferred from layered structures in stromatilitic limestones, which are known to be formed by bacterial mats in modern stromatilites. It would be therefore useful to have other tracers of bacterial activity, which do not necessarily rely on the bacteria being preserved in the rock record. Magnesium is the eighth most abundant element in the Earths crust, and the fourth most abundant species in seawater. As such it is an essential component of life, with pivotal roles in the generation of cellular energy as well as in plant chlorophyll and human dietary functions. Magnesium has three isotopes (with atomic masses of 24, 25 and 26) and inorganic processes that occur in nature produce differences in magnesium isotope ratios that are simply related to their relative atomic masses, such that all terrestrial materials essentially lie of a single line when the ratios of 26Mg/24Mg are plotted against 25Mg/24Mg. This line is known as the terrestrial mass fractionation line. However, recent experiments have discovered that some bacteria prefer to use 25Mg over 24Mg and 26Mg by 2-3 times to make ATP energy in their cells. The net effect of this is that these bacteria become enriched in 25 Mg and should lie above the terrestrial mass fractionation. This fractionation of 25Mg from the other isotopes of magnesium is known as a mass independent fractionation. Critically, this enrichment in 25Mg is a potential smoking gun for past bacterial activity, if it can be preserved in the geological record. Bacteria commonly play a role in inducing carbonate mineral precipitation. Recent experimental work by Pearce (co-I) has demonstrated that some carbonates produced by bacteria do indeed have an enrichment of 25Mg, the first terrestrial material to show a mass independent fractionation of magnesium. However, the material analysed in those initial experiments was probably a mixture of cellular and carbonate precipitate. The aim of this study is to bacterially precipitate carbonates with a wide range of Mg/Ca ratios (similar to those observed on Earth) and to measure the Mg isotope composition of both the bacterial and carbonate material using a new experimental technique developed at the Open University. This will allow us to assess the extent of bacterial uptake of 25Mg and how this signal is transferred to the carbonates. In parallel, we will study some ancient microbial carbonates, in this case well-preserved stromatilites from Scotland, to see if they do indeed record evidence of bacterial activity in their magnesium isotope composition. Finally, we will analyse some carbonates from Martian meteorites to see if they record any signs of bacterial activity on Mars.