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

细菌代表了一些最早的生命形式，已经在地球上存在了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的层状结构中推断出来的，这些叠层石是由现代叠层石中的细菌席形成的。因此，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。镁是地壳中含量第八多的元素，也是海水中含量第四多的元素。因此，它是生命的重要组成部分，在细胞能量的产生、植物叶绿素和人类饮食功能中起着关键作用。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，这与它们的相对原子质量有关，因此，当26Mg/24Mg的比率与25Mg/24Mg的比率相比较时，所有陆地物质基本上都位于一条直线上。这条线被称为地球质量分割线。然而，最近的实验发现，一些细菌更喜欢使用25Mg而不是24Mg和26Mg的2-3倍来在细胞中制造ATP能量。这样做的净效果是，这些细菌在25mg中变得富集，应该位于陆地质量分馏之上。这种将25Mg从镁的其他同位素中分离出来的方法被称为质量无关分离。关键的是，如果能在地质记录中保存下来，25毫克的富集可能是过去细菌活动的确凿证据。细菌通常在诱导碳酸盐矿物沉淀中起作用。皮尔斯（co-I）最近的实验工作表明，一些由细菌产生的碳酸盐确实富集了25Mg，这是第一个显示出镁的质量独立分选的陆地物质。然而，在最初的实验中分析的物质可能是细胞和碳酸盐沉淀的混合物。这项研究的目的是通过细菌沉淀具有广泛Mg/Ca比值的碳酸盐（类似于在地球上观察到的），并使用开放大学开发的一种新实验技术来测量细菌和碳酸盐材料的Mg同位素组成。这将使我们能够评估细菌摄取25Mg的程度以及这个信号是如何传递给碳酸盐的。与此同时，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石，看看它们是否确实记录了镁同位素组成中细菌活动的证据。最后，我们将分析火星陨石上的一些碳酸盐，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.