Effects of Environmental Growth Conditions on the Composition and Morphology of Bacterial Magnetosome Crystals and on the Subsequent Dissolution and Preservation of Magnetofossils

趋磁细菌(MB)是一类在代谢和形态上不同的革兰氏阴性原核生物。MB细胞通过生物控制的矿化过程使磁性矿物磁铁矿(Fe3O4)和/或云母(Fe3S4)的细胞内膜结合晶体矿化。这些结构被称为磁小体，使细胞在游泳时沿着地磁力线排列，并似乎与趋气性一起工作，帮助细胞在垂直化学(如氧气)/氧化还原梯度中定位和保持最佳位置。基于它们在自然生境中的高细胞数量及其催化的生物地球化学转化，MB是一个重要的环境细菌群，可能在Fe、S、N和C的生物地球化学循环中发挥重要作用。当MB细胞死亡和裂解时，它们的磁小体晶体可能被释放到周围环境中，最终在沉积物中被保存一段时间，似乎是被认为是“磁化石”。在大量最近的海洋和湖泊沉积物以及一些古代沉积物和陨石(即火星陨石ALH84001)中发现了磁化石，它们也被用作过去存在甲基溴的证据，作为生命的指示物，以及现代/古代环境条件的指示器。这些晶体作为磁性化石的使用是基于一些化学、结晶学和磁性标准，而它们作为环境指示剂的使用是基于仅在少数物种上进行的少量生理实验。关于MB合成Fe3O4的条件，以及磁小体Fe3O4晶体在释放到环境中时溶解或转化为其他矿物的条件，人们知之甚少。然而，目前已经清楚的是，高浓度的O2抑制了MB中磁铁矿的合成，并导致磁铁矿磁小体的氧化，一些MB可以厌氧合成Fe3O4磁小体。本提案中概述的研究的主要目标是确定不同形态的Fe3O4晶体是否可以可靠地用作磁化石(生命证据)或环境指示器。为此，我们将：1)研究相对大量的MB菌株合成不同晶体形态的Fe3O4的环境条件；以及2)检查假定Fe3O4晶体被溶解、转化和/或经历还原成岩作用的条件(例如，调查还原剂、铁载体等的影响)。这项工作的研究成果将促进微生物学、地质学、化学、环境科学和天体生物学的知识。他们还应该促进对火星陨石ALH84001和古代火星上生命证据有一定了解的普通公众的发现和学习。这一点现在尤其重要：最近涉及陨石和来自地球的古代岩石的研究和辩论(其中一些被公众视为)表明，我们需要明确明确的生物标志物，作为地球上以及外星栖息地和物质中生命存在的证据。此外，铁是生命的关键元素，往往是一种限制性营养物质，特别是在海洋系统中。这项研究的结果可能揭示在这种生境中铁循环以前未知的方面(例如，微生物Fe3O4还原和氧化)以及特定细菌在这种循环中的作用。

Magnetotactic bacteria (MB) are a metabolically and morphologically diverse group of gram-negative prokaryotes in the Domain Bacteria. Cells of MB biomineralize intracellular membrane-bounded crystals of the magnetic minerals magnetite (Fe3O4) and/or greigite (Fe3S4) via a biologically-controlled mineralization process. These structures, called magnetosomes, cause cells to align along geomagnetic field lines as they swim and appear to work in conjunction with aerotaxis in aiding cells in locating and maintaining an optimal position in vertical chemical (e.g. oxygen)/redox gradients. Based on their high cell numbers in natural habitats and the biogeochemical transformations they catalyze, MB are a significant environmental bacterial group that potentially have major roles in the biogeochemical cycling of Fe, S, N, and C. When MB cells die and lyse, their magnetosome crystals are presumably released into the surrounding environment and eventually end up in sediments where they appear to be preserved for some time as putative "magnetofossils". Magnetofossils have been found in a large number of recent marine and lacustrine sediments as well as in some ancient sediments and meteorites (i.e., Mars meteorite ALH84001) where they have also been used as evidence for the past presence of MB, as indicators of life, and as indicators of recent/ancient environmental conditions. The use of these crystals as magnetofossils is based on a number of chemical, crystallographic, and magnetic criteria while their use as environmental indicators is based on a small number physiological experiments performed on only a few species. Little is known about the conditions under which MB synthesize Fe3O4 and under which magnetosome Fe3O4 crystals are dissoluted or transformed to other minerals when they are released into the environment. It is presently clear, however, that high concentrations of O2 inhibit magnetite synthesis in MB and cause the oxidation of magnetite magnetofossils and that Fe3O4 magnetosomes can be synthesized anaerobically by some MB.The major goal of the research outlined in this proposal is to determine whether different morphological types of Fe3O4 crystals can be reliably used either as magnetofossils (evidence for life) or as environmental indicators. To do this, we will: 1) examine the environmental conditions under which a relatively large number of strains of MB that synthesize Fe3O4 of different crystal morphologies; and 2) examine the conditions under which the Fe3O4 crystals are supposedly dissoluted, transformed, and/or undergo reductive diagenesis (e.g., investigate the effect of reducing agents, siderophores etc.). Research results from this work should advance knowledge in Microbiology, Geology, Chemistry, Environmental Sciences, and Astrobiology. They should also advance discovery and learning for the general public who are somewhat familiar with Martian meteorite ALH84001 and the evidence for life on ancient Mars. This is especially important now: recent studies and debates, some viewed by the public, involving meteorites and ancient rocks from Earth show that we need clear unambiguous biomarkers as evidence for life in fossils on Earth and well as in extraterrestrial habitats and materials. In addition, Fe is a key element to life and is often a limiting nutrient particularly in marine systems. Results from this research may reveal previously unknown aspects of Fe cycling (e.g., microbial Fe3O4 reduction and oxidation) in such habitats and the role of specific bacteria in this cycling.