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

环境生长条件对细菌磁小体晶体的组成和形态以及磁化石的后续溶解和保存的影响

• 批准号: 0715492
• 负责人: Dennis Bazylinski
• 金额: --
• 依托单位: University of Nevada Las Vegas
• 依托单位国家: 美国
• 项目类别: Standard Grant
• 财政年份: 2006
• 资助国家: 美国
• 起止时间: 2006-10-01 至 2009-07-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=0715492&HistoricalAwards=false
• 关键词: Effects; Environmental; Growth; Conditions; Composition

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.

趋磁细菌（MB）是区域细菌中代谢和形态多样的革兰氏阴性原核生物。MB细胞通过生物控制的矿化过程将磁性矿物磁铁矿（Fe3O4）和/或灰长岩（Fe3S4）的胞内膜结合晶体生物矿化。这些结构被称为磁小体，它们使细胞在游动时沿着地磁力线排列，并与趋氧性一起帮助细胞在垂直化学（例如氧）/氧化还原梯度中定位和保持最佳位置。基于它们在自然栖息地的高细胞数量及其催化的生物地球化学转化，MB是一种重要的环境细菌群，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细胞死亡和分解时，它们的磁小体晶体可能被释放到周围环境中，并最终进入沉积物中，在那里它们似乎作为假定的“磁化石”保存了一段时间。在大量的近代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MB的证据，作为生命的指标，以及作为近代/古代环境条件的指标。这些晶体作为磁性化石的使用是基于许多化学、晶体学和磁性标准，而它们作为环境指标的使用是基于对少数物种进行的少量生理实验。MB合成Fe3O4的条件以及Fe3O4磁小体晶体在释放到环境中时溶解或转化为其他矿物的条件尚不清楚。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化石的氧化，并且某些MB可以厌氧合成Fe3O4磁小体。本文研究的主要目的是确定不同形态类型的Fe3O4晶体是否可以可靠地用作磁化石（生命证据）或环境指标。为此，我们将：1)研究相对大量的MB菌株合成不同晶体形态Fe3O4的环境条件；2)检查Fe3O4晶体可能溶解、转化和/或经历还原性成岩作用的条件（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方面的知识。他们还应该促进对火星陨石ALH84001和古代火星上存在生命的证据有所了解的公众的发现和学习。这一点现在尤其重要：最近的研究和辩论，其中一些是公众所看到的，涉及地球上的陨石和古代岩石，表明我们需要明确的生物标志物作为地球上化石以及地外栖息地和物质中生命的证据。此外，铁是生命的关键元素，通常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了以前未知的铁循环方面（例如，微生物Fe3O4的还原和氧化），以及特定细菌在这种循环中的作用。