How Geochemistry Provides Habitability: A Case Study of the Microbial Iron Cycle

该项目的目的是确定微生物可以在它们生活的地方生活的原因。重点是确定环境中的宜居条件，目的是解释温度和地球化学成分如何结合联合收割机以允许和支持微生物生命。一个适宜居住的环境必须满足两个条件：必须有能源，而且这些能源必须持续足够长的时间，让生命能够利用它们。会燃烧的东西不好吃。可居住性可以通过组合方法来量化，以计算微生物可用的化学能量的量，并测量在存在和不存在微生物的情况下产生的反应发生的速率。这种方法的重要性在于，它可以用于从土壤到地壳深处的各种环境中，从而扩大对我们星球如何支持生命的科学理解，甚至可以用于包括人类肠道在内的生物系统，其中可能有令人惊讶的应用程序来改善人类健康。在这项研究中，将研究支持使用铁反应作为其能量来源的微生物的环境，包括温泉，酸性矿井排水和由融雪提供的冷泉。通过研究不同环境中的相同过程，微生物铁循环的案例研究将成为未来研究其他化学能源的模板。最终，这些努力将使研究人员能够解释地球上发现的巨大微生物多样性的根本原因。第一，必须有能源。这需要存在处于不同氧化态的化合物，这些化合物彼此不处于热力学平衡。第二，一定有机械上的困难，使这些化合物不能反应，这意味着化学能不能自己消散。使用这个充满活力的参考框架，地球化学可居住性可以定义和量化的热力学和动力学限制在地球上和地球上的不同环境的组合存在。作为一个例子，整个生命系统发育树中的微生物通过溶解的还原铁与氧气在温度范围从冻结到沸腾和pH值在2和7之间的环境中反应来获得能量。然而，并非所有pH和温度的组合都适合居住。在高pH值环境中，这种反应本身迅速发生，这会阻止微生物利用它，并且这种动力学屏障发生的pH值随着温度的升高而降低。然而，在酸性环境中，非生物氧化反应速率显著减慢，允许微生物催化铁氧化并保存释放的一些能量。然而，增加酸度降低了能源产量，最终创造了一个能量边界的可居住性在最低值的pH值。结合这种充满活力和动力学的边界允许可居住性被映射为使用地球化学变量，包括pH值，温度和浓度的反应物和反应产物的个别反应。本研究的目的之一是为铁氧化还原反应的案例研究生成适居性图。地球化学数据，从实地考察，在温泉，酸性矿井排水，和冷泉喂养的融雪将被用来计算能源供应。生物和非生物铁氧化和还原速率的田间试验将确定动力学限制。补充实验室实验将提供非生物率。分子分析将揭示这些环境中可能负责驱动生物铁氧化还原循环的微生物。由此产生的多维可居住性地图的几个铁的氧化和还原反应将提供一个框架，为未来的研究许多其他化学无机营养代谢过程在整个地球表面和地下环境，这将定量限制在其他行星上的可居住性的讨论。

The objective of this project is to identify the reasons why microorganisms can live where they live. The focus is on identifying livable conditions in the environment, with the goal of explaining how temperatures and geochemical compositions combine to allow and support microbial life. Two things have to be true for an environment to be habitable: there have to be sources of energy, and those sources of energy have to persist long enough for life to take advantage of them. Things that burst into flame are not good to eat. Habitability can be quantified by combining methods to calculate the amounts of chemical energy available to microbes with measurements of the rates that resulting reactions happen with and without microbes present. The importance of this approach is that it can be used in diverse environments from soils to deep in the Earth's crust, allowing an expansion of scientific understanding of how our planet supports life, and even in biological systems including the human gut where there could be surprising applications to improve human health. In this study, environments that support microbes that use iron reactions as their source of energy will be studied including hot springs, acid mine drainage, and cold springs fed by snowmelt. By examining the same processes across diverse environments, this case study of the microbial iron cycle will serve as a template for future studies of other chemical energy sources. Ultimately these efforts will allow researchers to explain underlying reasons for the immense microbial diversity found on Earth.Two things have to be true for microbes to gain chemical energy from the environment. First, there must be a source of energy. This requires the presence of compounds in differing oxidation states that are out of thermodynamic equilibrium with one another. Second, there must be mechanistic difficulties that are keeping those compounds from reacting, which means that the chemical energy cannot dissipate by itself. Using this energetic reference frame, geochemical habitability can be defined and quantified by the combined presence of thermodynamic and kinetic limitations at diverse environments on and in the Earth. As an example, microorganisms across the phylogenetic tree of life gain energy by reacting dissolved reduced iron with oxygen in environments ranging in temperature from freezing to boiling and pH values between 2 and 7. However, not all combinations of pH and temperature are habitable. In high-pH environments this reaction occurs rapidly on its own, which prevents microorganisms from using it, and the pH where this kinetic barrier occurs decreases with increasing temperature. In acidic environments, however, the abiotic oxidation reaction rate is significantly slowed, allowing microorganisms to catalyze iron oxidation and conserve some of the energy released. However, increasing acidity lowers the energy yield, ultimately creating an energy boundary to habitability at the lowest values of pH. Combining such energetic and kinetic boundaries permits habitability to be mapped for individual reactions using geochemical variables that include pH, temperature, and concentrations of reactants and products of the reaction. It is a goal of this research to generate habitability maps for the case study of iron oxidation and reduction reactions. Geochemical data from fieldwork at hot springs, acid mine drainage, and cold springs fed by snowmelt will be used to calculate energy supplies. Field experiments of biotic and abiotic rates of iron oxidation and reduction will determine kinetic limitations. Complementary lab experiments will provide abiotic rates. Molecular analyses will reveal the microbes likely to be responsible for driving the biological iron redox cycle in these environments. The resulting multi-dimensional habitability maps for several iron oxidation and reduction reactions will provide a framework for future studies of many other chemolithotrophic metabolic process throughout surface and subsurface environments on Earth, which will quantitatively constrain the discussion of habitability on other planets.