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

这个项目的目的是找出微生物能在它们生活的地方生存的原因。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起来允许和支持微生物的生命。要使一个环境适合居住，必须具备两个条件：必须有能量来源，而且这些能量来源必须持续足够长的时间，以便生命能够利用它们。会着火的东西不好吃。可居住性可以通过结合计算微生物可用化学能的方法和测量在有和没有微生物存在的情况下产生反应的速率来量化。这种方法的重要性在于，它可以在从土壤到地壳深处的各种环境中使用，从而扩大对我们的星球如何支持生命的科学理解，甚至在包括人类肠道在内的生物系统中，可能会有令人惊讶的应用来改善人类健康。在这项研究中，将研究支持以铁反应为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。通过在不同环境中检查相同的过程，这个微生物铁循环的案例研究将作为未来其他化学能源研究的模板。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潜在原因。微生物要从环境中获得化学能，必须具备两个条件。首先，必须有能源。这需要处于不同氧化态的化合物的存在，这些化合物彼此之间处于热力学平衡之外。第二，一定有机械上的困难阻止这些化合物发生反应，这意味着化学能不能自己消散。使用这个能量参考系，地球化学的可居住性可以通过地球上和地球上不同环境中热力学和动力学限制的结合来定义和量化。例如，在整个生命进化树中，微生物在温度从冰点到沸点，pH值在2到7之间的环境中，通过溶解的还原铁与氧反应来获得能量。然而，并非所有的pH值和温度组合都适合居住。在高pH值的环境中，这种反应本身发生得很快，这阻止了微生物利用它，并且这种动力屏障发生的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明显减慢，使微生物能够催化铁氧化并保存一些释放的能量。然而，酸度的增加会降低能量产量，最终在pH值最低时形成可居住性的能量边界。结合这种能量和动力学边界，可以使用包括pH、温度、反应物和反应产物浓度在内的地球化学变量来绘制单个反应的可居住性。本研究的目标是为铁氧化和还原反应的案例研究生成可居住性图。从温泉、酸性矿井排水和融雪供给的冷泉的野外工作中获得的地球化学数据将用于计算能量供应。实地实验的生物和非生物速率的铁氧化和还原将确定动力学限制。补充实验室实验将提供非生物速率。分子分析将揭示在这些环境中可能负责驱动生物铁氧化还原循环的微生物。由此产生的几种铁氧化和还原反应的多维可居住性图将为未来研究地球表面和地下环境中的许多其他化学岩石营养代谢过程提供框架，这将定量地限制对其他行星可居住性的讨论。

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.