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

这个项目的目标是找出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是如何结合在一起允许和支持微生物生命的。要想让环境宜居，必须有两件事是正确的：必须有能源，这些能源必须持续足够长的时间，以便生命能够利用它们。起火的东西不好吃。通过将计算微生物可利用的化学能量的方法与在微生物存在和不存在的情况下所产生的反应发生的速率的测量相结合，可以对宜居性进行量化。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，允许扩大对我们的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，在这些系统中，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源的微生物的环境，包括温泉、酸性矿山排水和融雪供应的冷泉。通过研究不同环境中的相同过程，微生物铁循环的这一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在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量，必须有两件事是正确的。首先，必须有一个能源来源。这需要存在处于不同氧化状态的化合物，这些化合物彼此之间失去了热力学平衡。其次，一定存在阻止这些化合物反应的机械困难，这意味着化学能不能自行消散。利用这一能量参照系，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组合存在来定义和量化地球化学宜居性。例如，生命系统发育树上的微生物通过溶解的还原铁与氧气反应获得能量，环境温度从冻结到沸腾，pH值在2到7之间。然而，并不是所有的pH和温度组合都适合居住。在高pH环境中，这一反应会迅速发生，这会阻止微生物使用它，并且发生这种动力学障碍的pH会随着温度的升高而降低。然而，在酸性环境中，非生物氧化反应速度显著放缓，使微生物能够催化铁氧化，并节省一些释放的能量。然而，增加酸度会降低能量产量，最终在最低的pH值下形成宜居性的能量边界。结合这样的能量和动力学边界，可以使用包括pH、温度和反应物和反应产物的浓度在内的地化变量来绘制单个反应的宜居性地图。这项研究的一个目标是为铁氧化和还原反应的案例研究生成宜居图。将使用在温泉、酸性矿山排水和融雪供应的冷泉进行实地考察的地球化学数据来计算能源供应。铁的生物和非生物氧化还原速率的现场实验将确定动力学极限。互补性的实验室实验将提供非生物率。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。由此产生的几个铁氧化和还原反应的多维宜居性地图将为未来研究地球表面和亚表层环境中的许多其他化学物质营养代谢过程提供一个框架，这将定量地限制对其他行星宜居性的讨论。

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.