Microbial evolution and the isotopic record of early life - sulfur isotope constraints

在地球历史的最初40亿年里，生命是微观的。测定早期生命存在的标准方法是通过古代岩石中遗留的代谢废物的同位素组成。我的研究项目旨在回答一个简单的问题：微生物进化如何影响早期生命的同位素记录？目前的假设是，除了产生新的代谢外，它没有。这一假设基于“微生物代谢均变论”假说，该假说认为微生物在现代自然或实验室环境中的生理反应与岩石记录中所代表的古代环境中的生理反应是相同的。尽管这一假设是每一项研究的基础，这些研究使用来自古代岩石的稳定同位素测量来进行深时间的生物地球化学推断，但“微生物代谢均变论”的假设很少被陈述，几乎从未被评估过。考虑到地球上曾经生活过10^37到10^42个微生物，这个数字远远大于宇宙中恒星的估计数量（10^24），这是令人震惊的。我们将微生物进化实验与关键环境记录的实地研究结合起来，通过详细的生理研究直接验证这一假设。这项工作有望为早期地球生物地球化学领域做出重大贡献。多种S同位素分解微生物代谢途径的独特能力对拟议研究的成功至关重要。作为“发现”基金申请的一部分，提出的具体项目分为实验室研究和实地研究，旨在评估异化硫酸盐还原演化的同位素后果。实验室研究将分为生理实验，旨在解决细胞内代谢物浓度控制S同位素分异程度的假设，以及选择实验，试图评估代谢S同位素分异的进化轨迹是否再现生理轨迹。实地研究将分为具有活性异化硫酸盐还原作用的现代环境和代表这种环境的残余当量的古代岩石。现代研究的目标是假设简单的环境变化对代谢S同位素分馏的进化后果是可预测的，而古代研究将试图通过检查明确定义的化石环境中的S同位素组成来确定是否可以在地质时间尺度上分离出进化信号（例如，现在由硫酸盐蒸发岩代表）。这些问题的答案将为评估“微生物代谢均变论”假说提供一个强有力的框架。由于在拟议的研究中涉及的时间尺度和技术范围，作为研究计划的一部分培训的年轻科学家将获得真正的跨学科研究经验，为他们设定未来的地理生物学领域奠定良好的基础。

For the first 4 billion years of Earth history, life was microscopic. The standard way of assaying for the presence of early life is through isotopic composition of metabolic waste products left behind in ancient rocks. My research program is aimed at answering a simple question: how does microbial evolution affect the isotopic record of early life? The current assumption is that, except for the generation of new metabolisms, it does not. This assumption rests on the hypothesis of “microbial metabolic uniformitarianism”, which states that the physiological response of microbes in modern natural or laboratory environments is the same as it was in the ancient environments represented in the rock record. Although this assumption underlies every study that uses stable isotopic measurements from ancient rocks make biogeochemical inferences in deep time, the hypothesis of “microbial metabolic uniformitarianism” is rarely stated and almost never evaluated. This is astounding in light of the fact that 10^37 to 10^42 microbes have ever lived on Earth, numbers that a far greater than estimates for the number of stars in the universe (10^24). We are directly testing this hypothesis through a marriage of experimental microbial evolution with field studies on key environmental records, all informed by detailed physiological studies. This work promises to contribute significantly to the field of early Earth biogeochemistry. The unique ability of multiple S isotopes to take apart microbial metabolic pathways is critical to the success of the proposed research. The specific projects proposed as part of this Discovery grant application are divided between laboratory and field studies aimed at evaluating the isotopic consequences of evolution of dissimilatory sulfate reduction. The laboratory studies will be split between physiological experiments aimed at addressing the hypothesis that the concentration of intracellular metabolites controls the magnitude of S isotope fractionation, and selection experiments that attempt to assess whether evolutionary trajectories of metabolic S isotope fractionation recapitulate physiological ones. The field studies will be split between modern environments hosting active dissimilatory sulfate reduction and ancient rocks that represent relict equivalents of such environments. The modern studies will be targeted towards the hypothesis that evolutionary consequences of simple environmental changes on metabolic S isotope fractionation are predictable, while the ancient studies will try and determine whether a evolutionary signal can be isolated on geological timescales by examining S isotope compositions in well-defined fossil environments (now represent, for example, by sulfate evaporites). The answers to these questions will provide a robust framework for evaluating the hypothesis of “microbial metabolic uniformitarianism”. As a result of the range of timescales and techniques addressed in the proposed research, the young scientists trained as part of the research program will gain a truly interdisciplinary research experience, situating them well for setting the future course of the field of geobiology.