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

在地球历史的前40亿年里，生命是微小的。分析早期生命存在的标准方法是通过遗留在古代岩石中的新陈代谢废物的同位素组成。我的研究计划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，除了产生新的新陈代谢外，它不会。这一假设建立在“微生物代谢统一性”假说的基础上，该假说指出，现代自然或实验室环境中微生物的生理反应与岩石记录中代表的古代环境中微生物的生理反应是相同的。尽管这一假设是每一项使用古岩石的稳定同位素测量进行生物地球化学推断的研究的基础，但“微生物代谢均匀性”的假设很少被提出，也几乎从未得到过评估。鉴于地球上曾经生活过10^37到10^42个微生物，这一数字令人震惊，这个数字远远超过对宇宙中恒星数量的估计(10^24)。我们正在通过实验微生物进化与对关键环境记录的实地研究相结合的方式直接检验这一假设，所有这些都是由详细的生理研究提供信息的。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是拟议研究成功的关键。作为这项探索赠款申请的一部分，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变的同位素后果。实验室研究将分为旨在解决细胞内代谢物浓度控制S同位素分馏幅度的假设的生理实验和试图评估代谢S同位素分馏的进化轨迹是否概括生理轨迹的选择实验。实地研究将被分成两类：现代环境中存在活跃的异化硫酸盐还原作用，而古代岩石则代表了这种环境的残余等价物。现代研究将针对这样一个假设，即简单的环境变化对新陈代谢S同位素分馏的演化后果是可以预测的，而古代研究将试图通过检测定义明确的化石环境(现在以硫酸盐蒸发岩为代表)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演化信号。这些问题的答案将为评估“微生物代谢统一性”假说提供一个强有力的框架。由于拟议研究中涉及的时间范围和技术范围，作为研究计划一部分接受培训的年轻科学家将获得真正的跨学科研究经验，为设定地球生物学领域的未来方向奠定良好的基础。

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.