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

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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.