Geoscientists appreciate, if only from undergraduate textbooks, that functional groups of microbes in Earth’s subsurface arrange themselves into zones according to an energetic hierarchy, or a “thermodynamic ladder.” According to the theory of competitive exclusion, the functional group that derives the most chemical energy from its environment outcompetes other groups there, coming to overtake its habitat. We might observe sediments near an aquifer’s recharge populated by aerobes, followed downgradient by zones hosting iron-reducing bacteria, sulfate-reducers, and finally methanogens.

In this study we return to the Middendorf Aquifer in South Carolina—birthplace of the competitive exclusion theory—and the Mahomet-Teays Aquifer in central Illinois. There, we compile evidence of the nature of microbial zoning and the distribution of energy available to chemolithostatic life. Our field observations combined with a theoretical analysis of the effects of energetics on microbial kinetics, a set of long term laboratory experiments, and a series of bioreactive transport models portray the controls on the distribution of subsurface microbial life in a compelling, if surprising new light.

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