EcoHydrology, Thermodynamics, and Microbial Ecology at the onset of soil syntrophy

Syntrophy is metabolic cross-feeding in which an upstream organism can oxidize a substrate only because a partner continuously removes inhibitory products (often H2), making the overall reaction energetically favorable. In soils, moisture regulates anaerobic microbial interactions by shaping oxygen availability and gas diffusivity, while fermentation produces reduced intermediates, including volatile fatty acids (VFAs) such as butyrate and propionate, whose oxidation is endergonic under standard conditions and becomes feasible only when hydrogen is maintained sufficiently low by hydrogenotrophic methanogens. Here we present a minimalist predator–prey model that captures the key feedbacks among moisture, hydrogen dynamics, and methanogen biomass. Moisture modulate hydrogen production, leakage, and methanogenic growth, shifting the system between a hydrogen-accumulating, methanogen-free regime and a syntrophic coexistence regime in which methanogens depress hydrogen below the threshold required for VFA oxidation to become exergonic. The resulting moisture-driven transition is a transcritical bifurcation governed by a moisture-dependent methanogen reproduction number, providing a compact link between hydrologic variability and the onset and collapse of syntrophy in soils.