Mass-transfer-constrained thermodynamics links fluid motion to the preferential use of hydrogen and formate in syntrophic propionate oxidation

Syntrophic propionate oxidation in methanogenic environments depends on interspecies electron transfer through hydrogen and formate, yet the physical factors governing the relative use of these carriers remain poorly understood. Here, we examined how fluid motion alters electron-transfer energetics and pathway expression in the obligate syntrophic propionate oxidizer Pelotomaculum schinkii grown in coculture with Methanospirillum hungatei. A mass-transfer-constrained thermodynamic model was used to estimate H2 and formate concentrations at the P. schinkii cell surface and calculate the corresponding Gibbs free-energy change of H2- and formate-mediated propionate oxidation under different mixing conditions and growth stages. Transcriptomic analysis was used to assess expression of electron-transfer pathways. Under unmixed conditions, formate-mediated propionate oxidation was more thermodynamically favorable than the H2-mediated pathway, consistent with highly expressed genes involved in formate production. Mixing altered coculture activity and pathway energetics. H2 was more sensitive to mixing and certain conditions shifted the energetic advantage toward H2. Expression of the major hydrogenases and formate dehydrogenases generally tracked these pathway-specific energetic changes. These results show that fluid motion reshapes the near-cell thermodynamic favorability and enables condition- and growth-stage-dependent use of alternative electron-transfer pathways. Fluid motion should therefore be considered an ecological and engineering control on syntrophic metabolism.