Control of interspecies electron transfer flow during anaerobic digestion: Dynamic diffusion reaction models for hydrogen gas transfer in microbial flocs

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Abstract
Dynamic reaction diffusion models were used to analyze the consequences of aggregation for syntrophic reactions in methanogenic ecosystems. Flocs from a whey digestor were used to measure all model parameters under the in situ conditions of a particular defined biological system. Fermentation simulations without adjustable parameters could precisely predict the kinetics of H2 gas production of digestor flocs during syntrophic methanogenesis from ethanol. The results demonstrated a kinetic compartmentalization of H2 metabolism inside the flocs. The interspecies electron transfer reaction was mildly diffusion controlled. The H2 gas profiles across the flocs showed high H 2 concentrations inside the flocs at any time. Simulations of the syntrophic metabolism at low substrate concentrations such as in digestors or sediments showed that it is impossible to achieve high H2 gas turnovers at simultaneously low steady-state H2 concentrations. This showed a mechanistic contradiction in the concept of postulated low H2 microenvironments for the anaerobic digestion process. The results of the computer experiments support the conclusion that syntrophic H2 production may only be a side reaction of H2 independent interspecies electron transfer in methanogenic ecosystems.

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