Kinetics of Hydrogen Consumption by Rumen Fluid, Anaerobic Digestor Sludge, and Sediment †

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Michaelis-Menten kinetic parameters for H2 consumption by three methanogenic habitats were determined from progress curve and initial velocity experiments. The influences of mass transfer resistance, endogenous H2 production, and growth on apparent parameter estimates were also investigated. Kinetic parameters could not be determined for undiluted rumen fluid and some digestor sludge from gas-phase measurements of H2, since mass transfer of H2 across the gas-liquid interface was rate limiting. However, accurate values were obtained once the samples were diluted. H2 consumption by digestor sludge with a long retention time and by hypereutrophic lake sediment was not phase transfer limited. The Km values for H2 uptake by these habitats were similar, with means of 5.8, 6.0, and 7.1 μM for rumen fluid, digestor sludge, and sediment, respectively. Vmax estimates suggested a ratio of activity of approximately 100 (rumen fluid):10 (sludge):1 (sediment); their ranges were as follows: rumen fluid, 14 to 28 mM h−1; Holt sludge, 0.7 to 4.3 mM h−1; and Wintergreen sediment, 0.13 to 0.49 mM h−1. The principles of phase transfer limitation, studied here for H2, are the same for all gaseous substrates and products. The limitations and errors associated with gas phase determination of kinetic parameters were evaluated with a mathematical model that combined mass transport and Michaelis-Menten kinetics. Three criteria are described which can be used to evaluate the possibility that a phase transfer limitation exists. If it does not exist, (i) substrate consumption curves are Michaelis-Menten and not first order, (ii) the Km is independent of initial substrate concentration, and (iii) the Km is independent of biomass (Vmax) and remains constant with dilution of sample. Errors in the Michaelis-Menten kinetic parameters are caused by endogenously produced H2, but they were <15% for rumen fluid and 10% for lake sediment and digestor sludge. Increases in Vmax during the course of progress curve experiments were not great enough to produce systematic deviations from Michaelis-Menten kinetics.

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