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

Abstract

Michaelis-Menten kinetic parameters for H 2 consumption by three methanogenic habitats were determined from progress curve and initial velocity experiments. The influences of mass transfer resistance, endogenous H 2 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 H 2 , since mass transfer of H 2 across the gas-liquid interface was rate limiting. However, accurate values were obtained once the samples were diluted. H 2 consumption by digestor sludge with a long retention time and by hypereutrophic lake sediment was not phase transfer limited. The K m values for H 2 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. V max 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 H 2 , 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 K m is independent of initial substrate concentration, and (iii) the K m is independent of biomass ( V max ) and remains constant with dilution of sample. Errors in the Michaelis-Menten kinetic parameters are caused by endogenously produced H 2 , but they were <15% for rumen fluid and 10% for lake sediment and digestor sludge. Increases in V max during the course of progress curve experiments were not great enough to produce systematic deviations from Michaelis-Menten kinetics.