A mathematical model to describe in vitro kinetics of H2 gas accumulation

Abstract

Hydrogen (H2) is produced in the rumen during the microbial fermentation of dietary carbohydrates, and is consumed as an energy source by H2-using microbes, especially the methane (CH4)-forming methanogens. In vitro fermentation systems are used to study some aspects of rumen activity, and the kinetics of H2 accumulation in these systems is a balance between H2 production and consumption. The fate of H2 produced during fermentation is either as dissolved H2 in the liquid phase (dH2) or H2 gas (gH2). This study proposes a mathematical model of the processes leading to gH2 accumulation. The gH2 was mathematically divided into a dissolvable (x) and non-dissolvable (y) gH2 fractions. The gH2 in fraction x was assumed to be available to re-dissolve into the liquid phase and therefore could be consumed by H2-using microbes. The gH2 in fraction y represents gH2 that does not re-dissolve into the dH2 pool, and thus biologically unavailable for the H2-using microbes. Our model was developed to describe the changes in gH2 in an in vitro fermentation system, based on gH2 production and re-solution. Seven very different profiles of in vitro gH2 curves were selected to demonstrate the applicability of the new model, including gH2 accumulation profiles from two feeds and three methanogen inhibitors. The three inhibitors reduced CH4 production in different ways: by partial inhibition of methanogens (bromoethane sulphonate), by complete inhibition of methanogens (root of Rheum palmatum), or by acting as a hydrogen sink (nitrate added as NH4NO3). The new model fitted all seven curves of in vitro gH2 kinetics satisfactorily. The fitted parameters varied between the different in vitro gH2 curves, therefore allowing description and classification of the curves, and the underlying interpretations were consistent with current knowledge of H2 production and consumption. In summary, the mathematical model described here provides biologically meaningful parameters to interpret the process of in vitro gH2 accumulation. Use of this model in future studies with in vitro systems could confirm its wider application to describe gH2 kinetics when different inhibitors of methane formation are applied.