PERTURBATION OF SYNTROPHIC ISOBUTYRATE AND BUTYRATE DEGRADATION WITH FORMATE AND HYDROGEN

The effect of formate and hydrogen on isomerization and syntrophic deg radation of butyrate and isobutyrate was investigated using a defined methanogenic culture, consisting of syntrophic isobutyrate-butyrate de grader strain IB, Methanobacterium formicicum strain T1N, and Methanos arcina mazeii strain T18. Formate and hydrogen were used to perturb sy ntrophic butyrate and isobutyrate degradation by the culture. The reve rsible isomerization between isobutyrate and butyrate was inhibited by the addition of either formate or hydrogen, indicating that the isome rization was coupled with syntrophic butyrate degradation for the cult ure studied. Energetic analysis indicates that the direction of isomer ization between isobutyrate and butyrate is controlled by the ratio be tween the two acids, and the most thermodynamically favorable conditio n for the degradation of butyrate or isobutyrate in conjunction with t he isomerization is at almost equal concentrations of isobutyrate and butyrate. The degradation of isobutyrate and butyrate was completely i nhibited in the presence of a high hydrogen partial pressure (>2000 Pa ) or a measurable level of formate (10 mu M or higher). Significant fo rmate (more than 1 mM) was detected during the perturbation with hydro gen (17 to 40 kPa). Resumption of butyrate and isobutyrate degradation was related to the removal of formate. Energetic analysis supported t hat formate was another electron carrier, besides hydrogen, during syn trophic isobutyrate-butyrate degradation by this culture. (C) 1996 Joh n Wiley & Sons, Inc.