Reductive Acetogenesis in the Hindgut and Attempts to its Induction in the Rumen—A Review

Abstract

Fievez, V., Piattoni, F., Mbanzamihigo, L. and Demeyer, D. 1999. Reductive acetogenesis in the hindgut and attempts to its induction in the rumen-A review. J. Appl. Anim. Res., 16: 1-22. Hiridgut ferrneiitatiori differs froin ruinen fermentation b. y a. lower methane production and the presence of reductive acetogenesis (RA).Moreover, it has been showii tha.t with ageing RA is rephced coinpletely (ruinen) or partially (hindgut) A. IntroductionBesides methanogenesis (M), non assimilatory sulphate reduction and/ or reductive acetogenesis (RA) have been identified as major pathways of microbial competition for metabolic hydrogen, formed during bacterial fermentation in anaerobic microbial ecosystems. However, these alternative hydrogen sinks to M do not function in the rumen, although bacteria able to use them have been isolated . After a short introduction to the characteristics of RA, this paper intends to give a global review of the research carried out in our laboratory over the past five years, related both to the relative importance of RA and M and to the possibilities for induction of RA in the rumen. A.1. RA in the gastro-intestinal tractBacteria are defined as reductive acetogens when they are able to reduce 2 moles of CO, to 1 mole of acetate, either autotrophically or heterotrophically, e.g. homoacetogenesis (Fisher et al., 1932; Wieringa, 1936, Prins andLankhorst, 1977).The heterotrophic growth of hom,oacetogens is characterised by the (almost) complete conversion of monosaccharides to acetate (Ljungdahl, 1986 Moreover, most of the hoinoacetogens are able to reduce exogenous CO,> autotrophically with molecular H,: 2 CO, + 4H, 3 CH,COOH + 2HiO ( 3 ) . This ability of mixothrophic growth (i.e. autotrophically on CO,-H, --a n d heterotrophically on monosaccharides) enables these acetogens to survive in almost every anaerobic environment and could explain their rather high growth yields (Ljungdahl, 1986). A.2. Detection of the occurrence of RAThe amounts of end products formed in ruinen fermentation are related through the stoichiometery of the overall reaction concerned (Wolin, 1960). This allows calculation of the net amounts of "metabolic hydrogen" produced and recovered in reduced end products. The agreement of the amounts of end products formed within a fermentation, with the overall reactions proposed, can then be evaluated by calculation of a "hydrogen recovery (2Hr)" (Marty and Demeyer, 1973): 100 * (2P + 2B + 4CHJ / (2A + P + 4B), with A, P, B and CH, equal to the molar amounts of acetate, propionate, butyrate and methane, respectively.With rumen contents such recoveries range between 80 and 90% (Demeyer, 1991), whereas with hindgut contents from different species, values were much lower, suggesting the presence of other hydrogen consuming reactions, mainly RA (Henderson a n d Deineyer and De Graeve, 1991). However, this calculation does not allow to differentiate reaction (2) from (3) and, therefore, in this paper RA is used for both reactions. A.3. Competition and similarity betwee...

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