Glucose Fermentation Products of Ruminococcus albus Grown in Continuous Culture with Vibrio succinogenes: Changes Caused by Interspecies Transfer of H₂

ABSTRACT

The influence of a H₂-utilizing organism, Vibrio succinogenes, on the fermentation of limiting amounts of glucose by a carbohydrate-fermenting, H₂-producing organism, Ruminococcus albus, was studied in continuous cultures. Growth of V. succinogenes depended on the production of H₂ from glucose by R. albus. V. succinogenes used the H₂ produced by R. albus to obtain energy for growth by reducing fumarate in the medium. Fumarate was not metabolized by R. albus alone. The only products detected in continuous cultures of R. albus alone were acetate, ethanol, and H₂. CO₂ was not measured. The only products detected in the mixed cultures were acetate and succinate. No free H₂ was produced. No formate or any other volatile fatty acid, no succinate or other dicarboxylic acids, lactate, alcohols other than ethanol, pyruvate, or other keto-acids, acetoin, or diacetyl were detected in cultures of R. albus alone or in mixed cultures. The moles of product per 100 mol of glucose fermented were approximately 69 for ethanol, 74 for acetate, 237 for H₂ for R. albus alone and 147 for acetate and 384 for succinate for the mixed culture. Each mole of succinate is equivalent to the production of 1 mol of H₂ by R. albus. Thus, in the mixed cultures, ethanol production by R. albus is eliminated with a corresponding increase in acetate and H₂ formation. The mixed-culture pattern is consistent with the hypothesis that nicotinamide adenine dinucleotide (reduced form), formed during glycolysis by R. albus, is reoxidized during ethanol formation when R. albus is grown alone and is reoxidized by conversion to nicotinamide adenine dinucleotide and H₂ when R. albus is grown with V. succinogenes. The ecological significance of this interspecies transfer of H₂ gas and the theoretical basis for its causing changes in fermentation patterns of R. albus are discussed.