Degradation of cellulose by mixed cultures of fermentative bacteria and anaerobic sulfur bacteria

Abstract

Mixed cultures of anaerobic cellulolytic bacteria, sulfate reducers (Desulfovibrio vulgaris) and phototrophic bacteria (Chromatium vinosum) were studied. The part played by each microorganism in the food chain and its influence on the behaviour of the other two in this association were elucidated. The major products of cellulose fermentation were formic, acetic and lactic acids, ethanol, hydrogen and carbon dioxide. The hydrolysis of cellulose was slow; 1.37 mg equivalent of glucose per ml medium per hour was released. Desulfovibrio oxidized the organic acids, alcohol and hydrogen by dissimilatory sulfate reduction and sulfide and acetate were accumulated in the medium. The rate of sulfate reduction depends on the rate of cellulose fermentation.

In the presence of phototrophic bacteria, the end products of cellulose degradation and sulfate reduction disappeared from the medium and the protein content increased highly. The growth yield constant increased five times when the cellulolytic organism was associated with the two sulfur bacteria. The absence of Desulfovibrio perturbed the functioning of the trophic chain, as the rate of sulfate reduction partly determines the growth of Chromatium. Chromatium seems to hinder the anaerobic degradation of cellulose, whereas Desulfovibrio seems to enhance it. The microorganisms behave differently in pure and mixed cultures.