Associative cellulolysis and dinitrogen fixation by co-cultures of Trichoderma harzianum and Clostridium butyricum

Abstract

Lignocelluloses are a major source of carbon into ecosystems and represent the principal constituent of agricultural, forestry and municipal wastes. The large proportion of cellulose and hemicellulose in these wastes leads to a high C:N ratio which often results in N limitation during decomposition. This could be overcome if the decomposer organisms combined both the cellulolytic (cellulase) and N₂-fixing (nitrogenase) functions. With the exception of bacteria isolated from the specialized environment of marine shipworms^1,2, no organism possessing both cellulase and nitrogenase has yet been isolated from nature or genetically engineered. We now report the cooperative degradation of cellulose in which the fungus Trichoderma harzianum provides the cellulase function and an obligately anaerobic bacterium, Clostridium butyricum, provides the nitrogenase function. These co-cultures utilized cellulose as the sole carbon source for N₂ fixation, resulting in a substantial increase in the rate of substrate decomposition compared with the fungus alone. The co-cultures developed in apparently aerobic environments, demonstrating that aerobes and anaerobes can co-exist, the aerobe presumably providing respiratory protection to the anaerobe.