Why a co-substrate is required for anaerobic growth of Escherichia coli on citrate

Citrate was fermented by Escherichia coli if a second substrate, such as glucose, lactose or lactate, was available to the organism. The function of the second substrate was to provide reducing power for the formation of succinate from oxaloacetate. Citrate lyase, malate dehydrogenase, fumarase and fumarate reductase were present in cell extracts of E. coli at high activity. Oxaloacetate decarboxylate could not be detected, and it is assumed that this lack is the reason for the inability of E. coli to grow anaerobically with citrate as the only carbon and energy source.