Interspecies electron transfer (IET) is a feature of interest in anaerobic ecosystems especially in environments where electron acceptors are lacking. Recent experiments showed that some fermentative organisms can serve as potential electron sinks in such environments. However, the exact mechanism behind this phenomenon remains unclear, especially for fermentative Gram-positive bacteria. In a previous study[1], an interaction between the fermentative species Clostridium pasteurianum and the model electroactive species Geobacter sulfurreducens was demonstrated which resulted in a change of C. pasteurianum metabolic pathway on glycerol with an increase of the yield in 1,3-propanediol. The observed shift was attributed to IET, as a similar metabolic shift was observed when C. pasteurianum accepted electrons from a cathode. Through new co-culture setups and an RNAseq approach, we unveiled that the metabolic change in cocultures of C. pasteurianum and G. sulfurreducens is not due to an electron exchange but rather to a cobamide molecule produced by G. sulfurreducens, which probably interferes with the key enzyme glycerol dehydratase. Despite the lack of evidence for an electron transfer pathway, cocultures with addition of fumarate still point out to a partial IET. This work shows that Geobacter sp. plays a role in some fermentation setups beyond electron transfer.