The effects of adding biochar rice husk (R), white popinee (WP), bamboo (BB), or coconut (CT) on microbial community in fermentation broths from glucose were investigated. The added biochars acted as biofilm carriers on which Sporolactobacillus spathodeae, Clostridium sensu stricto 11 sp., Clostridium sensu stricto 12 sp., Clostridium sensu stricto 1 sp., and Clostridium sensu stricto 5 sp. were enriched. Fermentation reactions substantially increased the amounts of acid-producers in biofilm. The homoacetogens, Clostridium carboxidivorans and Clostridium drakei, were identified in the biofilm in the first two batches of fermentation with biochars as electron conductors between acid-producers and homoacetogens to assist homoacetogenesis. The heterotrophic bacteria overcompeted the acid-producers in the biofilm in long-term fermentation.