The relationship between instability of H2 production and compositions of bacterial communities within a dark fermentation fluidized-bed bioreactor

Microbial community composition dynamics was studied during H-2 fermentation from glucose in a fluidized-bed bioreactor (FBR) aiming at obtaining insight into the H-2 fermentation microbiology and factors resulting in the instability of biofilm processes. FBR H-2 production performance was characterized by an instable pattern of prompt onset of H-2 production followed by rapid decrease. Gradual enrichment of organisms increased the diversity of FBR attached and suspended-growth phase bacterial communities during the operation. FBA bacteria. included potential H-2 producers, H-2 consumers and neither H-2 producers nor consumers, and those distantly related to any known organisms. The prompt onset of H-2 production was due to rapid growth of Clostridium butyricum (99-100%) affiliated strains after starting continuous feed. The proportion trend of C. butyricum in FBR attached and suspended-growth phase communities coincided with H-2 and butyrate production. High glucose loading rate favoured the H-2 production by Escherichia coli (100%) affiliated strain. Decrease in H-2 production, associated with a shift from acetate-butyrate to acetate-propionate production, was due to changes in FBR attached and suspended-growth phase bacterial community compositions. During the shift, organisms, including potential propionate producers, were enriched in the communities while the proportion trend of C. butyricum decreased. We suggest that the instability of H2 fermentation in biofilm reactors is due to enrichment and efficient adhesion of H-2 consumers on the carrier and, therefore, biofilm reactors may not favour mesophilic H-2 fermentation.