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A microbial community is engaged in a complex economy of cooperation and competition for carbon and energy. In engineered systems such as anaerobic digestion and fermentation, these relationships are exploited for conversion of a broad range of substrates into products, such as biogas, ethanol, and carboxylic acids. Medium chain fatty acids (MCFAs), for example, hexanoic acid, are valuable, energy dense microbial fermentation products, however, MCFA tend to exhibit microbial toxicity to a broad range of microorganisms at low concentrations. Here, we operated continuous mixed population MCFA fermentations on biorefinery thin stillage to investigate the community response associated with the production and toxicity of MCFA. In this study, an uncultured species from the  Clostridium  group IV (related to  Clostridium  sp. BS-1) became enriched in two independent reactors that produced hexanoic acid (up to 8.1 g L −1 ), octanoic acid (up to 3.2 g L −1 ), and trace concentrations of decanoic acid. Decanoic acid is reported here for the first time as a possible product of a  Clostridium  group IV species. Other significant species in the community,  Lactobacillus  spp. and  Acetobacterium  sp., generate intermediates in MCFA production, and their collapse in relative abundance resulted in an overall production decrease. A strong correlation was present between the community composition and both the hexanoic acid concentration ( p  = 0.026) and total volatile fatty acid concentration ( p  = 0.003). MCFA suppressed species related to  Clostridium  sp. CPB-6 and  Lactobacillus  spp. to a greater extent than others. The proportion of the species related to  Clostridium  sp. BS-1 over  Clostridium  sp. CPB-6 had a strong correlation with the concentration of octanoic acid ( p  = 0.003). The dominance of this species and the increase in MCFA resulted in an overall toxic effect on the mixed community, most significantly on the  Lactobacillus  spp., which resulted in a decrease in total hexanoic acid concentration to 32 ± 2% below the steady-state average. As opposed to the current view of MCFA toxicity broadly leading to production collapse, this study demonstrates that varied tolerance to MCFA within the community can lead to the dominance of some species and the suppression of others, which can result in a decreased productivity of the fermentation.

A Clostridium Group IV Species Dominates and Suppresses a Mixed Culture Fermentation by Tolerance to Medium Chain Fatty Acids Products