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

Korneel Rabaey,Way Cern Khor,Ruben Props,Stephen J. Andersen,Marta Coma,Hugo Roume,Vicky De Groof

doi:10.3389/fbioe.2017.00008

Abstract

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.

Highlights

The conversion of a complex substrate in anaerobic digestion and fermentation relies on the competition for carbon and energy within a mixed microbial community to drive the conversion of diverse polymeric biomass to simpler end products, such as methane, acetic acid, and other volatile fatty acids (VFAs)
The VFA profile of the identical reactors began to diverge at day 20, with Reactor 1 progressively increasing to a maximum of 2.1 gC L−1 of octanoic acid by day 39
The octanoic acid concentration began to rise in Reactor 2 after day 60, which coincided with a decline in both hexanoic acid and the total VFA concentration from 7.4 gC L−1 on day 60 to a low of 5.0 gC L−1 by day 68, 32% less than the total average steady-state VFA concentration

Summary

Introduction

The conversion of a complex substrate in anaerobic digestion and fermentation relies on the competition for carbon and energy within a mixed microbial community to drive the conversion of diverse polymeric biomass to simpler end products, such as methane, acetic acid, and other volatile fatty acids (VFAs). Conditions cannot be ideal for each player, and some applied or inherent conditions can result in favorable (or unfavorable) conditions for some species to the detriment (or benefit) of others, impacting the role they play in this process Managing this principle is at the center of engineering a mixed microbial biotechnology (Angenent et al, 2016; Lindemann et al, 2016). VFAs are bulk chemicals that can be produced in a biorefinery context from organic waste streams This can include short chain fatty acids such as acetic and propionic acid, common biological intermediates such as lactic acid and succinic acid, and medium chain fatty acids (MCFAs) such as hexanoic and octanoic acid. The Clostridium group IV has been shown to generate hexanoic acid from lactic acid (Xu et al, 2015)

Methods

Results

Conclusion