Enrichment Versus Bioaugmentation-Microbiological Production of Caproate from Mixed Carbon Sources by Mixed Bacterial Culture and Clostridium kluyveri.

Roman Zagrodnik,Piotr Oleskowicz-Popiel,Mateusz Łężyk,Anna Duber

doi:10.1021/acs.est.9b07651

Abstract

Chain elongation is a process that produces medium chain fatty acids such as caproic acid, which is one of the promising products of the carboxylate platform. This study analyzed the impact of bioaugmentation of heat-treated anaerobic digester sludge with Clostridium kluyveri (AS + Ck) on caproic acid production from a mixed substrate (lactose, lactate, acetate, and ethanol). It was compared with processes initiated with non-augmented heat-treated anaerobic digester sludge (AS) and mono-culture of C. kluyveri (Ck). Moreover, stability of the chain elongation process was evaluated by performing repeated batch experiments. All bacterial cultures demonstrated efficient caproate production in the first batch cycle. After 18 days, caproate concentration reached 9.06 ± 0.43, 7.86 ± 0.38, and 7.67 ± 0.37 g/L for AS, Ck, and AS + Ck cultures, respectively. In the second cycle, AS microbiome was enriched toward caproate production and showed the highest caproate concentration of 11.44 ± 0.47 g/L. On the other hand, bioaugmented culture showed the lowest caproate production in the second cycle (4.10 ± 0.30 g/L). Microbiome analysis in both AS and AS + Ck culture samples indicated strong enrichment toward the anaerobic order of Clostridia. Strains belonging to genera Sporanaerobacter, Paraclostridium, Haloimpatiens, Clostridium, and Bacillus were dominating in the bioreactors.

Highlights

Production of biochemicals and biofuels from biomass feedstock by fermentation could be facilitated through the carboxylate platform, which is a promising technology within a bio-based economy concept.[1]
The substrate is converted to a mixture of short-chain carboxylates and alcohols. n-Caproate is a product of secondary fermentation, in a biochemical process of carboxylic acid chain elongation. n-Caproate is one of the most attractive medium chain fatty acids (MCFAs) because of its broad industrial and agricultural applications
Fermentation with anaerobic digester sludge (AS) or C. kluyveri (Ck) as inoculum was used as a reference

Summary

Introduction

Production of biochemicals and biofuels from biomass feedstock by fermentation could be facilitated through the carboxylate platform, which is a promising technology within a bio-based economy concept.[1] Chain elongation is an important part of this platform and an emerging biotechnological process which produces medium chain fatty acids (MCFAs) from volatile fatty acids (VFAs). MCFAs are straight-chain carboxylic acids with 6−12 carbon atoms and are promising precursors for the production of other added-value products.[2] During primary fermentation, the substrate is converted to a mixture of short-chain carboxylates and alcohols. N-Caproate is one of the most attractive MCFAs because of its broad industrial and agricultural applications It could be used as a feedstock in the chemical industry, feed additive, antimicrobial agent, or as a biofuel precursor.[3]. Acetyl-CoA is added to carboxylic acid, resulting in chain elongation by two carbon atoms per reaction cycle

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