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Gas fermentation is a promising way to convert CO-rich gases to chemicals. We studied the use of synthetic cocultures composed of carboxydotrophic and propionigenic bacteria to convert CO to propionate. So far, isolated carboxydotrophs cannot directly ferment CO to propionate, and therefore, this cocultivation approach was investigated. Four distinct synthetic cocultures were constructed, consisting of Acetobacterium wieringae (DSM 1911 T ) and Pelobacter propionicus (DSM 2379 T ), Ac. wieringae (DSM 1911 T ) and Anaerotignum neopropionicum (DSM 3847 T ), Ac. wieringae strain JM and P. propionicus (DSM 2379 T ), and Ac. wieringae strain JM and An. neopropionicum (DSM 3847 T ). Propionate was produced by all the cocultures, with the highest titer (∼24 mM) being measured in the coculture composed of Ac. wieringae strain JM and An. neopropionicum , which also produced isovalerate (∼4 mM), butyrate (∼1 mM), and isobutyrate (0.3 mM). This coculture was further studied using proteogenomics. As expected, enzymes involved in the Wood-Ljungdahl pathway in Ac. wieringae strain JM, which are responsible for the conversion of CO to ethanol and acetate, were detected; the proteome of An. neopropionicum confirmed the conversion of ethanol to propionate via the acrylate pathway. In addition, proteins related to amino acid metabolism and stress response were highly abundant during cocultivation, which raises the hypothesis that amino acids are exchanged by the two microorganisms, accompanied by isovalerate and isobutyrate production. This highlights the importance of explicitly looking at fortuitous microbial interactions during cocultivation to fully understand coculture behavior. IMPORTANCE Syngas fermentation has great potential for the sustainable production of chemicals from wastes (via prior gasification) and flue gases containing CO/CO 2 . Research efforts need to be directed toward expanding the product portfolio of gas fermentation, which is currently limited to mainly acetate and ethanol. This study provides the basis for a microbial process to produce propionate from CO using synthetic cocultures composed of acetogenic and propionigenic bacteria and elucidates the metabolic pathways involved. Furthermore, based on proteomics results, we hypothesize that the two bacterial species engage in an interaction that results in amino acid exchange, which subsequently promotes isovalerate and isobutyrate production. These findings provide a new understanding of gas fermentation and a coculturing strategy for expanding the product spectrum of microbial conversion of CO/CO 2 .

A syngas-degrading enrichment culture, culture T-Syn, was dominated by a bacterium closely related to Desulfofundulus australicus strain AB33T (98% 16S rRNA gene sequence identity). Culture T-Syn could convert high CO concentrations (from pCO ≈ 34 kPa to pCO ≈ 170 kPa), both in the absence and in the presence of sulfate as external electron acceptor. The products formed from CO conversion were H2 and acetate. With sulfate, a lower H2/acetate ratio was observed in the product profile, but CO conversion rates were similar to those in the absence of sulfate. The ability of D. australicus strain AB33T to use CO was also investigated. D. australicus strain AB33T uses up to 40% CO (pCO ≈ 68 kPa) with sulfate and up to 20% CO (pCO ≈ 34 kPa) without sulfate. Comparison of the metagenome-assembled genome (MAG) of the Desulfofundulus sp. from T-Syn culture with the genome of D. australicus strain AB33T revealed high similarity, with an ANI value of 99% and only 32 unique genes in the genome of the Desulfofundulus sp. T-Syn. So far, only Desulfotomaculum nigrificans strain CO-1-SRB had been described to grow with CO with and without sulfate. This work further shows the carboxydotrophic potential of Desulfofundulus genus for CO conversion, both in sulfate-rich and low-sulfate environments. [ABSTRACT FROM AUTHOR]

Syngas is a substrate for the anaerobic bioproduction of fuels and valuable chemicals. In this study, anaerobic sludge was used for microbial enrichments with synthetic syngas and acetate as main substrates. The objectives of this study were to identify microbial networks (in enrichment cultures) for the conversion of syngas to added-value products, and to isolate robust, non-fastidious carboxydotrophs. Enrichment cultures produced methane and propionate, this last one an unusual product from syngas fermentation. A bacterium closely related to Acetobacterium wieringae was identified as most prevalent (87% relative abundance) in the enrichments. Methanospirillum sp. and propionate-producing bacteria clustering within the genera Anaerotignum and Pelobacter were also found. Further on, strain JM, was isolated and was found to be 99% identical (16S rRNA gene) to A. wieringae DSM 1911T. Digital DNA-DNA hybridization (dDDH) value between the genomes of strain JM and A. wieringae was 77.1%, indicating that strain JM is a new strain of A. wieringae. Strain JM can grow on carbon monoxide (100% CO, total pressure 170 kPa) without yeast extract or formate, producing mainly acetate. Remarkably, conversion of CO by strain JM showed shorter lag phase than in cultures of A. wieringae DSM 1911T, and about four times higher amount of CO was consumed in 7 days. Genome analysis suggests that strain JM uses the Wood-Ljungdahl pathway for the conversion of one carbon compounds (CO, formate, CO2/H2). Genes encoding bifurcational enzyme complexes with similarity to the bifurcational formate dehydrogenase (Fdh) of Clostridium autoethanogenum are present, and possibly relate to the higher tolerance to CO of strain JM compared to other Acetobacterium species. A. wieringae DSM 1911T grew on CO in medium containing 1 mM formate. [ABSTRACT FROM AUTHOR]