Abstract
Carboxydotrophic bacteria (CTB) have received attention due to their ability to synthesize commodity chemicals from producer gas and synthesis gas (syngas). CTB have an important advantage of a high product selectivity compared to chemical catalysts. However, the product spectrum of wild-type CTB is narrow. Our objective was to investigate whether a strategy of combining two wild-type bacterial strains into a single, continuously fed bioprocessing step would be promising to broaden the product spectrum. Here, we have operated a syngas-fermentation process with Clostridium ljungdahlii and Clostridium kluyveri with in-line product extraction through gas stripping and product condensing within the syngas recirculation line. The main products from C. ljungdahlii fermentation at a pH of 6.0 were ethanol and acetate at net volumetric production rates of 65.5 and 431 mmol C·L−1·d−1, respectively. An estimated 2/3 of total ethanol produced was utilized by C. kluyveri to chain elongate with the reverse β-oxidation pathway, resulting in n-butyrate and n-caproate at net rates of 129 and 70 mmol C·L−1·d−1, respectively. C. ljungdahlii likely reduced the produced carboxylates to their corresponding alcohols with the reductive power from syngas. This resulted in the longer-chain alcohols n-butanol, n-hexanol, and n-octanol at net volumetric production rates of 39.2, 31.7, and 0.045 mmol C·L−1·d−1, respectively. The continuous production of the longer-chain alcohols occurred only within a narrow pH spectrum of 5.7–6.4 due to the pH discrepancy between the two strains. Regardless whether other wild-type strains could overcome this pH discrepancy, the specificity (mol carbon in product per mol carbon in all other liquid products) for each longer-chain alcohol may never be high in a single bioprocessing step. This, because two bioprocesses compete for intermediates (i.e., carboxylates): (1) chain elongation; and (2) biological reduction. This innate competition resulted in a mixture of n-butanol and n-hexanol with traces of n-octanol.
Highlights
Depletion of fossil energy carriers and concomitant emissions of greenhouse gases has stimulated research and development of energy systems that are sustainable and carbon neutral
For pre-culturing, C. ljungdahlii was routinely grown in modified P7 medium in 160 mL serum bottles with a working volume of 10 mL at 35◦C under a syngas headspace (Richter et al, 2013b), and C. kluyveri was grown in DSMZ52 medium in 160 mL serum bottles with a working volume of 20 or 50 mL at 35◦C under strictly anaerobic conditions without shaking
Together with the constant washout of 25% of bacterial cells in the effluent stream and the completely stirred conditions, we conclude that both strains showed sustainable growth and that they were resilient as a co-culture
Summary
Depletion of fossil energy carriers and concomitant emissions of greenhouse gases has stimulated research and development of energy systems that are sustainable and carbon neutral. Besides wind, solar, and hydroelectric power, is the conversion of producer gas into chemicals and fuels (Daniell et al, 2016). Recovery of carbon from syngas is a potential strategy to reduce consumption of fossil energy carriers and to reduce greenhouse gas emissions (Dürre, 2016). CTB are specific in their product portfolio, and have been suggested to tolerate gas contaminants and fluctuating H2/CO ratios better than metal catalysts (Dry, 2002; Liew et al, 2013). The spectrum of useful products from wild-type CTB is narrow with ethanol, 2,3-butanediol, and acetate as the only products that can be produced at a promising selectivity (mol carbon in product per mol carbon in substrate). Some CTB, such as C. carboxidivorans P7, produce n-butanol and n-hexanol from syngas, the achieved production rates and selectivities are low (Bruant et al, 2010; Ramió-Pujol et al, 2015)
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