Anaerobic fermentation of gaseous substrates – pure culture vs. open mixed culture applications

Bacteria employing the Wood-Ljungdahl pathway for carbon fixation, also known as homoacetogenic bacteria, hold great application potential for microbial synthesis of biofuels and commodity chemicals such as carboxylates and alcohols with two to four carbon atoms, starting from syngas (mixtures of carbon monoxide, hydrogen and carbon dioxide) or directly from H2/CO2 mixtures. As a competing anaerobic metabolic route, methanogenic archaea convert H2/CO2 or syngas to methane. Compared to established chemical synthesis of hydrocarbons from syngas (Fischer-Tropsch synthesis), the advantage of employing microbial syngas conversion is the higher tolerance of the biocatalysts for impurities in the raw (bio)-syngas and different gas compositions, making the gasification of woody biomass and high-solid organic waste to bio-syngas a suitable process step for microbial bio-refinery concepts and a complementary value chain to anaerobic digestion of wet biomass. Another advantage of microbial syngas fermentation over chemical syntheses is the moderate operating-temperature and –pressure of microbial syngas conversion. The latter also applies to microbial methanation of renewable hydrogen produced by water electrolysis utilizing excess electricity from fluctuating renewable energies (power-to-gas). Including the methanation step in power-to-gas concepts enables storage and distribution of a versatile chemical energy carrier (methane) in the natural gas grid and using it with the existing gas infrastructure for power and heat production or as vehicle fuel. Compared to the classical Sabatier process, i.e. the thermochemical conversion of H2/CO2 to methane, the microbial methanation is more flexible and robust in decentralized small-scale applications, and it can be integrated in biogas production, leading to an in situ upgrading of biogas to biomethane quality. Both microbial syngas fermentation and microbial power-to-gas processes are limited by the slow gas-to-liquid mass transfer. Specialized reactor types and process designs have been developed to overcome this limitation. Another limitation of syngas fermentation with pure cultures of homoacetogenic bacteria is the restriction to short-chain products of only two to four carbon atoms. To extend the application potential of syngas fermentation genetic engineering of production strains is intended. However, genetically modified organisms (GMO) are frequently not stable or less robust under real process conditions and maintaining a process sterile and contained causes additional costs. An alternative approach to pure culture and GMO applications are open mixed cultures which can be maintained under non-sterile and low-cost process conditions. This concept is well established in anaerobic digestion for biogas production and holds also great potential for the anaerobic fermentation of gaseous substrates by homoacetogenic bacteria or methanogenic archaea. Recent studies demonstrated that integrating syngas fermentation with the carboxylate platform improves the productivity of medium-chain alcohols. However, the underlying metabolic pathways in anaerobic consortia are highly complex and comprise competing or reversible reactions which are thermodynamically constrained. The detailed understanding of these metabolic pathways, their regulation and the involved organisms is a prerequisite for steering the process towards the desired products. Recent progress and open research questions in the field of anaerobic fermentation of gaseous substrates with pure or mixed cultures are discussed in this topic.