Gas Fermentation-A Flexible Platform for Commercial Scale Production of Low-Carbon-Fuels and Chemicals from Waste and Renewable Feedstocks

Abstract

There is an immediate need to drastically reduce the emissions associated with global fossil fuel consumption in order to limit climate change. However, carbon-based materials, chemicals, and transportation fuels are predominantly made from fossil sources and currently there is no alternative source available to adequately displace them. Gas-fermenting microorganisms that fix carbon dioxide (CO2) and carbon monoxide (CO) can break this dependence as they are capable of converting gaseous carbon to fuels and chemicals. As such, the technology can utilize a wide range of feedstocks including gasified organic matter of any sort (e.g., municipal solid waste, industrial waste, biomass, and agricultural waste residues) or industrial off-gases (e.g., from steel mills or processing plants). Gas fermentation has matured to the point that large-scale production of ethanol from gas has been demonstrated by two companies. This review gives an overview of the gas fermentation process, focusing specifically on anaerobic acetogens. Applications of synthetic biology and coupling gas fermentation to additional processes are discussed in detail. Both of these strategies, demonstrated at bench-scale, have abundant potential to rapidly expand the commercial product spectrum of gas fermentation and further improve efficiencies and yields.

Keywords: Clostridium; acetogens; carbon capture and utilization; coupled processes; gas fermentation; low-carbon fuels; syngas; synthetic biology.

Figure 1

Overview of feedstock and product options for gas fermentation. Feedstocks to the gas fermentation platform are highlighted in light blue (carbon and electron sources) and green (electron sources). Feedstocks shown are at various stages of commercial deployment. Synthesis of all products shown has been demonstrated including (1) native products (blue text), (2) synthetic products produced through genetic modification (red text), (3) products generated through secondary fermentation of co/mixed cultures (purple text), and (4) products achieved through additional catalytic upgrading (orange text). Acronyms: 2,3-BDO, 2,3-Butanediol; MEK, methyl ethyl ketone.

Figure 2

Overview of Wood-Ljungdahl pathway (WLP) and energy conserving mechanisms of acetogen C. autoethanogenum. The WLP is central to the gas fermentation platform for carbon fixation. Noteworthy enzymes are in labeled in blue. The enzymes involved in energy conservation are shown in purple. Acronyms: 2,3-BDO, 2,3-butanediol; AOR, aldehyde:ferredoxin oxidoreductase; ACS, acetyl-CoA synthase; CODH, carbon monoxide dehydrogenase; Nfn, transhydrogenase; PFOR, pyruvate:ferredoxin oxidoreductase; Rnf, Rhodocbacter nitrogen fixation; THF, Tetrahydrofolate; WGS, water-gas shift reaction.

Figure 3

Overview of the Triple Cross tool for precise genetic manipulations. The triple cross tool relies on two counter-selectable markers (CS1 and CS2) in combination with one antibiotic marker (AB). CS1 and AB1 are located between two homology arms (5′ and 3′) together with a shorter, third homology arm (X). Using selection for AB and against CS1, a direct double crossover event at homology arms 3′ and 5′ is forced in a first step, this is facilitated by having homology arms of different length. In an optional second step, the marker can be recycled using shorter third homology arm X and selection against CS2. Shown is deletion of a target gene, but the same technology can also be used to deliver genes by placing the respective sequence between homology arms X and 3′. Depending how the homology arms are placed, the gene can either be inserted at any given position in the genome or an existing sequence be replaced by a new one.