Gas fermentation – a biotechnological solution for today's challenges

Costs for biotechnological processes in general are largely dictated by substrate prices. Thus, especially production of bulk chemicals from sugar or molasses suffers economically, if these compounds increase in price. A fairly recent solution to deal with this problem is the use of cheap gases as a carbon and energy source. Both, aerobic and anaerobic gas fermentations have meanwhile reached commercial level (D€ urre and Eikmanns, 2015). Additional benefits are the use of non-food feedstocks and the reduction of greenhouse gas emissions, if sustainable compounds are produced. Anaerobic gas fermentation is a preserve of autotrophic acetogenic bacteria, mostly of the genus Clostridium. Typical representatives are Clostridium ljungdahlii and C. autoethanogenum that can use pure CO or syngas (a mixture of mostly CO and H2) as carbon and energy sources. Such gas mixtures are also waste emissions from steel manufacture and chemical production lines. They can easily be obtained from gasification of biomass, too. Metabolic engineering already allowed a large expansion of the product portfolio of these bacteria, which will offer alternatives for replacement of current crude oilbased chemical manufacturing processes. Commercial size plants are under construction in both Asia (e.g. in China Shougang Group and LanzaTech) and Europe (in Belgium ArcelorMittal and LanzaTech), situated within steel mills and operating by using the steel mill waste gases to produce ethanol. In addition, further processes have been developed for synthesis of jet fuel (press release of September 14th, 2016; http://www.lanzatech. com/low-carbon-fuel-project-achieves-breakthrough/). Pathways for commodity and specialty chemicals have already been implemented in C. ljungdahlii and C. autoethanogenum, such as e.g. butanol, acetone, and isopropanol (K€ opke et al., 2010; K€ opke and Liew, 2012; Simpson et al., 2012). Even CO2 can be used as a carbon source, if additional reducing power (e.g. H2) is provided. As an example, Acetobacterium woodii has been metabolically engineered to produce acetone in addition to acetate (Hoffmeister et al., 2016). Thus, we can envision a near future with carbon capture from waste gases for the production of hydrocarbon fuels and chemical building blocks. Having said this, I want, however, to emphasize what challenges still lie ahead. One of the major problems in anaerobic gas fermentation is the severe energy limitation that acetogens have to deal with. The central metabolic route for CO or CO2 fixation is the Wood-Ljungdahl pathway (Fig. 1A). One