Biotechnological fermentation is a well-established process, however, it is far from being fully understood and exploited. A new area of fermentation technology that has evolved over the recent decades is gas fermentation. Many microorganisms have been reported in literature to be capable of utilizing a variety of gases such as CO, CO 2 , H 2 , and CH 4 under anaerobic or aerobic conditions as their main carbon and/or energy source. Mostly waste stream gases from industrial plants or those that can be produced via the gasification of solids are investigated. This review focuses on the currently available scientific knowledge about gas fermentation processes, particularly anaerobic syngas fermentation and aerobic methane fermentation. Gas fermentation processes are compared with aerobic and anaerobic fermentation processes based on dissolved solid substrates. Also, the potential of gas fermentation when integrated into a biotechnological network of processes is outlined.

Synthesis gas fermentation using acetogenic clostridia is a rapidly increasing research area. It offers the possibility to produce platform chemicals from sustainable C1 carbon sources. The Wood-Ljungdahl pathway (WLP), which allows acetogens to grow autotrophically, is also active during heterotrophic growth. It acts as an electron sink and allows for the utilization of a wide variety of soluble substrates and increases ATP yields during heterotrophic growth. While glycolysis leads to CO 2 evolution, WLP activity results in CO 2 fixation. Thus, a reduction of net CO 2 emissions during growth with sugars is an indicator of WLP activity. To study the effect of trace elements and ventilation rates on the interaction between glycolysis and the WLP, the model acetogen Clostridium ljungdahlii was cultivated in YTF medium, a complex medium generally employed for heterotrophic growth, with fructose as growth substrate. The recently reported anaRAMOS device was used for online measurement of metabolic activity, in form of CO 2 evolution. The addition of multiple trace elements (iron, cobalt, manganese, zinc, nickel, copper, selenium, and tungsten) was tested, to study the interaction between glycolysis and the Wood ljungdahl pathway. While the addition of iron(II) increased growth rates and ethanol production, added nickel(II) increased WLP activity and acetate formation, reducing net CO 2 production by 28%. Also, higher CO 2 availability through reduced volumetric gas flow resulted in 25% reduction of CO 2 evolution. These online metabolic data demonstrate that the anaRAMOS is a valuable tool in the investigation of metabolic responses i.e. to determine nutrient requirements that results in reduced CO 2 production. Thereby the media composition can be optimized depending on the specific goal.

Syngas fermentation is a potential player for future emission reduction. The first demonstration and commercial plants have been successfully established. However, due to its novelty, development of syngas fermentation processes is still in its infancy, and the need to systematically unravel and understand further phenomena, such as substrate toxicity as well as gas transfer and uptake rates, still persists. This study describes a new online monitoring device based on the respiration activity monitoring system for cultivation of syngas fermenting microorganisms with gaseous substrates. The new device is designed to online monitor the carbon dioxide transfer rate (CO2TR) and the gross gas transfer rate during cultivation. Online measured data are used for the calculation of the carbon monoxide transfer rate (COTR) and hydrogen transfer rate (H2TR). In cultivation on pure CO and CO + H2, CO was continuously limiting, whereas hydrogen, when present, was sufficiently available. The maximum COTR measured was approximately 5 mmol/L/h for pure CO cultivation, and approximately 6 mmol/L/h for cultivation with additional H2 in the gas supply. Additionally, calculation of the ratio of evolved carbon dioxide to consumed monoxide, similar to the respiratory quotient for aerobic fermentation, allows the prediction of whether acetate or ethanol is predominantly produced. Clostridium ljungdahlii, a model acetogen for syngas fermentation, was cultivated using only CO, and CO in combination with H2. Online monitoring of the mentioned parameters revealed a metabolic shift in fermentation with sole CO, depending on COTR. The device presented herein allows fast process development, because crucial parameters for scale‐up can be measured online in small‐scale gas fermentation.

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