Online monitoring of microbial cultures is an effective approach for studying both aerobic and anaerobic microorganisms. Especially in small‐scale cultivations, several parallel online monitored experiments can generate a detailed understanding of the cultivation, compared to a situation where a few data points are generated from time course sampling and offline analysis. However, the availability of small‐scale online monitoring devices for acetogenic organisms is limited. In this study, the previously reported anaerobic Respiration Activity MOnitoring System (anaRAMOS) device was adapted for online monitoring of Clostridium ljungdahlii (C. ljungdahlii) cultures with fructose as the carbon source. The anaRAMOS was applied to identify conversion of different carbon sources present in commonly used YTF medium. An iron(II) deficiency was discovered in this medium for C. ljungdahlii. Addition of iron(II) to the YTF medium reduced the cultivation time and increased biomass yield of C. ljungdahlii cultures by 50% and 40%, respectively. The measurement of the carbon dioxide transfer rate was used to calculated the iron(II) contained in complex components. By demonstrating the application of the anaRAMOS device for medium optimization, it is proven that the described online monitoring device has potential for use in process development.
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.
Syngas fermentation is one possible contributor to the reduction of greenhouse gas emissions. The conversion of industrial waste gas streams containing CO or H 2 , which are usually combusted, directly reduces the emission of CO 2 into the atmosphere. Additionally, other carbon-containing waste streams can be gasified, making them accessible for microbial conversion into platform chemicals. However, there is still a lack of detailed process understanding, as online monitoring of dissolved gas concentrations is currently not possible. Several studies have demonstrated growth inhibition of Clostridium ljungdahlii at high CO concentrations in the headspace. However, growth is not inhibited by the CO concentration in the headspace, but by the dissolved carbon monoxide tension (DCOT). The DCOT depends on the CO concentration in the headspace, CO transfer rate, and biomass concentration. Hence, the measurement of the DCOT is a superior method to investigate the toxic effects of CO on microbial fermentation. Since CO is a component of syngas, a detailed understanding is crucial. In this study, a newly developed measurement setup is presented that allows sterile online measurement of the DCOT. In an abiotic experiment, the functionality of the measurement principle was demonstrated for various CO concentrations in the gas supply (0%-40%) and various agitation rates (300-1100 min −1). In continuous stirred tank reactor fermentation experiments, the measurement showed reliable results. The production of ethanol and 2,3-butanediol increased with increasing DCOT. Moreover, a critical DCOT was identified, leading to the inhibition of the culture. Thus, the reported online measurement method is beneficial for process understanding. In future processes, it can be used for closedloop fermentation control.
Clostridium autoethanogenum is capable of converting C1 carbon sources such as CO and CO2, as well as C5 carbon sources such as xylose, into various products, rendering it suitable for syngas conversion. However, pure gas fermentations generally have low volumetric productivity caused by low cell mass concentrations, resulting from low specific growth rates and low gas–liquid mass transfer. The strong dependency on gas–liquid mass transfer causes data generated in serum bottles to often deviate from experiments in stirred tank reactors. This study therefore characterizes growth and product formation in both the serum bottle and stirred tank reactor, while investigating a sequential and simultaneous feeding of xylose and gaseous carbon substrates. The ratio of the product was shown to be independent of the initial xylose concentration in serum bottles, while in stirred tank reactor experiments the product ratio changed under xylose-limited conditions. The product ethanol caused inhibiting effects which could be quantified in a kinetic model. The comparison of feeding strategies showed clearly that a fed-batch process with simultaneous xylose and CO feeding led to higher CO conversion when compared to CO conversion in a sequential cultivation strategy. Carbon can be directed toward acetate formation via fed-batch fermentation under C-limited conditions. Moreover, the combined feed of xylose and CO is an advantageous method to significantly enhance gas conversion in comparison to sequential feeding.
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