CO2 as carbon source for microbial production of bio-based

Reducing waste and emissions of greenhouse gases like CO2 is a major demand for industry. In this context great interest has emerged in biological CO2-fixing processes which are supposed to be very effective in reducing CO2 emissions. Acetogenic bacteria are able to use hydrogen gas for the reduction CO2. The reductive acetyl-CoA pathway enables the autotrophic production of biobased chemicals like acetate, ethanol, butyrate, butanol, 2,3- butanediol, hexanoate and hexanol. Metabolic engineering of acetogens is a promising approach to enlarge the natural product portfolio. Low product yields and selectivities as well as low biomass densities and inefficient utilization of gaseous substrates are some of the challenges that slow down commercialization so far. The reaction engineering analyses of acetogens and the application of bioreactor designs providing high gas-liquid mass transfer efficiencies will enable new gas fermentation processes overcoming the challenges for further commercialization in the near future. Microalgae consuming the greenhouse gas CO2 and using sunlight as energy source could become an important renewable source for biobased chemicals. Since the production cost of most microalgae products from current mass cultivation systems is still prohibitively high, further development is required. To advance economic microalgae mass production new open thin-layer cascade photobioreactors were designed recently up to a pilot scale (50 m2) for high-cell density cultivation of saline microalgae achieving up to 50 g L−1 dry cell mass, which was shown applying physically simulated climate conditions of a Mediterranean summer in Spain in the TUM-AlgaeTec-Center near by Munich, Germany. Finally, state-of-the-art of phototrophic CO2-fixation by microalgae and of autotrophic CO2-fixation by acetogens will be compared with respect to kinetics, process engineering aspects and productivities. References Riegler P, Chrusciel T, Mayer A, Doll K, Weuster Botz D (2019): Reversible retrofitting of a stirred-tank bioreactor for gas-lift operation to perform syngas fermentation studies. Biochem Eng J 141: 89-101. Doll K, Rückel A, Kaempf P, Weuster-Botz D (2018): Two stirred-tank bioreactors in series enable continuous production of alcohols from carbon monoxide with Clostridium carboxidivorans. Bioproc Biosys Eng 41:1403- 1416. Severin TS, Apel A, Brueck T, Weuster-Botz D (2018): Investigation of vertical mixing in open thin-layer cascade reactors using Computational Fluid Dynamics. Chem Eng Res Design 132: 436-444. Apel AC, Pfaffinger CE, Basedahl N, Mittwollen N, Goebel J, Sauter J, Brueck T, Weuster-Botz D (2017): Open thin-layer cascade reactors for saline microalgae production evaluated in a physically simulated Mediterranean summer climate. Algal Research 25 (2017): 381–390. Koller A, Loewe H, Schmid V, Mundt S, Weuster-Botz D (2017) Model-supported phototrophic growth studies with Scenedesmus obtusiusculus in a flat-plate photobioreactor. Biotechnol Bioeng 114: 308–320 Groher A, Weuster-Botz D (2016): Comparative reaction engineering analysis of different acetogenic bacteria for gas fermentation. J Biotechnol 228: 82-94. Kantzow C, Mayer A, Weuster-Botz D (2015): Continuous gas fermentation by Acetobacterium woodii in a submerged membrane reactor with full cell retention. J Biotechnol 212: 11-18