The Feasibility of Integrating an Autotrophic Acetobacterium woodii Process for CO2-Neutral Yeast Oil Production

The efficient production of microbial oils from agricultural residues and acetic acid has recently been shown with Cutaneotrichosporon oleaginosus. However, around 50% of the carbon is released as CO2 during aerobic yeast oil production. Anaerobic fermentation of CO2 and H2 with A. woodii enables the carbon-efficient production of acetate. The semi-continuous autotrophic production of acetate with A. woodii was studied in a stirred-tank bioreactor with continuous gassing, where the time of the repeated batch processes was adjusted to the batch process time for microbial oil production (6–7 days). Eight repeated batch processes with 80% medium exchange were performed with A. woodii within 48 days. After adaptation of the A. woodii cells, 48.29 ± 0.35 g L−1 acetate was achieved in the last four repeated batch processes with 70% H2 and 30% CO2 gassing. Acetic acid was extracted from the clarified and acidified fermentation broth with ethyl acetate, yielding 94.3% (w/w). Based on our process performance data with A. woodii and previously published data with C. oleaginosus, it was shown that, for providing enough acetic acid for microbial oil production, a 3.08 times higher bioreactor capacity is needed for the gas fermentation compared to the aerobic yeast fermentation. The lack of CO2 produced by C. oleaginosus may be compensated for by increasing the sugar supply (hydrolysate) during yeast oil production, or by the additional use of other biogenic CO2 sources. Thus, CO2-neutral production of microbial oils from sugars or hydrolysates of agricultural residues is possible by reusing the CO2 produced in the aerobic yeast oil production for the autotrophic production of acetic acid, which is fully recycled as an additional carbon source for yeast oil production.