White biotechnology is promising to transform CO₂ emissions into a valuable commodity chemical such as the biopolymer polyhydroxyalkanaotes (PHA). Our calculations indicated that the indirect conversion of acetic acid from CO₂ into PHA is an interesting alternative for the direct production of PHA from CO₂ in terms of CO₂ fixation, H₂ consumption, substrate cost, safety and process performance. An alternative cultivation method using acetic acid as an indirect sink of CO₂ was therefore developed and a proof-of-concept provided for the synthesis of both the homopolymer poly(3-hydroxybutyrate) (PHB) and the copolymer poly(3-hydroxybutyrate-co-3-hydroxyvalerate) (PHBV). The aim was to compare key performance parameters with those of existing cultivation methods for direct conversion of CO₂ to PHA. Fed-batch cultivations for PHA production were performed using a pH-stat fed-batch feeding strategy in combination with an additional Dissolved Oxygen (DO)-dependent feed. After 118 h of fermentation, 60 g/L cell dry matter (CDM) containing 72% of PHB was obtained, which are the highest result values reported so far. Fed-batch cultivations for PHBV production resulted in 65 g/L CDM and 48 g/L PHBV concentration with a 3HV fraction of 27 mol %. Further research should be oriented towards process optimisation, whole process integration and design, and techno-economic assessment.

As one of the key enabling technologies, industrial biotechnology has a high potential to tackle harmful CO₂ emissions and to turn CO₂ into a valuable commodity. So far, experimental work mainly focused on the bioconversion of pure CO₂ to chemicals and plastics and little is known about the tolerance of the bioprocesses to the presence of impurities. This work is the first to investigate the impact of real CO₂-rich off-gases on autotrophic production of polyhydroxybutyrate. To this end, two-phase heterotrophic-autotrophic fermentation experiments were set up, consisting of heterothrophic cell mass growth using glucose as substrate followed by autotrophic biopolymer production using either pure synthetic CO₂ or industrial off-gases sampled at two point sources. The use of real off-gases did not affect the bacterial performance. High biopolymer content (up to 73%) and productivities (up to 0.227 g/lh) were obtained. Characterisation of the polymers showed that all biopolymers had similar properties, independent of the CO₂ source. Moreover, the CO₂-derived biopolymers’ properties were comparable to commercial ones and biopolymers reported in literature, which are all produced from organic carbon sources.