Integrated bioconversion of syngas into bioethanol and biopolymers

•Syngas fermentation yields a mixture of acetic acid and alcohols.•N2 limitation in the syngas fermentation effluent favoured PHA accumulation.•Only acetic acid from syngas fermentation is totally used up for PHA production.•Substrates uptake rate ratio and storage rate strongly affect PHA accumulation.•A maximum PHA content of 24% of cell dry weight was reached.

Syngas bioconversion is a promising method for bioethanol production, but some VFA remains at the end of fermentation. A two-stage process was set-up, including syngas fermentation as first stage under strict anaerobic conditions using C. autoethanogenum as inoculum, with syngas (CO/CO2/H2/N2, 30/10/20/40) as gaseous substrate. The second stage consisted in various fed-batch assays using a highly enriched PHA accumulating biomass as inoculum, where the potential for biopolymer production from the remaining acetic acid at the end of the syngas fermentation was evaluated. All of the acetic acid was consumed and accumulated as biopolymer, while ethanol and 2,3-butanediol remained basically unused. It can be concluded that a high C/N ratio in the effluent from the syngas fermentation stage was responsible for non-consumption of alcohols. A maximum PHA content of 24% was reached at the end of the assay.