Establishing Mixotrophic Growth of <i>Cupriavidus necator</i> H16 on CO₂ and Volatile Fatty Acids

The facultative chemolithoautotroph <i>Cupriavidus necator</i> H16 is able to grow aerobically either with organic substrates or H₂ and CO₂ s and it can accumulate large amounts of (up to 90%) poly (3-hydroxybutyrate), a polyhydroxyalkanoate (PHA) biopolymer. The ability of this organism to co-utilize volatile fatty acids (VFAs) and CO₂ as sources of carbon under mixotrophic growth conditions was investigated and PHA production was monitored. PHA accumulation was assessed under aerobic conditions, with either individual VFAs or in mixtures, under three different conditions—with CO₂ as additional carbon source, without CO₂ and with CO₂ and H₂ as additional sources of carbon and energy. VFAs utilisation rates were slower in the presence of CO₂. PHA production was significantly higher when cultures were grown mixotrophically and with H₂ as an additional energy source compared to heterotrophic or mixotrophic growth conditions, without H₂. Furthermore, a two-step VFA feeding regime was found to be the most effective method for PHA accumulation. It was used for PHA production mixotrophically using CO₂, H₂ and VFA mixture derived from an anaerobic digestor (AD). The data obtained demonstrated that process parameters need to be carefully monitored to avoid VFA toxicity and low product accumulation.