Valorisation of CO2-rich Waste Gas into Polyhydroxyalkanoates (PHAs) by Cupriavidus necator

Polyhydroxyalkanoates (PHAs) are a family of biodegradable and biocompatible polymers that comprise sustainable alternatives to petroleum-based plastics. PHAs are produced by some microorganisms as carbon storage granules under nutrient limiting conditions. In particular, Cupriavidus necator has been widely studied as a promising platform for PHA production due to its ability to accumulate high levels of PHAs. The bacterium is able to grow both heterotrophically and autotrophically by consuming CO2. The latter is especially interesting as it presents a way to fix CO2 and convert it into useful products such as PHAs, a bioprocess that fits within the context of a circular economy.
The production of PHAs, and especially polyhydroxybutyrate (PHB) - the main type of biopolymer produced - in C. necator has been widely studied. However, the application of this carbon capture method to real-life waste gasses is scarce. Moreover, the process has a high hydrogen and oxygen requirement, posing the risk of explosiveness, and a relatively low carbon fixation rate. Thus, further need for optimization of the process is required.
In this work, we used C. necator for the valorisation of CO2-rich gas derived from the fermentation of grape must during wine production, a process which emits thousands of tonnes of CO2 per year. We fed fermentation gas to C. necator under phosphate limiting conditions obtaining 25% PHB (w/w). In order to increase this PHB content, we carried out a systematic comparison of several other nutrient stress conditions, obtaining an improved PHB content of 55%, and assessed growth under low H2 conditions. Finally, we are investigating the optimisation of the CO2 uptake rate of C. necator via genetic engineering, by developing mutant C. necator strains which overexpress key genes or regulatory elements of the main metabolic pathways involved in the autotrophic metabolism. These findings aim to better adapt the process for real CO2 capture applications.