Bioprocessing intensification of biomanufacture of microbial polyester with microbubble mediated gas fermentation, cell flotation and PHB harvesting

Sustainable bioprocessing for commodity chemicals/materials faces several challenges to be economically competitive with those petroleum-derived. Feedstocks used for fermentation are typically sugary materials which could be used for foodstuffs. Hence, gas fermentation, which uses waste gases, is a promising approach. Conventional downstream separations in bioprocessing are appropriate for high value added bioproducts, such as biopharmaceuticals and nutraceuticals. But these methods are too expensive for low value added commodity chemicals and materials. In BIOMPOLY, we propose microbubble mediated methodologies to enhance gas fermentation for faster metabolism, and then microbubble mediated downstream separation schemes, both aimed at reducing costs. The exemplar gas fermentation is to produce a bioplastic, polyhydroxybutyrate (PHB), which is one of a class of polyhydroxyyalkanoates (PHAs) that are biodegradable in all environments, including seawater. Although only ~100 kilotonnes annually are produced, PHAs have the potential to replace up to 90% of fossil polymers. Global PHB Production Market Value was $102 million in 2021 and is expected to be worth $283.5 million by 2028. A CAGR of 18.5% is set to grow heavily in the EU due to bans on single-use plastics. Conventional gas fermentation must take special care as the two major nutrient gases, methane and air, are potentially flammable in the headspace of the fermenter. Separate microbubble dispersions of methane-rich waste gases and of air can be designed and controlled so that one of the gases is outside the flammability limits, as the microbubble size can be controlled to be non-buoyant, so all the nutrient gas is used in the fermenter. Conventional downstream PHA extraction and purification is energy-intensive and involves the use of hazardous chemicals and solvents such as chloroform, dichloromethane, or a combination of solvents. Our methodologies use gentle, microbubble mediated cell lysis, so as not to damage the PHB nodules, with selective flotation by tuning microbubble sizes to harvest the cells and then separate the PHB from the remaining biomass. BIOMPOLY will involve two types of engineering biology from the unique UK laboratory producing methanotrophs (methane and potentially CO2 consuming bacteria) that generate PHBs currently. The methanotroph strain will be genetically engineered to maximize the yield, increase the growth rate, and stop the PHB from being consumed by the bacterial host. Secondly, we will symbiotically engineer a co-culture of microalgae with the methanotroph strain, to consume the CO2 produced by the methanotroph, and produce oxygen for the methanotroph. Both approaches aim to match microbubble size distribution to the size distribution of the microbes, so that complexes of microbes and microbubbles will be formed, called Desai artificial lichen (DAL). DALs exploit the gas exchange across the microbubble, which is orders of magnitude faster than dissolving the practically insoluble nutrient gases like methane, in the fermentation media. From observations in the literature, some DAL complexed microbial consortia have much faster coupled metabolisms than those without the local gas exchange. By matching the microbubble/microbe size distributions, we aim to optimize the formation of such complexes. The co-culture will have the benefit of not needing to introduce air microbubbles, so an intrinsically safe design. Techno-economic and sustainability assessment of the potential upscaled BIOMPOLY will be undertaken -- necessary to allow decision makers to determine likely commercial feasibility so that an impact plan can be developed.