ABSTRACT
The sustainable high-capacity production of the biodegradable biopolymer polyhydroxyalkanoate (PHA) requires the availability of large amounts of carbon-neutral carbonaceous substrates. Harnessing organic waste, such as municipal solid and agricultural waste, sewage sludge, and biomass, for bioconversion to bioplastics, presents a cost-efficient opportunity. Nevertheless, these carbon sources present a large diversity in terms of composition and, therefore, would lead to inconsistent polymer quality. The preliminary transformation of such waste streams by gasification into syngas leads to a more defined gaseous substrate consisting primarily of CO, H2, CO2, and CH4. The components of this gas mix can serve as substrates in so-called syngas fermentation. A few types of bacteria can assimilate CO as carbon and/or an energy source. This includes acetogens, which use CO and CO2 as carbon and energy sources to produce acetyl-CoA and finally acetate in the Wood–Ljungdahl pathway. These bacteria do not possess the enzymes for the production of PHA in their wild-type form, but this hurdle has been overcome recently by genetic engineering. Carboxydobacteria can grow solely on CO in aerobic conditions and can produce PHA. Their growth rate may, however, vary as a function of the CO concentration in the gas mixture. CO-tolerant hydrogen-oxidizing bacteria can grow in syngas on CO2 and H2, in aerobic conditions at low CO concentration, but de facto cannot assimilate CO. They can accumulate high PHA content. Finally, Rhodospirillum rubrum is the most studied bacterial species for the conversion of syngas to PHA but exhibits only a modest growth rate and limited PHA accumulation capacity. Genetic engineering of these bacteria was also investigated and may lead to higher PHA production rates. The underlying processes of waste and biomass gasification and gas fermentation will be discussed in detail in this chapter, together with the metabolic pathways involved in the assimilation of syngas as carbon and energy sources.
