No abstract

Production of biodegradable thermoplastics, polyhydroxyalkanoates (PHAs), from organic wastes may provide multiple benefits to environmental conservation efforts. In this study, microfiltration-coupled reactors were set up to study the dynamic behavior of a typical PHA-producing bacteria, Ralstonia eutropha, fed with a real acidic solution from starch acidogenesis in a continuous flow system. The majorfermentation acids (butyric and acetic acids) were utilized by the PHA producers at a high conversion rate (>95%) when the cells were suspended in a small volume of mineral solution (pH 7), but at a low conversion rate (<10%) when the cells were suspended in an acidic solution (pH 4). The acids were consumed mainly for PHA synthesis and maintenance energy, which resulted in slow growth of PHA-producing cells and a washout of the cells in the continuous flow system. PHAs, however, were continuously synthesized and accumulated during the washout. A simple dynamic model is proposed for estimation of specific growth rates and PHA formation rates during the washout at two hydraulic retention times (HRT) or dilution rates. The net specific growth rate of PHA-producing cells was near zero at a hydraulic retention time (HRT) of around 30 h, but it increased to 0.01 h(-1) when the HRT was reduced to 18 h. The model also reveals that PHA was synthesized faster based on the active biomass (ABM) during the short HRT (10.3 mg PHA/g ABM.h) than during the long HRT (3.4 mg PHA/g ABM.h).

Short chain fatty acids such as acetic, propionic, and butyric acids can be synthesized into polyhydroxyalkanoates (PHAs) by Ralstonia eutropha. Metabolic carbon fluxes of the acids in living cells have significant effect on the yield, composition, and thermomechanical properties of PHA bioplastics. Based on the general knowledge of central metabolism pathways and the unusual metabolic pathways in R. eutropha, a metabolic network of 41 bioreactions is constructed to analyze the carbon fluxes on utilization of the short chain fatty acids. In fed-batch cultures with constant feeding of acid media, carbon metabolism and distribution in R. eutropha were measured involving CO2, PHA biopolymers, and residual cell mass. As the cells underwent unsteady state metabolism and PHA biosynthesis under nitrogen-limited conditions, accumulative carbon balance was applied for pseudo-steady-state analysis of the metabolic carbon fluxes. Cofactor NADP/NADPH balanced between PHA synthesis and the C3/C4 pathway provided an independent constraint for solution of the underdetermined metabolic network. A major portion of propionyl-CoA was directed to pyruvate via the 2-methylcitrate cycle and further decarboxylated to acetyl-CoA. Only a small amount of propionate carbon (<15% carbon) was directly condensed with acetyl-CoA for 3-hydroxyvalerate. The ratio of glyoxylate shunt to TCA cycle varies from 0 to 0.25, depending on the intracellular acetyl-CoA level and acetic acid in the medium. Malate is the node of the C3/C4 pathway and TCA cycle and its decarboxylation to dehydrogenation ranges from 0.33 to 1.28 in response to the demands on NADPH and oxaloacetate for short chain fatty acids utilization.