Corn stover hydrolysate and levulinic acid: Mixed substrates for short-chain polyhydroxyalkanoate production

The plastic disposal problem has been augmented over the years due to the surge in utilization of single-use plastics, the indiscriminate discarding of plastics, and space limitations associated with landfill sites. Therefore, in this study the biodegradable, biorenewable, and biocompatible plastic substitutes, poly(hydroxyalkanoate) (PHA) biopolymers were synthesized from low-value and high-volume corn stover in combination with levulinic acid (LevA). Acid hydrolysis liberated the fermentable sugars from the cellulosic and hemicellulosic fractions of the corn stover. This hydrolysate was used as a feedstock in combination with LevA to produce a terpolyester composed of 3-hydroxybutyric acid (3-HB), 3-hydroxyvaleric acid (3-HV), and 4-hydroxyvaleric acid (4-HV) by the bacterium Azohydromonas lata, and a copolymer of 3-HB and 3-HV by the bacterium Burkholderia sacchari. Detoxification of the hydrolysate through ‘overliming’ was necessary to induce bacterial growth and PHA production in A. lata, while B. sacchari was able to utilize the hydrolysate without detoxification achieving a maximum PHA titer of 1.2 g PHA/L. The inclusion of higher LevA in the medium affected the monomeric composition of the copolymers produced by B. sacchari resulting in a 3-HB:3-HV ratio of 39:61 when grown in the presence of 0.4% (w/v) LevA. In contrast, the monomeric compositions were relatively unaffected by the LevA media concentration for the PHA biopolymers produced by A. lata. The results of this study should help reduce the overall costs to synthesize bacterially derived PHA biopolymers and increase their applicability, thus reducing our dependence on recalcitrant petroleum-based plastics.