Upcycling agricultural waste to biodegradable polyhydroxyalkanoates by combined ambient alkaline pretreatment and bacterial fermentation

Medium-chain-length polyhydroxyalkanoates (mcl-PHAs), polyesters produced by bacterial fermentation of sugars and/or lipids, are potential bioplastic alternatives to petroleum-derived plastics. Lignocellulose is an abundant renewable source of sugars for bacterial fermentation. However, chemical or enzymatic release of the sugars requires a pretreatment step to dispel the rigid structure of the lignocellulose and enhance sugar access. Most pretreatments of lignocellulose involve high temperature and/or pressure, which increases processing and production costs and discourages commercialization. Here we demonstrate a chemical-biological pathway for mcl-PHAs production from rice straw and hemp hurd. We combined ambient alkaline pretreatment, enzymatic hydrolysis, and bacterial fermentation. The alkaline pretreatment reduced the particle size of pretreated solids and partially fractionated hemicellulose and lignin; these effects increased cellulose accessibility to enzymes and enabled a high sugar release (78–83% glucose yield) at a high solid loading (9 wt%). Therefore, we obtained 47 wt% and 69 wt% mcl-PHAs (with respect to gram of dry cell weight) from pretreated rice straw and hemp hurd, respectively. These findings constitute an energy-efficient pretreatment process that can be extended to other sources of lignocellulose, such as woody biomass and dedicated bioenergy crops. Moreover, mcl-PHAs from lignocellulose provide revenue to the agricultural sector, mitigate global warming from fossil fuel processing, and potentially reduce plastic pollution.