Polyhydroxyalkanoate production by Bacillus thuringiensis: an aspect of biorefining pulp and paper mill sludge.

Abstract

The substantial success of plastic as a material is owed to its unparalleled designs with unique properties and proved versatility in an extensive range of applications. Unfortunately, the reliance on single-use plastic commodities consequently results in the incorrect disposal and accumulation of this waste at staggering rates in our environment and landfill sites. In this regard, there is a vested interest in replacing petrochemical plastics with natural, biodegradable plastics (bioplastics). Of the many natural polymers available, microbially synthesized polyhydroxyalkanoates (PHAs) have gained popularity. Eco-friendly PHA-based bioplastics are characteristically as robust and as durable as their oil-based equivalents. Pulp and paper mill sludge (PPMS) is another solid waste stream that is predominantly disposed of via landfilling. The environmentally hazardous gases and leachate emitted from PPMS together with limited landfill space availability and the implementation of strict waste management legislation may not make landfilling practicable in the future. However, this carbohydrate-rich biomass has favorable traits that make it applicable as a feedstock for microbial biomass and PHA production. Hence, in the interest of addressing the issues mentioned above, this study aimed to beneficiate PPMS into PHAs by applying it as the sole feedstock for microbial cell proliferation and subsequent PHA production. Presently, to the best of the author’s knowledge, there are no reports on PHA production as a route for valorization of PPMS from South African pulp and paper mills. Thus, the novelty of the present study is marked by the unique ways of incorporating PPMS as a low-cost substrate as well as the various fermentative strategies navigated to enhance both microbial cell biomass and PHA productivity. In the present study, it was established that Bacillus thuringiensis had promising PHA-producing capability. The strain synthesized a copolymer and terpolymer using untreated (raw) neutral semi-sulphite chemical pulping and cardboard recycling mill (NSSC-CR) and prehydrolysis kraft and kraft pulping mill (PHKK) PPMS in a consolidated bioprocessing fermentation. A separate hydrolysis and fermentation strategy was pursued whereby a glucose-rich hydrolyzate was obtained from enzymolysis of PPMS and subsequently utilized in a cyclic fed-batch fermentation (CFBF) strategy to obtained enhanced yields of cell biomass and PHAs. Response surface methodology (RSM) was first implemented to optimize the conditions for enzymatic saccharification of de-ashed PHKK PPMS. The optimized variables were; pH 4.89; 51°C; hydrolysis time 22.9 h; 30 U/g β-glucosidase and 60 U/g cellulase; and 6.4% of dried de-ashed PPMS fiber resulting in a hydrolyzate comprising of 48.27% glucose. Thereafter, CFBF was pursued where the glucose-rich hydrolyzate was employed as the sole carbon source for cell proliferation and PHA production. The statistically optimized fermentation conditions to obtain high cell density biomass (OD600 of 2.42) were; 8.77 g L-1 yeast extract; 66.63% hydrolyzate (v/v); a fermentation pH of 7.18; and an incubation time of 27.22 h. The CFBF comprised of three cycles and after the third cyclic event, maximum cell biomass (20.99 g L-1) and PHA concentration (14.28 g L-1) were achieved. This cyclic strategy yielded an almost 3-fold increase in biomass concentration and a 4-fold increase in PHA concentration compared with batch fermentation. The properties of the synthesized PHAs were similar to commercial polyhydroxybutyrate (PHB) and polyhydroxybutyrate-co-valerate (PHBV) and also displayed slightly higher thermostability and lower crystallinity compared with commercial PHB and PHBV. This is the first report detailing the proof of concept of using PPMS from South African pulp and paper making mills for cell biomass and PHA production by B. thuringiensis. In addition, this study reports on the practicality and novelty of utilizing PPMS either in its raw, untreated state or as enzymatically saccharified glucose-rich hydrolyzate as cheap substrates applicable for both cell biomass and PHA production using different fermentation strategies. Finally, to the best of our knowledge, this is also the first report that has successfully applied B. thuringiensis in a CFBF strategy coupled with glucose-rich hydrolyzate as the sole carbon source for the production of high cell density biomass and enhanced PHA production. From this study, it is intended that innovative insights and prospective solutions to valorizing pulp and paper mill sludge are provided, whilst simultaneously generating a value-added product.