Polyhydroxyalkanoate production from rice straw hydrolysate obtained by alkaline pretreatment and enzymatic hydrolysis using Bacillus strains isolated from decomposing straw

Doan Van Thuoc,Nguyen Thi Chung,Rajni Hatti-Kaul

doi:10.1186/s40643-021-00454-7

Doan Van Thuoc, Nguyen Thi Chung + Show 1 more

Open Access

https://doi.org/10.1186/s40643-021-00454-7

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Abstract

Rice straw is an important low-cost feedstock for bio-based economy. This report presents a study in which rice straw was used both as a source for isolation of bacteria producing the biodegradable polyester polyhydroxyalkanoate (PHA), as well as the carbon source for the production of the polymer by the isolated bacteria. Of the 100 bacterial isolates, seven were found to be positive for PHA production by Nile blue staining and were identified as Bacillus species by 16S rRNA gene sequence analysis. Three isolates showed 100% sequence identity to B. cereus, one to B. paranthracis, two with 99 and 100% identity to B. anthracis, while one was closely similar to B. thuringiensis. For use in PHA production, rice straw was subjected to mild alkaline pretreatment followed by enzymatic hydrolysis. Comparison of pretreatment by 2% sodium hydroxide, 2% calcium hydroxide and 20% aqueous ammonia, respectively, at different temperatures showed maximum weight loss with NaOH at 80 °C for 5 h, but ammonia for 15 h at 80 °C led to highest lignin removal of 63%. The ammonia-pretreated rice straw also led to highest release of total reducing sugar up to 92% on hydrolysis by a cocktail of cellulases and hemicellulases at 50 °C. Cultivation of the Bacillus isolates on the pretreated rice straw revealed highest PHA content of 59.3 and 46.4%, and PHA concentration of 2.96 and 2.51 g/L by Bacillus cereus VK92 and VK98, respectively.

Highlights

Rice straw is an obvious choice to be used as residual lowcost feedstock for the bio-based economy, especially in South- and South-East Asian countries that are the main producers of rice, the third most important grain crop in the world after wheat and corn (Binod et al 2010)
Isolation and identification of PHA‐producing bacteria from decomposing rice straw More than 100 bacterial colonies were isolated from decomposing rice straw, among which seven isolates were found to show significant PHA accumulation by Nile blue staining, and were used in this study
The sequence of strain VK24 showed a high level of similarity (98.2%) with that of B. thuringiensis LDC 507

Summary

Introduction

Rice straw is an obvious choice to be used as residual lowcost feedstock for the bio-based economy, especially in South- and South-East Asian countries that are the main producers of rice, the third most important grain crop in the world after wheat and corn (Binod et al 2010). Many studies claim that 25–35% of the straw may be available for biofuels and other products after leaving that is needed to conserve soil quality and for competitive uses (Bakker et al 2013). There have been many studies on the use of rice straw for the production of second-generation biofuel, and more recently even to other products like insulation materials, composites, biodegradable plastics and chemicals (Abraham et al 2016; Bilo et al 2018; Goodman 2020; Overturf et al 2020). As a major part of the production cost of PHA is attributed to the carbon source, use of rice straw could provide a potential low-cost alternative besides lowering the environmental impact caused by its burning (Obruca et al 2015; Heng et al 2016)

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