Fengxue Xin

State of the art review of biofuels production from lignocellulose by thermophilic bacteria

Biofuels, including ethanol and butanol, are mainly produced by mesophilic solventogenic yeasts and Clostridium species. However, these microorganisms cannot directly utilize lignocellulosic materials, which are abundant, renewable and non-compete with human demand. More recently, thermophilic bacteria show great potential for biofuels production, which could efficiently degrade lignocellulose through the cost effective consolidated bioprocessing. Especially, it could avoid contamination in the whole process owing to its relatively high fermentation temperature. However, wild types thermophiles generally produce low levels of biofuels, hindering their large scale production. This review comprehensively summarizes the state of the art development of biofuels production by reported thermophilic microorganisms, and also concludes strategies to improve biofuels production including the metabolic pathways construction, co-culturing systems and biofuels tolerance. In addition, strategies to further improve butanol production are proposed.

Metabolic construction strategies for direct methanol utilization in Saccharomyces cerevisiae

The aim of this study was to metabolically construct Saccharomyces cerevisiae for achievement of direct methanol utilization and value added product (mainly pyruvate) production. After successful integration of methanol oxidation pathway originated from Pichia pastoris into the chromosome of S. cerevisiae, the recombinant showed 1.04g/L consumption of methanol and 3.13% increase of cell growth (OD600) when using methanol as the sole carbon source. Moreover, 0.26g/L of pyruvate was detected in the fermentation broth. The supplementation of 1g/L yeast extract could further improve cell growth with increase of 11.70% and methanol consumption to 2.35g/L. This represents the first genetically modified non-methylotrophic eukaryotic microbe for direct methanol utilization and would be of great value concerning the development of biotechnological processes.

Progress of succinic acid production from renewable resources: Metabolic and fermentative strategies

Succinic acid is a four-carbon dicarboxylic acid, which has attracted much interest due to its abroad usage as a precursor of many industrially important chemicals in the food, chemicals, and pharmaceutical industries. Facing the shortage of crude oil supply and demand of sustainable development, biological production of succinic acid from renewable resources has become a topic of worldwide interest. In recent decades, robust producing strain selection, metabolic engineering of model strains, and process optimization for succinic acid production have been developed. This review provides an overview of succinic acid producers and cultivation technology, highlight some of the successful metabolic engineering approaches.

Succinic acid production from fruit and vegetable wastes hydrolyzed by on-site enzyme mixtures through solid state fermentation

In this study, a novel biorefinery concept of succinic acid (SA) production from fruit and vegetable wastes (FVWs) hydrolyzed by crude enzyme mixtures through solid state fermentation was designed. Enzyme complex solid mashes from various types of FVWs were on-site produced through solid-state fermentation by Aspergillus niger and Rhizopus oryzae. This solid was then added to FVW suspensions and undergo hydrolysis reaction to generate fermentable sugars and other essential nutrients for bacterial growth and product formation. The subsequent fungal hydrolysis produced 12.00g/L glucose and 13.83g/L fructose using 10% mass ratio (w/v) of FVW. Actinobacillus succinogenes used this FVW hydrolysate as the sole feedstock and produced 27.03g/L of succinic acid with high yield and productivity of 1.18gSA/g sugar and 1.28gL-1h-1, respectively. This work demonstrated that FVWs can be biotransformed to value added products which have considerable potential economics and environmental meaning.

Guidance for engineering of synthetic methylotrophy based on methanol metabolism in methylotrophy

Methanol is increasingly becoming an attractive substrate for production of different metabolites, such as commodity chemicals, and biofuels via biological conversion, due to the increment of annual production capacity and decrement of prices. In recent years, genetic engineering towards native menthol utilizing organisms – methylotrophy has developed rapidly and attracted widespread attention. Therefore, it is vital to elucidate the distinct pathways that involve methanol oxidation, formaldehyde assimilation and disassimilation in the different methylotrophies for future synthetic work. In addition, this will also help to genetically construct some new and non-native methylotrophies. This review summarizes the current knowledge about the methanol metabolism pathways in methylotrophy, discusses and compares different pathways on methanol utilization, and finally presents the strategies to integrate the methanol metabolism with other chemicals, biofuels or other high value-added product formation pathways.

Recent advances on conversion and co-production of acetone-butanol-ethanol into high value-added bioproducts

Abstract Butanol is an important bulk chemical and has been regarded as an advanced biofuel. Large-scale production of butanol has been applied for more than 100 years, but its production through acetone–butanol–ethanol (ABE) fermentation process by solventogenic Clostridium species is still not economically viable due to the low butanol titer and yield caused by the toxicity of butanol and a by-product, such as acetone. Renewed interest in biobutanol as a biofuel has spurred technological advances to strain modification and fermentation process design. Especially, with the development of interdisciplinary processes, the sole product or even the mixture of ABE produced through ABE fermentation process can be further used as platform chemicals for high value added product production through enzymatic or chemical catalysis. This review aims to comprehensively summarize the most recent advances on the conversion of acetone, butanol and ABE mixture into various products, such as isopropanol, butyl-butyrate and higher-molecular mass alkanes. Additionally, co-production of other value added products with ABE was also discussed.

Enhanced biobutanol production with high yield from crude glycerol by acetone uncoupled Clostridium sp. strain CT7

This study reports a unique acetone uncoupled Clostridium species strain CT7, which shows efficient capability of glycerol utilization with high butanol ratio. Medium compositions, such as substrate concentration, micronutrients and pH show significant effects on butanol production from glycerol by strain CT7. To further maximize butanol production, fermentation conditions were optimized by using response surface methodology (RSM). Final butanol production of 16.6g/L with yield of 0.43g/g consumed glycerol was obtained, representing the highest butanol production and yield from glycerol in the batch fermentation mode. Furthermore, strain CT7 could directly convert crude glycerol to 11.8g/L of butanol without any pretreatment. Hence, strain CT7 shows immense potential for biofuels production using waste glycerol as cheap substrate.

Expression of global regulator IrrE for improved succinate production under high salt stress by Escherichia coli

Poor high salt stress resistance remained as a main hurdle limiting the efficient bio-based succinic acid production. In this study, the metabolically engineered E. coli not only showed improvement of high salt stress tolerance through expression of a global regulator IrrE, but also could use seawater for succinic acid fermentation. The recombinant strain showed an increased 1.20-fold of cell growth rate and 1.24-fold of succinic acid production. Expression levels of genes related glucose uptake and succinic acid synthesis were up-regulated, and more glycerol and trehalose were accumulated. Moreover, no significant differences were observed in cell growth even when tap water was replaced by 60% artificial seawater. In the fermentation using Yellow Sea seawater, 24.5 g/L succinic acid was achieved with a yield of 0.88 g/g. This strategy set up a platform for improving abiotic stress tolerances and provide a possible approach for fermentation processes with low cost.

Characterization of a β-glucosidase from Paenibacillus species and its application for succinic acid production from sugarcane bagasse hydrolysate

In this study, a β-glucosidase from Paenibacillus sp. M1 was expressed in E. coli BL21(DE3), purified and characterized. The specific activity of purified BglA was 137.64U·mg-1 protein with optimal temperature and pH of 50°C and 6.0. Furthermore, BglA shows excellent adaption to various environmental factors such as temperature, pH and metal ions. Engineered E. coli Suc260 was further reconstructed by overexpressing the β-glucosidase for achieving direct cellobiose utilization, which could efficiently utilize the pretreated sugarcane bagasses hydrolysate (SBH) consisting of 25.30g·L-1 cellobiose, 9.70g·L-1 glucose, 5.90g·L-1 arabinose and 7.10g·L-1 xylose. As a result, 26.50g·L-1 and 24.30g·L-1 succinic acid were produced by strain Suc260(pTbglA) from cellobiose and SBH with corresponding yields of 88.30% and 89.20% using dual-phase fermentation, respectively. This study indicated that incomplete enzymatic hydrolysate of SCB will be a potential feedstock for succinic acid production.

Current advance in biological production of malic acid using wild type and metabolic engineered strains

Malic acid (2-hydroxybutanedioic acid) is a four-carbon dicarboxylic acid, which has attracted great interest due to its wide usage as a precursor of many industrially important chemicals in the food, chemicals, and pharmaceutical industries. Several mature routes for malic acid production have been developed, such as chemical synthesis, enzymatic conversion and biological fermentation. With depletion of fossil fuels and concerns regarding environmental issues, biological production of malic acid has attracted more attention, which mainly consists of three pathways, namely non-oxidative pathway, oxidative pathway and glyoxylate cycle. In recent decades, metabolic engineering of model strains, and process optimization for malic acid production have been rapidly developed. Hence, this review comprehensively introduces an overview of malic acid producers and highlight some of the successful metabolic engineering approaches.