Microbial recycling of glycerol to biodiesel

Recent advances in biofuel production through metabolic engineering

Rising global energy demands and climate crisis has created an unprecedented need for the bio-based circular economy to ensure sustainable development with the minimized carbon footprint. Along with conventional biofuels such as ethanol, microbes can be used to produce advanced biofuels which are equivalent to traditional fuels in their energy efficiencies and are compatible with already established infrastructure and hence can be directly blended in higher proportions without overhauling of the pre-existing setup. Metabolic engineering is at the frontiers to develop microbial chassis for biofuel bio-foundries to meet the industrial needs for clean energy. This review does a thorough inquiry of recent developments in metabolic engineering for increasing titers, rates, and yields (TRY) of biofuel production by engineered microorganisms.

Engineering of glycerol utilization in Gluconobacter oxydans 621H for biocatalyst preparation in a low-cost way

Background

Whole cells of Gluconobacter oxydans are widely used in various biocatalytic processes. Sorbitol at high concentrations is commonly used in complex media to prepare biocatalysts. Exploiting an alternative process for preparation of biocatalysts with low cost substrates is of importance for industrial applications.

Results

G. oxydans 621H was confirmed to have the ability to grow in mineral salts medium with glycerol, an inevitable waste generated from industry of biofuels, as the sole carbon source. Based on the glycerol utilization mechanism elucidated in this study, the major polyol dehydrogenase (GOX0854) and the membrane-bound alcohol dehydrogenase (GOX1068) can competitively utilize glycerol but play no obvious roles in the biocatalyst preparation. Thus, the genes related to these two enzymes were deleted. Whole cells of G. oxydans ∆GOX1068∆GOX0854 can be prepared from glycerol with a 2.4-fold higher biomass yield than that of G. oxydans 621H. Using whole cells of G. oxydans ∆GOX1068∆GOX0854 as the biocatalyst, 61.6 g L⁻¹ xylonate was produced from 58.4 g L⁻¹ xylose at a yield of 1.05 g g⁻¹.

Conclusion

This process is an example of efficient preparation of whole cells of G. oxydans with reduced cost. Besides xylonate production from xylose, other biocatalytic processes might also be developed using whole cells of metabolic engineered G. oxydans prepared from glycerol.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-1001-0) contains supplementary material, which is available to authorized users.

Synthesis of the Fatty Esters of Solketal and Glycerol-Formal: Biobased Specialty Chemicals

The caprylic, lauric, palmitic and stearic esters of solketal and glycerol formal were synthesized with high selectivity and in good yields by a solvent-free acid catalyzed procedure. No acetal hydrolysis was observed, notwithstanding the acidic reaction conditions.

A novel biocatalyst for efficient production of 2-oxo-carboxylates using glycerol as the cost-effective carbon source

BACKGROUND

The surplus of glycerol has increased remarkably as a main byproduct during the biofuel's production. Exploiting an alternative route for glycerol utilization is significantly important for sustainability of biofuels.

RESULTS

A novel biocatalyst that could be prepared from glycerol for producing 2-oxo-carboxylates was developed. First, Pseudomonas putida KT2440 was reconstructed by deleting lldR to develop a mutant expressing the NAD-independent lactate dehydrogenases (iLDHs) constitutively. Then, the Vitreoscilla hemoglobin (VHb) was heterologously expressed to further improve the biotransformation activity. The reconstructed strain, P. putida KT2440 (ΔlldR)/pBSPPcGm-vgb, exhibited high activities of iLDHs when cultured with glycerol as the carbon source. This cost-effective biocatalyst could efficiently produce pyruvate and 2-oxobutyrate from dl-lactate and dl-2-hydroxybutyrate with high molar conversion rates of 91.9 and 99.8 %, respectively.

CONCLUSIONS

The process would not only be a promising alternative for the production of 2-oxo-carboxylates, but also be an example for preparation of efficient biocatalysts for the value-added utilization of glycerol.