Fuelling the future: microbial engineering for the production of sustainable biofuels

Key Points

  • Various renewable resources, including plant biomass, atmospheric CO2 and methane from landfill gas, can be used through several pathways for sustainable biofuel production.

  • These metabolic pathways for biofuel production include ethanol pathways, keto acid pathways, isoprenoid pathways, CoA-dependent reverse β-oxidation and fatty acid biosynthesis.

  • Recent research in this area has included metabolic engineering of microorganisms to improve their production of biofuels.

  • There are several possible strategies for the optimization of biofuel production, including the redesign of central metabolism for carbon conservation, the balancing of redox cofactors and prolonging the production phase.

Abstract

Global climate change linked to the accumulation of greenhouse gases has caused concerns regarding the use of fossil fuels as the major energy source. To mitigate climate change while keeping energy supply sustainable, one solution is to rely on the ability of microorganisms to use renewable resources for biofuel synthesis. In this Review, we discuss how microorganisms can be explored for the production of next-generation biofuels, based on the ability of bacteria and fungi to use lignocellulose; through direct CO2 conversion by microalgae; using lithoautotrophs driven by solar electricity; or through the capacity of microorganisms to use methane generated from landfill. Furthermore, we discuss how to direct these substrates to the biosynthetic pathways of various fuel compounds and how to optimize biofuel production by engineering fuel pathways and central metabolism.

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Figure 1: Overview of biofuel production from sunlight and atmospheric carbon.
Figure 2: Three modes of cellulose degradation in different organisms.
Figure 3: Biosynthetic pathways of biofuels.

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Acknowledgements

The authors appreciate support from the US Department of Energy (DOE) BioEnergy Science Center, the University of California, Los Angeles (UCLA)–DOE Institute of Genomics and Proteomics, the DOE (grant DE-SC0012384), the US National Science Foundation (grant MCB-1139318) and the DOE Advanced Research Project Agency-energy REMOTE programme.

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The authors are affiliated with the University of California Los Angeles (UCLA). UCLA has filed patents on some of the technologies discussed here.

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Glossary

Thermohaline circulation

Large-scale ocean currents that result from water density gradients, which are under the control of temperature (thermo) and salinity (haline).

Lignocellulose

The major component of the plant cell wall, which consists of cellulose, hemicellulose and lignin.

Cellulolytic organisms

Organisms that can digest cellulose.

Lithoautotrophic

Capable of using CO2 as the carbon source and inorganic compounds as reducing equivalents.

Methylotrophs

Capable of utilizing reduced C1 compounds.

Alkanes

Organic molecules with the general formula CnH2n+2.

Cellulose

A long-chain linear polysaccharide of β(1–4)-linked D-glucose; this polysaccharide is a major energy and carbon storage material in plants.

Hemicellulose

A group of heteropolymers in plant cell walls, commonly consisting of mainly pentoses (such as xylose, arabinose and mannose) and some hexoses (such as glucose).

Lignin

A group of heterogeneous crosslinked polymers that are rich in phenolic moieties and provide structural support in plant cell walls.

Synthetic gas

(Syngas). A mixture of CO and H2.

Fischer–Tropsch reaction

A process that converts a mixture of H2 and CO to liquid fuels using cobalt, iron, or ruthenium as catalysts at 150–350 °C.

Wood–Ljungdahl pathway

A CO2-fixing pathway that is found in certain bacterial and archaeal species and that synthesizes acetyl-CoA; also known as the reductive acetyl-CoA pathway.

Cellulosomes

Cell surface-attached multi-enzyme complexes that are produced by several cellulolytic bacteria for lignocellulose degradation.

Microfibrils

Fine fibre-like strands formed of multiple cellulose polymers that are aligned in parallel via hydrogen bonding, creating structural crystallinity and mechanical strength.

Water splitting

A chemical reaction that separates water molecules into O2 and H2.

3-hydroxypropionate/4-hydroxybutyrate CO2 fixation pathway

A cyclic pathway that was discovered in the archaeon Metallosphaera sedula and uses 3-hydroxypropionate and 4-hydroxybutyrate as key intermediates to convert CO2 into acetyl-CoA.

Acetogenic

Able to produce acetate as a major metabolite in anaerobic respiration.

Acetyl-CoA

An acetyl group linked to CoA by a thioester bond; this molecule acts as the acetyl group donor in cells.

Keto acid

An organic acid that contains a ketone group.

Malate shunt

A pathway that converts phosphoenolpyruvate to pyruvate through oxaloacetate and malate; in this pathway, the reducing equivalent NADH is converted to NADPH.

Electron-bifurcating enzymes

Multi-enzyme complexes that were recently discovered in anaerobic microorganisms and are capable of coupling a thermodynamically unfavourable redox reaction with a thermodynamically favourable one.

Acyl carrier protein

A protein molecule that transfers the acyl group during fatty acid biosynthesis.

Sesquiterpenes

A class of C15 isoprenoid compounds described by the general chemical formula C15H24.

Embden–Meyerhof–Parnas pathway

(EMP pathway). The most common native glycolytic pathway for converting glucose to pyruvate and thereby generating reducing equivalents and ATP; the resultant pyruvate is then converted to acetyl-CoA.

Non-oxidative glycolysis pathway

(NOG pathway). A pathway that can be constructed, through metabolic engineering, to enable the digestion of glucose or other sugars in a redox-neutral manner into acetyl-phosphate or acetyl-CoA without losing carbon to CO2. This is in contrast to native glycolysis, which is oxidative and loses carbon to CO2 when producing acetyl-CoA.

Reverse glyoxylate shunt

(rGS). A synthetic pathway that is designed to convert malate to acetyl-coA without carbon loss, using glyoxylate as a key pathway intermediate.

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Liao, J., Mi, L., Pontrelli, S. et al. Fuelling the future: microbial engineering for the production of sustainable biofuels. Nat Rev Microbiol 14, 288–304 (2016). https://doi.org/10.1038/nrmicro.2016.32

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