Engineering cyanobacteria for direct biofuel production from CO2

https://doi.org/10.1016/j.copbio.2014.09.007Get rights and content

Highlights

  • We review the recent progress in the production of solar biofuels.

  • Articles demonstrating proof of concept and optimisation studies are reviewed.

  • At this time, high-value-added compounds show greater promise than fuels.

For a sustainable future of our society it is essential to close the global carbon cycle. Oxidised forms of carbon, in particular CO2, can be used to synthesise energy-rich organic molecules. Engineered cyanobacteria have attracted attention as catalysts for the direct conversion of CO2 into reduced fuel compounds.

Proof of principle for this approach has been provided for a vast range of commodity chemicals, mostly energy carriers, such as short chain and medium chain alcohols. More recently, research has focused on the photosynthetic production of compounds with higher added value, most notably terpenoids. Below we review the recent developments that have improved the state-of-the-art of this approach and speculate on future developments.

Section snippets

Introduction: ‘light-driven conversion’

Cyanobacteria are photosynthetic prokaryotes that can use photon energy to ultimately transfer electrons from water to carbon dioxide, generating more reduced molecules in the process. The introduction of heterologous, mostly catabolic, pathways into the metabolism of cyanobacteria allows production of a wide range of fuel and commodity products from CO2, light and water [1, 2].

In the recent past this approach has matured so that by now for a large range of compounds proof of principle has been

Biofuels and other products of engineered cyanobacteria

With fuel applications in mind, there are several interesting classes of molecules available. Figure 2 gives an overview of heterologous pathways that have been introduced into cyanobacteria and the corresponding product titres that were achieved. Hydrogen, although versatile in its applications, will not be discussed in this review.

Short chain alcohols can be used as drop-in automotive fuels. The highest titres for a photosynthetically produced biofuel have been reported by Gao et al. for

General challenges for the design of production systems

A compound of interest can be produced through the introduction of a reaction that converts an endogenous metabolite. If the compound of interest is an endogenous metabolite, then its intracellular concentrations can be increased by removal of a consuming reaction. Many compounds are excreted from and/or leak out of the cytoplasm into the culture medium. For cyanobacteria, currently little is known about the underlying processes. It was shown that exporter proteins can lead to extracellular

The ideal cyanobacterial host

The ideal production host is hard to specify. Rather, different desired properties can be formulated, depending on the nature of the process.

Generally, genetic manipulation should be straightforward and engineered strains should show a high degree of stability. Growth should be fast and robust, as well as photosynthetic efficiency and carbon fixation rate could be high. To use off-gas, growth at elevated CO2 concentrations should be possible. Growth in seawater would reduce competition with the

Scientific and economic prospects

Commercial fuels usually are mixtures of compounds. Similarly, production of biofuel mixtures by photosynthetic microbes could be advantageous; even more so as many enzymes show inherent promiscuity which may lead to product diversification. Mixtures with different chain lengths were reported for alkanes [22], fatty acids [17, 18, 19, 20••, 58, 76] and fatty alcohols [23] producing strains. Furthermore, Synechococcus 7942 engineered to produce 2-methyl-1-butanol accumulated 1-propanol and

References and recommended reading

Papers of particular interest, published within the period of review, have been highlighted as:

  • • of special interest

  • •• of outstanding interest

Acknowledgements

The authors thank Filipe Branco dos Santos and Pascal van Alphen for helpful discussions and S. Andreas Angermayr for critical review of the manuscript. This review was written within the research programme of BioSolar Cells, co-financed by the Dutch Ministry of Economic Affairs.

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