New Microbial Fuels
In an effort to combat climate change, aid energy independence and counteract diminishing supplies of fossil fuels, there has been a resurgence of research on renewable and carbon-neutral energy sources. Biofuel production captures the energy of the sun as chemical energy in the bonds of biologically produced materials. All routes to biofuels hence start with photosynthesis, and it is at that point where they diverge. There are basically three routes to convert renewable resources into energy-rich, fuel-like molecules or fuel precursors:
- Direct production by photosynthetic organisms such as plants and algae
- Fermentative or nonfermentative production by heterotrophic microorganisms such as bacteria, yeast or fungi
- Chemical conversion of biomass to fuels
On the first route, production of fuels directly from CO2 using photosynthetic metabolism appears desirable as it is carbon-neutral in the strictest sense and does not rely on expensive feedstocks. However, this approach faces several major hurdles, most importantly scalability and land use issues. This revies focusses on the fermentative and nonfermentative metabolism of heterotrophic microorganisms and the different fuels they can supply. The emphasis is on the pathways leading to the new generation of microbial fuels, in particular on the key biocatalysts that convert metabolic intermediates into fuel-like molecules, and on the parameters that govern their cost-effective production. Because much of this research has been carried out in biotechnology companies and is unpublished, the citation of some recent patent literature is included.
The exploitation of diverse metabolic pathways leading to energy-rich, fuel-like hydrocarbons opens up a path to develop renewable fuels that go far beyond the restrictions of bioethanol and plant-derived biodiesel. As novel biocatalysts leading to a greater variety of hydrocarbon products are being discovered, microbiologists will have an even more expansive tool box at their disposal to design better fuel to fit the need of different engines. The metabolic efficiency of a particular pathway has a profound impact on the economics of fuel production in a microbial host. Furthermore, microbial fuels that are easy to recover and do not require additional chemical conversion have the best chances to be developed in cost-effective and unsubsidized commercial processes.
New microbial fuels: a biotech perspective. Curr Opin Microbiol. 2009 12(3): 274-81
Bioethanol and plant oil-derived biodiesel are generally considered first generation biofuels. Recognizing their apparent disadvantages, scientists and engineers are developing more sustainable and economically feasible second generation biofuels. The new microbial fuels summarized here have great potential to become viable replacements or at least supplements of petroleum-derived liquid transportation fuels. Yields and efficiencies of the four metabolic pathways leading to these microbial fuels - mostly designed and optimized in Escherichia coli and Saccharomyces cerevisiae using modern tools of metabolic engineering and synthetic biology - and the robustness of the biocatalysts that convert the metabolic intermediates to, in some cases, finished and engine-ready fuels, will determine if they can be commercially successful and contribute to alleviating our dependence on fossil fuels.
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Tags: Bacteria, Biology, Biotechnology, Environment, fuel, Microbiology, Science
