Metabolic engineering of microorganisms for biofuel production

Microorganisms directly and indirectly contribute to production of diverse biofuels. Heterotrophic microorganisms are being used for commercial production of biofuels such as biogas and fuel alcohols from organic matter. Photosynthetic microorganisms convert inorganic carbon and water to potential fuels (e.g. fuel alcohols, biohydrogen) and fuel precursors (e.g. biomass, starch, lipids). Only a few microbial processes are used for commercial production of biofuels, but this will certainly change with the enhanced production capabilities being achieved through microbial metabolic engineering. Processes that previously required multiple steps of feedstock pretreatment and subsequent conversion to fuel are being consolidated into single-step microbial processes using metabolically engineered species. Microorganisms with the ability to produce fuels from feedstock they could not use previously, are being engineered. This review discusses some of the metabolic engineering approaches being used to enhance the commercialization potential of microbial biofuels including fuel alcohols, biodiesel and biohydrogen. At present, all biogas production relies on native populations of methanogens and this does not seem likely to change in the near term. Potential fuels from microalgae, cyanobacteria and other photosynthetic bacteria, whether native or engineered, have distant prospects of commercial use. Metabolically engineered yeasts surface displaying various hydrolytic enzymes appear to hold the greatest potential for near term commercial use in generating bioethanol from starch, pretreated lignocellulose and other polysaccharides. The bacterium Zymomonas mobilis metabolically engineered to make bioethanol from pentose sugars is already being commercialized. Other similar examples are likely to emerge as more engineered microorganisms become available.