Bacteria that can produce ‘high-energy’ biofuels, such as biodiesel, direct from simple sugars, or light up in coordinated waves could provide a turning point in the fortunes of synthetic biology – a field that has so far failed to fulfil its early promise.
Researchers from the University of California, US, engineered E coli to produce biodiesel, or fatty esters, from glucose, without the need for other carbon sources (Nature 2010, 463, 559). They then engineered the same strain to express hemicellulases, enzymes that break down hemicellulose, a step towards bacteria that can produce biodiesel direct from cellulosic biomass.
Global biodiesel consumption is more than 2bn gallons/year. Companies make fatty esters commercially by reacting triglycerides from vegetable – or less commonly animal – oils with alcohols.
But using vegetable oils as a fuel feedstock is economically and environmentally problematic, critics say. For example, as farmers switch to crops for biodiesel, food production could fall, exacerbating food scarcity and driving up prices. In addition, high demand for energy and land scarcity could encourage farmers in ecologically rich, but economically poor, regions to cut down forests to make way for crops.
Biodiesel can be made from cellulosic biomass, waste plant material, such as wood chips, or nonfood crops, such as grasses. But, using current methods, it is more difficult and more expensive.
Scientists hope to bring the cost down by getting bacteria to do the hard work. E coli, for example, will grow on plant sugars. In earlier work, scientists made E coli that produced ethanol, which they used to chemically convert fatty acids into fatty esters (Microbiology 2006, 152, 2529). But fatty acids are not a commercially viable feedstock, which limits the potential of this approach.
Therefore, the Californian researchers redirected the E coli fatty acid metabolism towards fatty esters and other commercially useful fuel and chemical products. Crucially, they encouraged the bacteria to produce more fatty acids, while expressing key enzymes that caused esterification.
Commercially viable bacteria that performed all the steps in the chain inside their cells – bacteria for ‘consolidated bioprocessing’ – would make biofuel production easier and cheaper, the researchers say in the paper. ‘This engineering strategy supports yields of these products within an order of magnitude of that required for commercial production,’ they add.
Synthetic biology
They carried out the work with a grant
from LS9, a US biotech company
focusing on ultraclean fuels and
sustainable chemicals.
‘Significant hurdles remain to demonstrate yield, productivity and cost performance consistent with fuel economics,’ says Johan van Walsem, vp for strategy and commercial development at US biotechnology company Metabolix. ‘Both capital and operating costs are likely to be much higher than that of relatively simple ethanol fermentation and remains a hurdle to commercialisation. The same technology can also provide valueadded chemicals, such as detergent alcohols, providing a different commercial entry point.’
Synthetic biology is a busy, diverse field, in which scientists combine science and engineering to make biological machines that perform functions using the minimum of parts. Advances have been made towards the production of clean, sustainable biofuels, cheap drugs and synthetic organs, but the challenges have proved greater than many initially envisaged.
In other research, scientists at the same institution have engineered E coli that produce fluorescent molecules in synchrony to generate coordinated waves of light (Nature 2010, 463, 326). They exploited a bacterial communication system known as quorum sensing to create a complex network of genes, proteins and signalling molecules that act as a ‘molecular clock’. In natural systems, quorum sensing is used to trigger group behaviour, such as plaque formation, when a critical population is reached, which makes cooperation worthwhile. In previous work, scientists made E coli with visible internal time-keeping, but a universal time was needed for the group activity – and that meant effective communication.
The work could improve our understanding of sleeping and learning, as well as the symptoms of certain disorders, such as Parkinson’s, Huntington’s and Alzheimer’s disease. It could also lead to cell implants that can deliver drugs at specific times, in precise doses, says Martin Fussenegger, a biotechnology professor at ETH Zurich in Switzerland, in a Nature N&V article (Nature 2010, 463, 301). ‘The use of the rhythmic synthesis of molecules… as a pacemaker to coordinate the behaviour of individual oscillators in a growing population of cells is a quantum leap in molecular-clock design,’ he adds.
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