Genetically modified bacteria that munch on sugar to produce refinable fuels could bring down the cost of switching to cleaner energy.
Although many biodiesels produced from crops and cooking fat can be fed directly into car and truck engines, they are not suitable for existing refineries and pipelines, and so require a separate distribution network. Efforts to produce "drop-in" biofuels that can use the existing fuel infrastructure have so far involved prohibitively expensive chemical conversion steps, says Steve del Cardayre at biofuel developer LS9, based in San Francisco.
Organisms such as cyanobacteria could be the answer. It has been known for years that some naturally produce alkanes – the primary hydrocarbon component of gasoline, diesel and jet fuel and thus a potential drop-in biofuel – through photosynthesis. If you could identify the genes responsible it would be possible to manipulate bacteria that may be more suited to industrial production to produce alkanes, says del Cardayre. "People have looked for these genes for over 20 years," he says.
So a team led by Andreas Schirmer at LS9 compared the genomes of 10 cyanobacteria of various strains known to produce alkanes with one type of bacterium that does not. When the team subtracted the genome of the non-producing strain from those of the producing strains, they were left with a shortlist of 17 genes found only in the alkane producers.
The functions of some of the genes were already understood, leaving two prime suspects for a role in alkane production. So the team then inserted them into a new host, a strain of Escherichia coli – chosen because it breeds readily in laboratory conditions and so is a good candidate for industrial-scale processes. As hoped, they found that the re-engineered bacterium began making enzymes that produced alkanes.
"We have a one-step process to make alkane" in an industrial process, says Schirmer. "Basically, in goes the feedstock – sugar – and out comes the vehicle-ready fuel. It's really simple," he adds.
The bacteria can be grown on any sugar, including those produced from second-generation cellulose-based sources such as grasses and plant waste, which do not compete for land with food crops.
The team have so far produced roughly 10 litres of alkane in a 1000-litre pilot fermentation tank at the company's laboratories, and are now attempting to increase the yield in the hope of scaling up to a larger demonstration plant in a couple of years. Once the technology is fully developed, the company expects the alkane to cost around $50 per barrel, says del Cardayre.
Journal reference: Science, DOI:10.1126/science.1187936
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