Modified microbes can absorb CO2 and can secrete the primary component of oil

19/5/2011 New Scientist Microbes with tweaked DNA can convert sunlight and carbon dioxide into oil-I’M STARING at a tank filled with bubbling liquid the colour of steamed spinach. Swimming inside are photosynthetic cyanobacteria. Although their kind are extremely common, these bacteria differ from their wild counterparts. Their DNA has been tweaked so that, given light, water and carbon dioxide, they secrete alkanes - the primary components of diesel.

Biotechnology company Joule Unlimited of Cambridge, Massachusetts, which grows the bacteria, envisions a future in which swathes of desert are packed with photobioreactors. These huge tanks, churning with the same spinach-green soup now in front of me, will pump out alkanes as the sun shines. There’s another advantage to this solar-powered oil: the CO2 the bacteria inhale could come from industry. Firms would no longer have to invest in technology to bury the greenhouse gas safely.

Understandably, such an optimistic vision has attracted plenty of press interest. But until now Joule has been tight-lipped about what the media has dubbed a “magic bug”. My conversations at Joule, and a look at some of the firm’s recently acquired patents, offer tantalising hints that the underlying science is sound.

Biofuels have been around for a while. Joule’s system is an unusual member of the “third generation” of the technology. The first-generation fuels stalled largely because they had to compete with the food industry. Their feedstock, a mixture of sugars, starches and oils, came from sugarcane and corn. The second generation produced fuels from inedible cellulose and non-food crops, which are difficult to break down cost-effectively into the simple molecules found in fuels.

The latest biofuels are derived from microbes that can live on land unfit for crops and generate nearly engine-ready chemicals. Competition between third-generation biofuel developers is fierce (see “Invest and hope for the best”), but Joule has made impressive strides towards commercial success.

Most of its competitors use algae that squirrel away oil within their cells, but Joule’s magic bugs are cyanobacteria that secrete the alkanes they generate, which float to the surface of the reactor for convenient collection. Even better, the alkanes have chains of carbon 13 to 19 molecules long, an ideal length for diesel, says Dan Robertson, Joule’s senior vice-president of biological sciences.

Previous scientific studies provide evidence that some microbes, including a number of cyanobacteria, can synthesise alkanes. The genetic pathways involved have been unclear, but Joule’s patents and a 2010 paper published by biotech rival LS9, based in San Francisco (Science, DOI: 10.1126/science.1187936), suggest that both firms have pinpointed the genes and enzymes involved in species such as Thermosynechococcus elongatus, which inhabits hot springs. Robertson says that Joule has enhanced the expression of these genes and encouraged the microbes to secrete their alkanes. It helps that the DNA in prokaryotic cyanobacteria is easier to manipulate because it is not protected by a nucleus.

Microbiologists have become so adept at this kind of manipulation that they can coax bacteria to divert “the vast majority of their energy” into producing useful chemicals, says Cameron Coates, who is trying to produce fuels from microbes at the Scripps Institution of Oceanography in San Diego, California. Robertson says that Joule’s microbes can convert 90 per cent or more of the carbon they fix during photosynthesis into alkanes or alcohols that can be used in fuels.

“Anything over 80 per cent to me is a surprise,” says Himadra Pakrasi, who researches cyanobacteria at Washington University in St Louis, Missouri. Coates agrees: “I haven’t seen any peer-reviewed publication that backs up that figure. We won’t know if they are accomplishing those yields until they show it at a large scale.”

Coates may not have long to wait. Joule already has a pilot plant covering 0.8 hectares in Leander, Texas. On 5 May, the firm announced that it had secured 486 hectares in Lea County, New Mexico, for a plant to produce ethanol and diesel. The project may be scaled up to 2000 hectares.

With its engineered microbes, Joule claims to be able to produce ethanol at a rate of 93,000 litres per hectare per year, suggesting its New Mexico site will generate 45 million litres per year, rising to nearly 200 million litres if the site is expanded to 2000 hectares.

But what works in theory might not work in practice: so far no company has been able to mass-produce fuels using engineered microorganisms.

“From a scientific view a lot of what they say is possible, but it needs to be tested on a big scale,” says Louis Sherman, who studies cyanobacteria at Purdue University in West Lafayette, Indiana. “The theory sounds nice, but I want to see what happens after they have been in operation for a year or two. There are problems you can’t anticipate.”
Invest and hope for the best

The third generation of biofuels is attracting millions of dollars of investment. In 2009 oil giant ExxonMobil committed $600 million to develop algal biofuels with Craig Venter, the genome-sequencing pioneer. Such major investments are rare, but Venter can still expect some stiff competition.

Using cyanobacteria that secrete rather than store their oil, as does Joule of Cambridge, Massachusetts, saves the money that would be wasted cracking open oily cells. But it is not the only option.

San Francisco-based biotech firm LS9 is also trying to harness cyanobacteria’s knack of turning sunlight and CO2 into oil. But instead of enhancing the expression of these genes in cyanobacteria themselves - as Joule is doing - LS9 decided to transfer the genes to the laboratory workhorse Escherichia coli. Unlike cyanobacteria, E. coli must be fed to grow. But Stephen del Cardayre, vice-president of research and development at LS9, says the firm’s approach has advantages. “One reason we chose E. coli is that it is one of the fastest growing organisms known,” he says.

More speculatively, bacteria living in oxygen-poor soils or ocean sediments could provide an alternative route to third-generation fuels, according to Derek Lovley at the University of Massachusetts, Amherst. Lovley and colleagues have been generating electricity using Geobacter. To produce energy through cellular respiration, Geobacter transfers electrons to metals like iron in its oxygen-poor environments. Poke in a pair of electrodes and you can harness those electrons to generate a current, says Lovley.

By lowering the potential of the electrodes, Lovley discovered he could reverse the flow of electrons and force the bacteria to offload them onto carbon inside their cells to form acetate. With engineering, the bacteria could be made to turn that acetate into biofuel - an idea so new that Lovley is still working on proving the concept.

Promising as these technological advances seem, commercial success is not guaranteed. In 2006, high-profile start-up GreenFuels Technologies built an ambitious pilot plant at the Redhawk power plant in Arizona, designed to turn the plant’s waste CO2 into algal biofuel New Scientist, 6 October 2006, p 28. Although GreenFuels Technologies raised more than $70 million in investments, it went bust in May 2009 because of the cost of maintaining its growth chambers and the difficulty of handling unpredictable algal growth.
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