Engineered Organisms for Making Cheap Sugar

In a bid to make biofuels cheaper, a startup called Proterro, based in Princeton, New Jersey,  is developing a way to cut the cost of making sugar,
a basic building block for ethanol. The company is engineering photosynthetic microorganisms to secrete large amounts of sugar, and it is designing a  bioreactor for growing the organisms using small amounts of water.
Photosynthetic microorganisms, such as algae, are usually prized for their ability to produce oils. Proterro chose to focus on sugar production because that's the source for biofuel ethanol, and it's also the starting point for new processes for making other types of biofuels.
Today, almost all of the sugar for biofuels is made from corn or sugarcane, and several companies are developing processes for making sugar from abundant cellulosic materials such as grass and wood chips.  But as a feedstock to make biofuels, "sugar is still too expensive," says Kef Kasdin, Proterro's CEO. Only sugar from sugarcane is cheap enough to make economic sense, and that can only be grown inexpensively in some locations, such as Brazil.
Proterro's microorganisms, a type of cyanobacteria, can produce far higher yields of sugar per acre than sugarcane and other conventional sources, Kasdin says. Sugarcane plants use water and energy from the sun to produce a lot of biomass that isn't sugar, and then that bulky biomass has to be transported, and the sugar extracted, which contributes to its cost. In Proterro's system, more of the water and energy in sunlight is directed into making sugar instead of supporting biomass, and the organisms don't need to be harvested—instead, they continuously secrete sugar in a form that's easy to use to make biofuels.
Proterro's microbes naturally produce sucrose when the water that they're growing in becomes too salty—it's a defense mechanism to keep water from being sucked out of them into the surrounding water via osmosis. The company has identified the genes that trigger this mechanism, and engineered the organisms to switch it on. The researchers have also engineered the organisms to secrete the sugar, which makes it easier to collect. In conventional approaches to making fuels using algae or cyanobacteria, the organisms have to be harvested and dewatered—the oil or sugar is then isolated from the rest of the biomass, which is one reason algae fuels are expensive.
Growing cyanobacteria and other photosynthetic microorganisms such as algae also usually requires a large amount of water—they're grown in ponds or in large clear containers filled with water. Proterro is developing a new bioreactor that does not immerse the cyanobacteria in water. Instead, they're grown on fabric that's soaked with water using a drip-feed system. "The organisms don't need to be submerged, just fed with a trickle of water," Kasdin says.
The approach is similar to one being developed by Joule Unlimited, except Joule has engineered its organisms to produce fuel directly. Kasdin says Proterro chose a process that the organisms do naturally, which it hopes will make the organisms easier to develop and maintain. She also says that Proterro's low water bioreactors could also cut costs compared to Joule's, which immerse the organisms in water.
Proterro, which was founded in 2008 and has raised $5 million in venture funding, is still at an early stage. It's demonstrated engineered versions of the sugar secreting cyanobacteria, and made prototype bioreactors.  But it's still not clear  exactly how high the yields will be, or how much less water will be used than conventional ways of making sugar—or even it if can make sugar cheaper than it can be made from sugarcane.
The fact that the organisms don't need much water and can secrete the sugar makes the technology attractive, says Jianping Yu, a scientist at the National Renewable Energy Laboratory in Golden, Colorado. What's more, he says, it's already been demonstrated that algae and cyanobacteria can produce high yields per acre. But he says the company will face many challenges. The biggest comes from the fact that by secreting sugar, the organisms create an ideal environment for many other organisms. These other organisms could choke out the cyanobacteria, making it necessary to clean the bioreactors frequently and grow new batches of cyanobacteria. That would make the system too expensive.
"Biofuels companies that stick closer to established industrial processes have a better chance of success," says Mark Bünger, a research director at Lux Research. "The biofuels arena has a lot of challenges, and Proterro is trying to tackle them all at once. It's a big, bold bet."
sources : here and here