Proterro: Sugar From Sunlight and CO2

Ethanol is pretty easy to make from pure sugar. It’s getting the sugar — either from corn and sugarcane, or hard-to-digest cellulosic materials — that’s the challenging part. But startup Proterro says it has engineered a microbe that can turn sunlight and carbon dioxide into sugar, and has combined that tech with a bioreactor based on the world’s oldest design — the leaf.

It’s an elegant idea that sounds a lot like the genetic engineering work being done on algae by Solazyme, yeast by Amyris or bacterium by LS9. But in fact, those companies have engineered microbes that turn sugars into different fuels, whereas Proterro’s engineered cyanobacteria are meant to continuously churn out sugar itself, CEO Kef Kasdin to us explained this week.

Proterro has raised $5 million from Braemar Ventures and Battelle Ventures. The latter is the investment arm of the company that runs many of the Department of Energy’s national laboratories. Kasdin herself is chief greentech investor for Battelle Ventures, and had struggled to find an answer for the challenge of the high cost of feedstock in biofuel production.

The answer came in the form of bacteria that generate sugar not to feed themselves, but to protect themselves from dehydration, Kasdin explained. That allows the bacteria to live on, day after day, creating sugar, unlike many algae strains that must be crushed to yield biofuel precursors, she noted.

The challenge, she said is to organize Proterro’s bacteria in a physical system that allows them access to both sunlight and atmospheric CO2. Many algae-to-biofuel plants suffer from using water as their medium, which tends to block light at depth and doesn’t absorb CO2, she noted. Using a fabric-thin, solid substrate could avoid those problems, however, she said.

“Think of a leaf as a very thin-film photobioreactor, where the photosynthetic cells are at the surface, protected by an outer layer,” she said. That gives direct access to sunlight and CO2 without all that interfering water. Just what an industrial-scale photobioreactor made up of lots of “leaves” of microbe-filled material might look like, Kasdin wasn’t able to say just yet — the Proterro team is still working on different physical models. But Proterro is looking at a “fabric-like” solid film that can be vertically packed densely enough to produce enough sugar per acre to compete with traditional sugar crops such as sugarcane, she said.

Those bioreactors, in turn, can stand alone or be incorporated into biofuel plants. While scaling up a microbial-scale production model to industrial-scale systems will be a challenge, Proterro believes it will be able to produce sugar at significantly lower cost than Brazilian sugarcane producers. Of course, for a company with no employees or production facilities, it still has a long way to go, and Kasdin declined to say when the company might be rolling out its first bioreactor test model.

Of all the would-be biofuel breakthroughs out there, Proterro’s approach seems most closely matched by Joule Unlimited, a startup that promises its genetically modified organisms can turn sunlight and carbon dioxide directly into fuel. That claim has been met with its share of skepticism. Proterro, on the other hand, promises only sugar from its microbes, a proposition that may sit well with biofuel makers hungry for a cheap and reliable feedstock, if it can scale to match their appetites.

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Image courtesy of Proterro.