Where other companies are growing sugars via crop improvements, or extracting them, Proterro is making them via synthetic biology. Specifically, they have engineered a cyanobacteria to make sucrose (via a series of steps you can read all about here), ultimately from CO2, water, sunlight and nutrients – and secrete it into a sugarwater stream that can be purified into affordable sucrose.
It’s been well-known in the literature for some time that certain cyanobacteria can accumulate disaccharides, via photosynthesis.
So, why aren’t there a raft of companies chasing this type of solution? First, there’s a ton of science in engineering up the yield to attractive rates. More importantly, there’s been a tremendous skepticism that such a system would not simply become a restaurant for a host of other invading microorganisms that would dine out on the sugar. Sort of like the plight of 9th century Englishmen, who every time they accumulated some wealth would lose it to Viking raiders.
Contamination, contamination, contamination. It’s been on everyone’s mind, lately – especially after problems arose in scale-up at Amyris and Gevo. In recent months at some high-profile ventures, no one has been more popular than the autoclave salesman, offering the latest in sterilization equipment and strategies. Excepting, perhaps, the antibiotics salesman.
Proterro’s work-around for contamination
Let’s make one thing clear – this is a solid-phase approach. Most biofuels ventures — and all first-gen ones, worked in the liquid phase. Big fermentation vats, lots of liquids, hot temperatures, enzymes, yeasts, distillation, and so on.
In recent years, we’ve seen an explosion in gas-phase innovation. From gas fermentation technologies like Coskata, ZeaChem, LanzaTech and INEOS Bio – to thermochemical players like KiOR, Enerkem, Anellotech, Rentech, Velocys with everything from improved F-T technologies to catalytic fast pyrolysis.
But very few are working in solid phase – where essentially instead of working with a liquid vat, microbes are growing on a fabric or film — and the organisms are provided with a moist environment instead of a liquid immersion. Think terrestrial grasses vs, say, sea grasses.
To date, Proterro and BioProcess Algae have been working on these lines, that we know of. When it comes to water, you dip or drip – and a little dab’ll do ya.
Saves you headaches in two ways, One, you don’t have the tremendous problems of moving lots of water. Two, you don’t have extraction. Third, if the residence time is short (that is, the period when the microbe is actively making sucrose), its tougher for other microbes- competitors, parasites, predators or viruses, to take hold.
“Sugar is a food source for microbes,” Proterro CEO Kef Kasdin told the Digest. “We knew we had to design for this from the beginning, and mitigate the likelihood that other microbes will take the sugars. This is what we did do in our prototype — and
ran several tests for over three months with no contamination.”
“There are no antibiotics,” she continued. “It’s done through passive systems and overall design. First, our system doesn’t haven’t be opened up to inoculate. Second, because we are talking about a plastic enclosure and a fabric. We start in a collapsed state, like one of those kids’ bounce houses when its uninflated. We sterilize in a low-cost way in that form.
“Then, we feed in the inoculum through a design which allows us to not ever have to open the box. Then, we expand the plastic enclose with slightly positive pressure as the gas is brought in. Now, we have a closed system, and not in a vat of water, but constantly flowing, fast enough to make it tough for anything to take hold because the residence time is short.”
About the CO2
Other ventures have struggled with the logistics of CO2, so let’s look at that. It’s one thing to use CO2, another to use coal-stack flue gas, for example. Aside from the contaminants, there’s the problem that CO2 from a coal-fired power plant arrives in a constant, low-pressure stream, mixed with air. You have to isolate the CO2, possibly bring up the pressure, scrub out the toxins — and then you still have to store CO2 at night because organisms generally need it during the daylight.
In Proterro’s case? “We’re looking at all options, but for now we are focused on obtaining CO2 from fermentation sources, such as ethanol plants. To get to a scale we need, that’s not sufficient CO2, but we expect to augment with other sources, and a combustion gas like flue gas is an option. With flue gas, based on the literature and others who have some before us, we don’t expect that what you find in scrubbed flue gas will be harmful.
Speaking of scale, let’s look at that. A 100 million gallons ethanol plant gives you enough CO2 to make 10 million gallons (equivalent) of sucrose. Kasdin says that the economics could be attractive even at that scale compared to other sugar sources, but the chase for 5-cent sugars requires more scale.
Now, where is Proterro in its journey towards scale? Unlike fermentation systems where you need to make bigger and bigger fermentation runs — for instance, scaling up to 500,000 liters as Solazyme did recently in Clinton, Iowa — Proterro uses a modular approach. It has a small unit that brings in the CO2 and water, hosts the reactions, and distributes the sugarwater.
So, at Proterros, a pilot unit is one in which the modular photobioreactor is completed and at the desired scale, but it is not hooked up for the inputs and outputs.
In the demonstration, Proterro will hook up all the inputs and the outputs and demonstrate a completed system which can then be manufactured and expanded in a modular fashion to make up a complete at-scale facility.
We continue the Proterro story in today’s Digest, here:
More background on the story from the Digest
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