December 12, 2007
Genetically Engineered Organisms To Convert Cellulose To Diesel

MIT's Technology Review has placed David Berry of biotech energy start-up LS9 on a list of top 35 innovators for 2007 for an on-going attempt to genetically engineer organisms to feed on plant cellulose and produce gasoline or diesel fuel.

Berry took the lead in designing a system that allowed LS9 researchers to alter the metabolic machinery of ­microorganisms, turning them into living hydrocarbon refineries. He began with biochemical pathways that microbes use to convert ­glucose into energy-storing molecules called fatty acids. Working with LS9 scientists, he then plucked genes from various other organisms to create a system of metabolic modules that can be inserted into microbes; in different combinations, these modules induce the microbes to produce what are, for all practical purposes, the equivalents of crude oil, diesel, gasoline, or hydrocarbon-based in­dustrial chemicals.


Nonetheless, LS9 has no products so far and many ­hurdles to surmount. Berry's system, for example, is designed to exploit glucose-based feedstocks such as cellulose. Berry says he is "agnostic" about what source of cellulose might drive the LS9 system on an industrial scale; he lists switchgrass, wood chips, poplar trees, and Miscanthus, a tall grass similar to sugarcane, as potential sources of biomass. But a cost-­effective and efficient source of cellulose is one of the more significant bottlenecks in the production of any biofuel.

Producing gasoline or diesel has a lot of advantages over producing ethanol. Unlike ethanol both gasoline and diesel can be transported via pipelines. They also let you go much further between fill-ups than ethanol. So a reader asked whether this approach can work. I can't say for sure but the question should be considered in parts:

1) Can they genetically engineer the sorts of organisms that they want to genetically engineer to perform the way they want those organisms to perform?

2) Will the resulting process be cost competitive?

3) If they manage to create a cost competitive process will the result be a good thing?

I'm much more optimistic on the first point than on the second point. Worse, I'm more optimistic on the second point than on the third.

On the first point: Sure, with enough genetic engineering talent and time you can modify organisms to eat cellulose and produce diesel. Will this particular crew succeed? Hard to know.

But if they succeed in the genetic engineering task will the resulting fuel be cheap enough? They have going for them the rising cost of corn driven by both corn ethanol subsidies and rising world demand for food. But their approach starts with an inefficiency: They first grow plants to produce cellulose. Therefore some of the initial plant energy gets lost as they feed the cellulose to genetically engineered organisms. The conversion from cellulose to diesel fuel will have some inefficiency associated with it. Will the conversion process cost more than half the cellulose energy?

A larger fraction of the sun's energy would get converted to diesel fuel if they geneticallly engineered organisms to convert the sun's energy directly into diesel fuel rather than first into cellulose. But that approach would raise costs since then plants would need to be grown in elaborate diesel fuel collection systems. Far easier to harvest existing trees, bushes, and grasses to get cellulose.

That depends on the cost of the cellulosic material, the efficiency of the conversion of it into less oxidized hydrocarbons (out with the oxygens and in with the hydrogens), and the cost of vats and other equipment.

How much of the existing biomass out there in nature eill get diverted to this purpose? The human footprint is already much too big and growing. The other species are already too squeezed.

I'm skeptical of this approach because it seems inefficient. Plants are inefficient converters of sunlight into chemical energy. Then there's an additional step of harvesting the plants, transporting them to vats, and then using the cellulose to feed microorganisms where part of the energy gets lost running the metabolism of these plants.

Most estimates I've come across on the efficiency of conversion of light energy into chemical energy by plants end up with a conversion efficiency of 1% or less. Keep in mind that most photons are at frequencies that plant chloroplasts can't use. Plus, seasonal plants aren't even alive part of the year to absorb photons and convert them into chemical energy. According to this report sugarcane is the most efficient converter of light energy into chemical energy.

Tropical grasses that are C4 plants include sugarcane, maize, and crabgrass. In terms of photosynthetic efficiency, cultivated fields of sugarcane represent the pinnacle of light-harvesting efficiency. Approximately 8% of the incident light energy on a sugarcane field appears as chemical energy in the form of CO2 fixed into carbohydrate. This efficiency compares dramatically with the estimated photosynthetic efficiency of 0.2% for uncultivated plant areas. Research on photorespiration is actively pursued in hopes of enhancing the efficiency of agriculture by controlling this wasteful process. Only 1% of the 230,000 different plant species known are C4 plants; most are in hot climates.

Since the conversion efficiency into chemical cellulose energy is so low in the first place the harvesting, transportation and conversion of cellulose to diesel or gasoline makes a low initial efficiency even lower by the time the final usable chemical product comes out of the conversion process. That means if we shift to biomass energy to push our vehicles around more land must get shifted to provide energy for humans.

Share |      Randall Parker, 2007 December 12 10:22 PM  Energy Biomass

James Bowery said at December 17, 2007 3:34 PM:

I have a lot of trouble accepting the assertion that sugarcane is more efficient than algae at converting solar energy into biomass calories. I didn't see any reference to algae's efficiency, compared to sugar cane, in the linked article.

Randall Parker said at December 17, 2007 7:07 PM:


I spent over an hour trying to find data on incident sunlight-to-carbohydrate conversion efficiency of various plants and didn't come up with much. I've read this sort of data before. Next time I come across good references I'm going to post on it because the information is hard to find.

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