Fill the tank, please. With bacteria!

I apologize if you've already heard about this, but the paper is really cool and I couldn't resist discussing it here.

Escherichia coli, or E. coli for brevity, is a bacterium normally associated with "bad" things like food poisoning. Even though most strains are actually harmless, even the CDC has a page dedicated to E. coli outbreaks. Since it's part of our gut flora, the lower intestines in particular, it's usually not a good sign when E. coli is found in places like restaurants and cafeterias. (Yuck!)

What's less known to the public is that E. coli is one of the most studied bacteria and makes a great model for mutations, gene duplications, and horizontal gene transfer.

What's even less known is that this amazing bacterium has the potential to save our planet from further drilling. How? By producing fuel. Yes, you read that right: through a combination of gene modifications, researchers from the University of Exeter [1] induced "petroleum-replica hydrocarbons" production in E. coli. These hydrocarbons are structurally and chemically similar to fossil fuels.

In their paper, Howard et al. argue against the current biofuels because they bring additional costs in downstream processing and are not 100% compatible with the engines on the market.

"To overcome the end-user blend wall, it is essential to generate precise chemical replacements to fossil fuels through sustainable means.Retail transport fuels are composed primarily of hydro- carbons (n-alkanes) of various carbon chain lengths (Cn), branched hydrocarbons (iso-alkanes), and unsaturated hydrocarbons (n- alkenes). The ideal biofuels are therefore n-alkanes, iso-alkanes, and n-alkenes that are chemically and structurally identical to the fossil fuels they are designed to replace [1]."

Gasoline, diesel and jet fuels are made primarily of molecules called alkanes, or saturated hydrocarbons. Most people are familiar, or at least have heard of methane, the simplest alkane molecule. These molecules are naturally produced not just by bacteria, but also by plants and insects when they metabolize fatty acids. In 2010 Schirmer et al. described in a Science paper [2] an alkane biosynthesis pathway in cyanobacteria, commonly known as blue-green algae.

"The pathway consists of an acyl-acyl carrier protein reductase and an aldehyde decarbonylase, which together convert intermediates of fatty acid metabolism to alkanes and alkenes [2]."

Understanding how alkanes are produced and, in particular, which genes are involved in their production, was the first step. The second step was answering the question: can we tweak this pathway to produce alkanes that can replace our current fuels?

Seen under this light, the PNAS study published last March 15 [1] is a bioengineering success story. Howard et al. designed a novel metabolic pathway that forced E. coli to use free fatty acids instead of fatty acid compounds as in cyanobacteria, and produce fuel-like alkanes, what the authors call "industrially relevant, petroleum replica fuel molecules." Once finalized, this type of biofuel will be compatible with current engines and will not need to be blended with other petroleum derived chemicals.

A bit of perspective: though derived from natural and biological sources, biofuels still contribute to pollution, carbon emissions, and global warming. Despite the amicable "bio" prefix, they all come with a non-null carbon footprint, some more than others. The true efficiency of any kind of fuel is the energy they produce minus the energy and costs it takes to derive them. For example, producing biofuels from crops drains precious resources, first and foremost, water, but also arable land, forests when arable land is not available, and food sources in underdeveloped countries.

So here's where biofuels from bacteria have a striking advantage: E. coli is one of the cheapest and easiest bacterium to grow in a lab. It doesn't drain water reservoirs and it doesn't need deforestation to grow. Contrary to most biofuels out there, that have high production and energy costs, the carbon footprint of biofuels derived from bacteria only comes from carbon emissions when you burn them.

And while this is an excellent thing, I still think that the real change we need to make to preserve our planet is to switch to renewable energy.


[1] Howard, T., Middelhaufe, S., Moore, K., Edner, C., Kolak, D., Taylor, G., Parker, D., Lee, R., Smirnoff, N., Aves, S., & Love, J. (2013). Synthesis of customized petroleum-replica fuel molecules by targeted modification of free fatty acid pools in Escherichia coli Proceedings of the National Academy of Sciences DOI: 10.1073/pnas.1215966110

[2] Schirmer, A., Rude, M., Li, X., Popova, E., & del Cardayre, S. (2010). Microbial Biosynthesis of Alkanes Science, 329 (5991), 559-562 DOI: 10.1126/science.1187936