Biofuels from brown macroalgae
From ars technica by Kyle Niemeyer
Biofuels may hold the key to reducing our dependence on foreign oil and cutting down on our greenhouse gas emissions. Ethanol is currently the biofuel of choice, with almost all gasoline bought at the pump in the United States containing 10 percent ethanol. Right now, though, most ethanol comes from corn and sugarcane, and there are concerns that growing our fuel from these crops could drive up food prices (“food versus fuel”).
Biofuels made from macroalgae, aka seaweed, avoid this problem. Seaweeds do not require arable land, fertilizer, or fresh water, and they are already cultivated as food (though not a staple crop like corn), animal feed, fertilizers, and sources of polymers. Traditionally, scientists ignored seaweed as a biofuel source because its main sugar component was too difficult to process. A recent paper published by Science describes how researchers genetically-engineered a microbe that is capable of producing ethanol from seaweed.
The so-called second generation of bioethanol is derived from inedible crops like wood and switchgrass, or the inedible portions of food crops like corn (the leaves and stalks). However, this cellulosic material is difficult to process due to the presence of lignin in the cell walls—although we reported on some attempts to genetically modify switchgrass to make this easier. Seaweed doesn’t contain lignin, making processing a lot easier and enabling higher yields: a Department of Energy study showed that, under ideal conditions, seaweed could produce twice the ethanol that we get from sugarcane and five times the amount from corn.
You may be asking “This sounds great, why aren’t we making ethanol from seaweed?” Well, there is a catch. Seaweeds contain three primary sugars: alginate, mannitol, and glucose. Right now, existing industrial microbes can’t metabolize alginate, so ethanol yields are severely limited.
At this point, most people would stop and say “Well, maybe seaweeds aren’t the best way to produce biofuels.”
On the other hand, if you were Adam Wargacki and a team of 13 others from the Bio Architecture Lab, you wouldn’t stop. Instead, you’d look to the well-known bacterium Escherichia coli (E. coli), which has a natural ability to metabolize mannitol and glucose. Since we know of enzymes that can process alginate (alginate lyase and oligoalginate lyase), Wargacki et al apparently thought “We can make this work.”