Jan 20 2012

Biofuels from Seaweed

We are at a time in history when collectively we are re-examining the flow of energy through our civilization. It’s a fascinating question from a purely scientific point of view, but also with profound practical implications.

For the last century we have relied heavily on fossils fuels – energy stored in hydrocarbons that we pull out of the ground. This is a cheap and convenient source of large amounts of energy, and it’s difficult to imagine that we could have gotten to this point in our technological development without it.

But looking to the future we can see the light at the end of the tunnel of our fossil fuel society. Experts still debate how much of the stuff there is left in the ground. Do we have 50 years left or 500? Probably the truth is somewhere in between, but in any case the supply is finite and will be exhausted eventually. There is also the issue of releasing all that sequestered carbon back into the atmosphere and its long term effects on the climate. Further, some countries with large stores of crude oil, perhaps the most important form of fossil fuel, tend to be politically unstable.

There would be multiple advantages to finding cheap, renewable, and clean alternatives to fossil fuels. Nuclear is probably a necessary option for the foreseeable future. Solar, wind, geothermal, wave power, and hydroelectric all have good features, but are not presently productive enough to take over for fossil fuel and will take decades at least to develop to that point. Solar in particular seems promising, in my opinion, but we are decades and hundreds of billions of dollars away from developing a solar infrastructure that can run our society.

One option that gets a lot of attention but, in my opinion, has not been ready for prime time is biofuels – essentially making fuel from plant or animal sources. There are multiple practical limitations that limit the usefulness of biofuels as a source of energy.

Corn-based ethanol is the most common biofuel in the US, and it is still controversial whether or not there is more energy output than input in making corn ethanol. The corn is fertilized with fossil-fuel based fertilizer, for one thing. By the time you count up all the energy that going into growing, converting, and transporting the ethanol it may be a net loser of energy. Even if you take the most optimistic interpretation, it’s pretty close to the break-even line and therefore not a significant source of new energy.

There are a few variables to manipulate to try to make biofuels more advantageous. The plant source is important, as some contain more energy than others, are easier to grow, use less land, and require less fertilizer. Researchers are also working to refine the process of conversion to allow for cheap and large volume mass production.

Land-based plants are problematic because many of them use up farm land and therefore increase the price of food. They also require fertilizer and irrigation.

For that reason scientists have been looking toward water-based plants, such as algae, as the plant source for biofuels. Vats or even lakes of algae could be a source of bio-matter. But perhaps better still are the world’s oceans, which are, of course, huge and could potentially produce enough plant matter to meet the world’s biofuel needs.

With that in mind, researchers have recently published a paper in which they describe a DNA fragment that codes for an enzyme that can convert seaweed (macroalgae) polysaccharides into ethanol. Here is the abstract:

Prospecting macroalgae (seaweeds) as feedstocks for bioconversion into biofuels and commodity chemical compounds is limited primarily by the availability of tractable microorganisms that can metabolize alginate polysaccharides. Here, we present the discovery of a 36–kilo–base pair DNA fragment from Vibrio splendidus encoding enzymes for alginate transport and metabolism. The genomic integration of this ensemble, together with an engineered system for extracellular alginate depolymerization, generated a microbial platform that can simultaneously degrade, uptake, and metabolize alginate. When further engineered for ethanol synthesis, this platform enables bioethanol production directly from macroalgae via a consolidated process, achieving a titer of 4.7% volume/volume and a yield of 0.281 weight ethanol/weight dry macroalgae (equivalent to ~80% of the maximum theoretical yield from the sugar composition in macroalgae).

Pretty cool – fuel from seaweed.  Seaweed would not take up farm land or forests, would not require fertilizer or irrigation, and can be farmed from the ocean. Biofuels are carbon neutral, because burning them releases the carbon that was captured by them in the first place.

The non-trivial questions that always remain: Can a manufacturing process be developed with bacteria engineered with this DNA fragment that can mass produce ethanol from seaweed in a cost effective and energy effective process? Probably – but the process and infrastructure still needs to be developed.

Also, thinking about this from an ecosystem perspective, no matter how you slice it such a process would involve pulling large amounts of nutrients and energy out of the sea. If done on a sufficiently large scale to make a difference to our dependence on fossil fuels, what effect would this have on the oceans’ ecosystems? Would the seaweed be capturing extra energy from the sun, or would this process represent a net deficit of energy from the ocean. Would the nutrients that go into the seaweed ultimately flow back to the sea, or will they be a slow drain of life from the oceans?

Here’s a thought – perhaps seaweed farms could be grown in the Gulf of Mexico and take advantage of the nitrogen run off from farming, replacing the harmful algae blooms that otherwise form. It would be nice if things work out that way – a convenient win-win.

The bigger issue is – the cycle of energy, carbon, and nutrients on a large scale has to be considered and, hopefully, engineered to be environmentally friendly and sustainable. Our civilization uses so much energy, which is increasing steadily, that any significant contribution to that energy will likely have a large impact on the world. We are therefore getting to the point that we have to think carefully about the engines of our civilization and how they can coexist with the rest of the world.

I don’t know what will ultimately become of biofuels from seaweed, but the idea has a lot of interesting advantages. I don’t think land-based biofuels will ever be a major player in our energy infrastructure. It’s more of a feel-good science project for environmentalists. But seaweed may have potential.


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