Turning brown seaweed into biofuels is one option that has been proposed to help meet the world's growing energy demands from renewable sources. Recent research has overcome a major barrier to converting the majority of sugars in seaweed into bioethanol and other valuable products by using genetically engineered bacteria to break down the seaweed.
Harvesting algae as a feedstock to produce biofuels could be part of a future mix of renewable energy sources. One type of algae suitable for this purpose, macroalgae (seaweed), grows naturally in the sea, which could help overcome the contentious issue of whether land should be used to grow crops for food or fuel – an issue that concerns biofuels produced from land-based crops, such as maize. Unlike many other biofuels, its cultivation requires no fertilisers or freshwater. An additional advantage of macroalgae is that it does not contain lignin, the stiff plant material found in land crops that require energy-intensive pre-treatment to break down the biomass before conversion to biofuels. Seaweed simply needs to be milled or crushed before fermentation.
However, industrial microorganisms that are commonly used in fermentation processes are not able to digest all of the sugars found in algae, in particular, the most abundant one, alginate. This means the process is inefficient and the full potential of seaweed to produce ethanol cannot be achieved.
This study describes how the researchers have bioengineered Escherichia coli, a common bacterium found in the human digestive system and used in the laboratory and industry, to digest all of the sugars found in seaweed, including alginate. The technique converts seaweed into ethanol or other useful chemicals in a single process. However, the study does not address environmental impacts of large scale application of the technology.
The researchers identified the genes in a marine microbe, Vibrio splendidus that are responsible for breaking down the complex sugars in seaweed into simpler sugars. These genes were inserted into samples of E. coli. The genetically modified E. coli strain was further engineered to turn the simple sugars into ethanol. The advantage of this bioengineering technique is that the genetically modified E. coli bacteria can also be engineered to turn the sugars into other valuable chemicals instead of ethanol.
Laboratory tests on the widely available brown kombu seaweed (Saccharina japonica) suggest the engineered strain of E. coli could achieve 80% of the theoretical maximum yield of ethanol at temperatures between 25°-30° C. The potential yield of ethanol from brown seaweed using this technique is twice that of sugarcane and five times that of maize.
Seaweed grows along the coastline in many parts of the world, and is already cultivated for food, but is not a staple crop. The researchers estimate that seaweed farms along 3% of the world’s coastlines could produce 60 billion gallons (about 227 billion litres) of ethanol a year, using this technique. It is unlikely any accidental release of engineered E. coli could damage seaweed growing in the sea, as the microbes are not suited to the ocean environment.
This preliminary research in the laboratory has demonstrated a new way to produce bioethanol using brown seaweed as the feedstock. The technique overcomes the limitations of using marine organisms to ferment seaweed under ordinary industrial conditions. Further work is being carried out to scale up the technology to commercial production. A pilot facility using aquafarmed seaweed is under development in Chile.
Source: Wargacki, A.J., Leonard, E., Win, M-N. et al. (2012) An Engineered Microbial Platform for Direct Biofuel Production from Brown Macroalgae. Science. 335: 308-313.. 15:1-3.