Bioethanol fuel is mainly produced by the sugar fermentation process, although it can also be manufactured by the chemical process of reacting ethylene with steam. The main sources of sugar required to produce ethanol come from fuel or energy crops. These crops are grown specifically for energy use and include corn, maize and wheat crops, waste straw, willow and popular trees, sawdust, reed canary grass, cord grasses, jerusalem artichoke, myscanthus and sorghum plants.
Recent research has turned to macroalgae as a potential source of sugars. The most abundant sugars in brown algae are alginate, mannitol, and glucan; whereby the degradation of these polysaccharides requires specific enzymes for the release of monosaccharides. Monosaccharides are most efficiently fermented into ethanol by Saccharomyces cerevisiae and Escherichia coli strains of bacteria.
Using brown seaweeds as a source of bioethanol could reduce pressure on food crops, and help draw down CO2 from the oceans. While biofuels still release CO2, they reduce the demand for finite fuel resources.
Here is a link to a chapter in Advances in Feedstock Conversion Technologies for Alternative Fuels and Bioproducts that outlines the process.