Cellulosic biofuel is a renewable fuel converted from cellulosic biomass, the structural non-food portion of plants, using microbial platforms. Lignocellulosic biomass is the most abundant and renewable organic material in the world. Sources of lignocellulosic biomass such as wood, grass, forestry waste and agricultural residues do not compete with food economics.
Lignocellulosic biomass is woody material made up of three main types of carbon-based polymer which are cellulose, hemicellulose and lignin. Cellulose is a sugar and a polymer of glucose that can be fermented to make ethanol or the longer-chain alcohol butanol.
For economic viability the microbial cells need to perform well utilizing a broad range of carbon substrates and must be able to handle toxic compounds derived from lignocellulosic biomass. Acids, furans and phenolic compounds are present after pretreatment of lignocellulosic biomass necessary to free cellulose and hemicellulose but these substances can affect productivity of fermentation. For example, acetate and furfural were found to decrease the growth rates of the ethanol-producing bacterium Zymomonas mobilis. Microbial engineering has been used to expand or improve the usability of different carbons sources such as glucose, xylose and acetic acid and tolerance to acetic acid, phenolics and furfural.
Bioethanol is ethanol that is produced from biomass and used as fuel. Ethanol is used as an alternative fuel to gasoline for internal combustion engines and can be mixed with gasoline.
The yeast Saccharomyces cerevisiae is an attractive ethanol-producing microbial platform since it has well-characterized physiology, proven industrial feasibility and genetic tools are available. One drawback for S. cerevisiae is that it does not utilize xylose. Cellulosic biomass contains xylose and arabinose, which are referred to as C5 sugars. Microbial engineering strategies led to strains with increased intracellular xylose transport. Directed evolution approaches lead to mutant xylose transporters with improved xylose utilization rates. A mutant glucose/xylose co-transporter evolved form CiGXS1 of Candida intermedia. Several metabolic engineering strategies have been used to improve bioethanol production in S. cerevisiae, which target genes that affect the xylose transporter and xylose catabolism.
Acetic acid is an inhibitor found in lignocellulosic hydrolysates after pretreatment of lignocellulosic biomass. Using a synthetic biology approach acetic acid has become a non-sugar carbon source for an engineered strain of S. cerevisiae. This was accomplished by constructing a synthetic acetate catabolic pathway based on endogenous acetyl-CoA synthetase (ACS) and heterologous NADH-specific acetylated acetaldehyde dehydrogenase (AADH). The strategy improved cellulosic bioethanol production efficiency and also allowed in situ detoxification of acetic acid. Various genes have been targeted to improve stress tolerance associated with inhibitors found in lignocellulosic hydrolysates.
Biodiesel is commonly extracted from Triacylglycerols (TAG) stored in plants or animal fats but prokaryotes can also accumulate storage lipids or synthesize TAGs using organic compounds. Yarrowia lipolytica is an oleaginous yeast and heterotrophic microbial platform being developed for biodiesel production. Up to 90% of the cell mass is lipid when grown on glucose. Y. lipolytica can produce 98.9g/L of fatty acid methyl ester (FAME) from glucose with a yield of 0.269 g/g and productivity of 1.3 g/L/h.
Oleaginous microbes accumulate more than 20 percent of their biomass as lipid and can range from 20-70 percent of the cell’s biomass and are usually TAGs that are stored as a reserve supply of carbon and energy. TAG accumulates after synthesis of phospholipids during the exponential growth phase and during the stationary phase when cellular growth is impaired under conditions of excess carbon and limited nitrogen. Slow growth rate is requirement for lipid accumulation.
Eukaryotic organisms such as yeast, fungi and algae generally produce polyunsaturated fatty acid triacylglycerols similar to vegetable oils which can be used for biodiesel production. Most bacteria produce glycolipids, lipoproteins and wax esters. Some strains of bacteria in the Actinomycetes group show TAG accumulation which come from Mycobacterium, Streptomyces, Rhodococcus and Nocardia species.
Cellulosic Biofuel - an overview | ScienceDirect Topics
Wallace E. Tyner
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