Abstract
The production of fuels from biomass is categorized as first-, second- or third-generation depending upon the source of raw materials, either food crops, lignocellulosic material, or algal biomass, respectively. Thus far, the emphasis has been on using food crops creating several environmental problems. To overcome these problems, there is a shift toward bioenergy production from non-food sources. Algae, which store high amounts of carbohydrates, are a potential producer of raw materials for sustainable production of bioethanol. Algae store their carbohydrates in the form of food storage sugars and structural material. In general, algal food storage polysaccharides are composed of glucose subunits, however they vary in the glycosidic bond that links the glucose molecules. In starch-type polysaccharides (starch, floridean starch, and glycogen), the glucose subunits are linked together by α-(1→4) and α-(1→6) glycosidic bonds. Laminarin-type polysaccharides (laminarin, chrysolaminarin, and paramylon) are made of glucose subunits that are linked together by β-(1→3) and β-(1→6) glycosidic bonds. In contrast to food storage polysaccharides, structural polysaccharides vary in composition and glycosidic bond. The industrial production of bioethanol from algae requires efficient hydrolysis and fermentation of different algal sugars. However, the hydrolysis of algal polysaccharides employs more enzymatic mixes in comparison to terrestrial plants. Similarly, algal fermentable sugars display more diversity than plants, and therefore more metabolic pathways are required to produce ethanol from these sugars. In general, the fermentation of glucose, galactose, and glucose isomers is carried out by wild type strains of Saccharomyces cerevisiae and Zymomonas mobilis. In these strains, glucose enters glycolysis, where is it converted to pyruvate through either Embden-Meyerhof-Parnas pathway or Entner-Doudoroff pathway. Other monosaccharides must be converted to fermentable sugars before entering glycolysis. In contrast, microbial wild type strains are not capable of producing ethanol from alginate, and therefore the production of bioethanol from alginate was achieved by using genetically engineered microbial strains, which can simultaneously hydrolyze and ferment alginate to ethanol. In this review, we emphasize the enzymatic hydrolysis processes of different algal polysaccharides. Additionally, we highlight the major metabolic pathways that are employed to ferment different algal monosaccharides to ethanol.
Highlights
During the last decade, growing concerns over depleting fossil oil reserves and increasing greenhouse gas emissions have led to the use of food crops as biomass for making what is called firstgeneration biofuel
To overcome these environmental problems, there is a shift toward the production of biofuel from non-food biomass sources, such as lignocellulosic and algal biomass sources, which are known as the second- and third-generation biofuel crops, respectively (Badger, 2002; Zheng et al, 2009; Brennan and Owende, 2010)
This review focuses on the enzymatic hydrolysis steps of the major algal carbohydrates and their fermentation process to ethanol
Summary
During the last decade, growing concerns over depleting fossil oil reserves and increasing greenhouse gas emissions have led to the use of food crops as biomass for making what is called firstgeneration biofuel. Expansion in biofuel production from food crops has drawn attention to several environmental impacts, such as the conversion of agricultural land from producing food crops to producing biofuel crops and the deforestation of hundreds of thousands of acres (Groom et al, 2008; Searchinger et al, 2008; Naik et al, 2010) To overcome these environmental problems, there is a shift toward the production of biofuel from non-food biomass sources, such as lignocellulosic and algal biomass sources, which are known as the second- and third-generation biofuel crops, respectively (Badger, 2002; Zheng et al, 2009; Brennan and Owende, 2010). The production of biofuel from algae has several advantages over the first- and second-generation of biomass sources [discussed by John et al (2011)].
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