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As carbon-neutral and sustainable, biofuels (i.e., ethanol, biodiesel, and bio-jet fuel) are produced through contemporary biological processes, rather than geological processes [190, 191]. Because of the need for increased energy security, increased petroleum price, and negative impacts of fossil fuels on the environment, biofuels have been gaining increased public and scientific attention [191].

Biofuels are classified into first, second and third-generation biofuels depending on the carbon source of feedstock. The first-generation feedstock is based on the starch obtained from wheat, corn, barley, etc. The second-generation feedstock includes biomass rich in lignin and cellulose such as rice wheat straw, straw, and sugarcane bagasse. Finally, the third-generation feedstock comprises polysaccharides, such as starch, laminarin, and floridean starch [190]. In the extracellular cell wall of plant cells, cellulose is generally embedded in a lignin matrix. This structure is called lignocellulose and it constitutes the essential part of the woody cell walls of plants. In addition to their other biological functions, such as providing resistance against the penetration of microorganisms or degrading enzymes, cellulose and lignin together provide the plant structure. Cellulose can be fermented by many microorganisms to produce biofuels, such as bioethanol. Lignin is a large 3D polymer of phenylpropanoid molecules and it is an abundant source of high energy because of having a high C/O ratio [192, 193]. Lignocellulose is not a part of the human diet as a food since it cannot be readily digested by the microorganisms reside in the human gut, so lignocellulose has been proposed to be used as a renewable source because of its availability and structural characteristics. However, it has been noticed that the production of lower-cost cellulosic biofuels was challenging because lignocellulosic residues are a complex of carbohydrates and polyphenol polymers usually associated with proteins. Consequently, the necessity of the use of several steps including pre-treatments and enzymatic digestions to extract fermentable carbohydrates from this complex network increases the cost of ethanol production drastically [193, 194].

The world’s energy demand is increasing and so, industrial and scientific communities try to introduce new, cost-efficient, and sustainable energy production approaches to the global energy market. More recently, green algae and cyanobacteria have been also gained attention as sources for biofuel production because of their high polysaccharide and lipid composition, but still this “third-generation bioethanol production technology” needs to be improved [195]. Plant genetic engineering is one of the new approaches in the field. Genetic manipulations of plants to increase polysaccharide content and overall biomass, and to produce cellulases and hemicellulases for reducing the need for pretreatment processes seem promising for the fabrication of biofuels [196]. To obtain a higher monomeric sugar release from plants, increasing cellulose content, reducing cellulose crystallinity, altering the amount or composition of noncellulosic polysaccharides and/or lignin seems to be key options. Modification of chemical linkages within and between lignocellulosic biomass components has also been suggested to improve the ease of deconstruction [197]. Besides, some bacteria and fungi can produce and secrete relatively higher extracellular cellulase which can solubilize crystalline cellulose and cellulase enzymes secreted by these microorganisms are feasible for large scale production. For instance, the marine macroalgal cell wall is predominantly comprised of cellulose with the complex chain of glycosidic linkages. Bioethanol production from macroalgae relies on breaking this complex chain into a simple glucose molecule by using cellulases. To obtain cellulases, cellulose-degrading bacteria can be isolated from wide-ranging sources from marine habitats to herbivore residues and gastrointestinal region [190]. In addition to investigation and characterization of new cellulose/lignocellulose-degrading enzymes with metagenomics approach, improvement of bacterial strains for extraction and fermentation steps is another area of much interest to produce optimal yields of biofuel from lignocellulosic biomass [198, 199].