In contrast, cultivation of microalgae in open ponds involves lower capital and operating costs but offers low productivity. Additionally, microalgal cultures growing in open ponds are exposed to contaminants and affected by seasonal variations Chisti, In both systems, microalgal density must be controlled to maintain a viable culture Wang et al.
Other challenges associated with biofuel from microalgae have been discussed in detail elsewhere Hannon et al. Macroalgae refer to the macroscopic seaweeds. They are characterized by forming multicellular specialized tissues and defined structures that are comparable to plant leaves and roots John and Anisha, ; Murphy et al.
Macroalgae are less versatile than microalgae and are distributed primarily over green, red, and brown algae Jung et al. In comparison to terrestrial plants, macroalgae grow faster and produce more biomass per area due to their high photosynthetic efficiency Murphy et al.
Although commercial third-generation biofuels are derived from microalgal biomass, seaweeds specifically red and brown macroalgae serve as an unexploited potential source for bioethanol production due to two facts. First, macroalgae combine high biomass productivity with low capital and operating costs owing to the fact that macroalgae are harvested from naturally occurring stocks or aquacultured sea farms.
Such cultivation systems require capital and operating costs that are significantly lower than the microalgal open ponds, nevertheless they provide high biomass productivity Carlsson et al.
Second, macroalgae are cultivated worldwide on a large scale for non-biofuel purposes. The remainder of the biomass, which is rich in carbohydrates, can be hydrolyzed to produce ethanol. In fact, the worldwide biomass production from macroalgae greatly surpasses that of microalgae. For example, in , approximately 9 million and 6. In comparison, a total of only 6.
The potential application of macroalgae for biofuel production has been reviewed by others Murphy et al. The production of biofuels from macroalgae has several environmental advantages [discussed by Hughes et al. First, in contrast to microalgal feedstocks, which are used for simultaneous production of bioethanol and biodiesel, macroalgae accumulate considerable amounts of carbohydrates, and thus can be used to produce bioethanol only [see Table 1 in Singh et al.
Third, the macroalgal carbohydrates content varies depending on the alga growth stage and seasonal variations Suutari et al.
Fourth, macroalgae accumulate lower amounts of glucan food reserves i. Therefore, the industrial production of bioethanol from macroalgae requires fermentation of both glucose- and non-glucose-based sugars Yanagisawa et al. Spirulina sp. Its biomass is used primarily for human and animal consumption; however, only a small portion is directed toward biofuel production Ciferri, ; Wikfors and Ohno, ; Habib et al.
Additionally, several cyanobacterial strains of Synechococcus species have been genetically modified for enhanced commercial production of bioethanol [reviewed by Dexter et al.
The production of biofuel from cyanobacteria has several advantages [discussed by Quintana et al. Among these advantages is the fact that many cyanobacteria, e. However, there are several disadvantages of using cyanobacterial biomass for biofuel production. For example, in contrast to microalgae, which store high amounts of lipids and carbohydrates, cyanobacteria do not accumulate significant amounts of lipids, and therefore they are not suitable for biodiesel production Quintana et al.
Other challenges that constrain bioethanol production from cyanobacteria have been discussed by other reports Nozzi et al. Similar to plants, photosynthesis in algae is divided into two steps: the light-dependent reactions and Calvin cycle.
In the light-dependent reactions, light energy is absorbed at the thylakoid membranes in the chloroplasts, where it is converted into adenosine triphosphate ATP and the reduced form of nicotinamide—adenine dinucleotide phosphate NADPH. For reviews on algal photosynthesis, we recommend the reader to refer to Moroney and Ynalvez Algae produce a wide range of polysaccharides depending on the algal species Table 2.
This diverse collection of polysaccharides functions primarily as food reserves or structural material Figure 1. Here, we describe the most economically important algal sugars, which have received considerable amount of research interest.
The advantages and disadvantages of employing these sugars for bioethanol production are highlighted in Table 3. For more information about algal polysaccharides, we refer the reader to previously published reviews Peat and Turvey, ; Percival, , ; Avigad and Dey, ; Grant Reid, ; Synytsya et al.
Table 2. Chemical structure and distribution of food reserves and structural polysaccharides among different groups of algae. Figure 1. Overview of ethanol production from major algal carbohydrates.
A Algae store simple sugars in the form of simple and complex food reserves See Food Reserves and as structural polysaccharides See Structural Polysaccharides. The chemical structures of the listed polysaccharides are presented in Table 1. DEHU, 4-deoxy- l -erythrohexoseulose uronic acid. Table 3. The advantages and disadvantages of employing different algal sugars for the production of third-generation biofuels.
Food reserves are easily fermented into ethanol and thus are the primary source for industrial third-generation bioethanol.
In contrast, the hydrolysis of structural carbohydrates is challenging due to their rigidity. Therefore, optimization of the hydrolysis process of structural carbohydrates carries the promise of maximizing ethanol yield. In this section, we will first review the major algal food reserves.
Additionally, we will discuss major algal structural polysaccharides because of their potential in enhancing the yield of bioethanol from algal feedstock. The majority of algae store their food reserves in the form of starch-type polysaccharides such as starch, floridean starch, and glycogen Viola et al.
Additionally, brown algae accumulate large amounts of mannitol, which functions as an antioxidant and regulator of cell osmolarity Davis et al. In contrast to plants, which store starch granules in the amyloplast, most algae lack the amyloplast, and therefore store starch grains in the chloroplast Busi et al.
Exceptions to this are the red algae, Dinophyta Dinoflagellates , and Glaucophyta, which store their food reserves in the cytosol Radakovits et al. Floridean starch is another main food reserve polysaccharide Table 2.
It is a starch derivative that is synthesized by red algae Rhodophyta. Its granules differ from starch by lacking amylose and thereby are composed completely of amylopectin Viola et al. The granules are similar in structure to plant starch but more variable in size diameter: 0. Laminarin and chrysolaminarin are the third major food reserves. Laminarin is synthesized by brown algae, and it forms either a G-chain — with glucose molecule at the reducing end — or an M-chain — with mannitol at the reducing end Kadam et al.
Chrysolaminarin is the food reserve polysaccharide in diatoms, and it is comprised only of glucose molecules G-chains at the reducing end Beattie et al. Glycogen is the food reserve form in cyanobacteria. Glycogen and amylopectin one of starch granule constituents are similar in structure; however, glycogen is more branched and forms smaller granules diameter is 42 nm in comparison to starch granules diameter —, nm Ball et al.
In addition to the previously described major polysaccharide forms, other granule forms exist among algae but to a lesser degree. For instance, algae of the class Euglenophyta store their food reserves in the cytoplasm as paramylon. Mannitol is a sugar alcohol of the aldohexose d -mannose. In brown algae, it serves as a storage sugar, and an antioxidant, and protects against osmotic stress.
It accumulates in the cell as a monosaccharide i. Mannitol is produced in brown algae from fructosephosphate, which is reduced to mannitolphosphate via Mannitolphosphate 5-dehydrogenase EC1. In the second step, mannitolphosphate is converted to d -mannitol by mannitolphosphatase EC3. Structural polysaccharides are another putative source to increase bioethanol yield from algae. Their main function is to confer rigidity to the algal cell wall. In contrast to plants, which usually have a lignocellulosic cell wall, the composition of algal cell wall varies among algal groups.
Cellulose is the major algal cell wall polysaccharide, and it is present in most algal groups. In addition to cellulose, algae incorporate significant amounts of other polysaccharides into their cell wall, which can be converted into ethanol. Such polysaccharides can be specific to an algal group, such as the red algae, which contain agarose and carrageenan; and the brown algae, which are rich in alginate Vreeland and Kloareg, ; Murphy et al.
Variations in algal cell wall contents can also be found within families and genera of the same group. For example, the cell wall of the green seaweeds Ulva and Enteromorpha sp. Cellulose chains aggregate together by intra- and inter-molecular hydrogen bonds to form cellulose microfibrils. Microfibrils are packed together to form fibrils, which in turn aggregate to form cell wall fibers Brown and Saxena, ; Pu et al.
With exception to diatoms, cellulose is found in the majority of algal cell walls. Agarose is a non-sulfated, non-water-soluble linear galactan that is composed of repeating disaccharide units of d -galactose d -Gal and 3,6-anhydro- l -galactose l -AnGal.
The repeating disaccharide unit is called agarobiose or neoagarobiose depending on 1 the position of each sugar in the disaccharide, 2 the bond that links the monomers within the disaccharide, and 3 the bond that links the disaccharides to form agarose.
Carrageenan is a sulfated water-soluble linear galactan of carrabiose or neocarrabiose subunits Table 2. Carrabiose and neocarrabiose are similar in structure and linkage to agarobiose and neoagarobiose, respectively De Ruiter and Rudolph, ; Renn, ; Delattre et al.
The main function of alginate is to provide the cell wall with elasticity and rigidity to survive aquatic habitats Dornish and Rauh, Additionally, alginate is found in the matrix of some bacterial biofilms. Although its function in bacterial biofilms is not yet fully understood, alginate has been shown to play a role in bacterial pathogenesis and epiphytism Halverson, Ulvan is a water-soluble cell wall polysaccharide, which is found in green seaweeds, such as Ulva and Enteromorpha sp.
Jiao et al. It is comprised of sulfated rhamnose, glucuronic acid, iduronic acid, xylose, and sulfated xylose. The ratio and linkage of ulvan constituent monosaccharides vary among species [refer to Lahaye and Robic ]. Fucoidan is a cell wall polysaccharide that is found in the members of family Laminariaceae of brown algae. Fucoidan structure is heterogeneous and varies among algal species.
The process of bioethanol production from algal polysaccharides consists of three major steps: biomass pretreatment, enzymatic hydrolysis of algal polysaccharides, and fermentation of sugar monomers to ethanol.
The pretreatment step disrupts algal cell and releases intracellular sugars. Additionally, the pretreatment step reduces algal cell wall crystallinity making its polysaccharides accessible for enzymatic hydrolysis. Algal biomass is pretreated by physical, chemical, or biological methods.
This review will not cover the pretreatment step since it has been covered in detail by other reviews [refer to Harun et al. Subsequently, algal biomass is degraded by lytic enzymes into simple sugars and uronic acid monomers. In contrast, industrial production of ethanol from alginate requires engineering of alginate degradation, uptake, and metabolic pathways See Fermentation of Algal Simple Sugars and Uronic Acids to Bioethanol.
Isoamylases E. Figure 2. Schematic diagrams for the enzymatic hydrolysis of algal polysaccharides. At the industrial scale, the enzymatic hydrolysis of starch is carried out at elevated temperatures, and it is divided into three steps: starch gelatinization, liquefaction, and saccharification. Saccharification is the conversion process of oligosaccharides to primarily glucose along with other disaccharides i. Similar to starch, gelatinized floridean starch is easier to degrade than granules and requires the same procedure of starch degradation described above Yu et al.
Glycogen is similar in structure to amylopectin, and therefore the hydrolysis of glycogen requires the same amylolytic enzymes, which are used to breakdown starch. In fact, two reports found that only two enzymes, i. However, neither the synergistic effect of these isoamylases on glycogen hydrolysis nor the final products of hydrolysis were analyzed in these studies. The hydrolysis products are fermented to ethanol using budding yeast.
Additionally, extraction of glycogen from cyanobacteria is simpler than algae as it only requires breaking down the cyanobacterial cell wall with lysozyme Aikawa et al. Degradation of the M-chain type of laminarin i. Mannitol is readily dissolved from algal biomass, and therefore conversion of mannitol to ethanol requires no pretreatment steps, which simplify bioethanol production process Wang et al.
In order to be fermented, mannitol must be converted to fructosephosphate fructoseP. Mannitol metabolic pathways vary among organisms. For example, non-lactic-acid bacteria and homofermentative lactic-acid bacteria assimilate mannitol via phosphoenolpyruvate-dependent mannitol phosphotransferase system to mannitolphosphate mannitolP , which is dehydrogenated to fructoseP by mannitolphosphate dehydrogenase M1PDH, EC 1.
In fungi, algae, and heterofermentative lactic-acid bacteria, mannitol is dehydrogenated by mannitol 2-dehydrogenase [M2DH, EC 1. Similar to plants, the commercial production of biofuel from algal cellulose remains a challenge since cellulose is embedded in a multilayered intricate rigid matrix of sugars and polymers, which protect cellulose from enzymatic degradation Domozych et al.
However, the hydrolysis process of algal cell wall to ethanol remains simpler than lignocellulosic biomass because algal cell wall lacks or contains low amounts of lignin. Nevertheless, algal pretreatment is required to remove non-cellulosic cell wall matrix and reduce algal cellulose crystallinity making it accessible for enzymatic hydrolysis.
Subsequently, cellulose is hydrolyzed by cellulolytic enzymes into glucose, which is fermented into ethanol. Endoglucanase EC 3. Exoglucanase EC 3. However, enzymatic hydrolysis of agarose yields low concentrations of galactose due to low solubility of agarose Yun et al. To increase the yield, an agarose acid and heat pretreatment step also known as chemical liquefaction has been introduced preceding the enzymatic hydrolysis step.
The degradation of carrageenan is one of the least studied among the major cell wall carbohydrates in algae. Carrageenolytic enzymes are not common among microbes.
Only few microbes have been reported to excrete carrageenases. The majority of these microbes are marine bacteria Michel et al. The degradation of alginate into unsaturated alginate oligomers degree of polymerization 2—4 is carried out by endo-alginate lyases. The enzymatic hydrolysis of ulvan and fucoidan has been studied less intensively than other polysaccharides, due to several reasons.
First, both sugars can be easily degraded into monomeric sugars by acid treatment Davis et al. Second, ulvan and fucoidan are species-specific and family-specific cell wall polysaccharides, respectively, and therefore only few microbes display activities against ulvan Lahaye et al. The hydrolysis of major algal polysaccharides releases several simple sugars, such as glucose, mannose, fructose, galactose, and uronic acids. These monomers are fermented to produce ethanol.
Simple sugars are readily fermented to ethanol using microbial wild-type strains. In contrast, fermentation of uronic acid monomers requires genetically engineered microbes that can hydrolyze alginate to KDG and ferment it to ethanol.
The classical budding yeast Saccharomyces cerevisiae is the most commonly used microbe for fermenting sugars to bioethanol.
Additionally, the Gram-negative rod-shaped bacterium Zymomonas mobilis is used for fermentation, but to a lesser extent than the budding yeast. Owing to their diversity, several metabolic pathways are required to convert algal sugars to ethanol. While glucose enters glycolysis directly, galactose must be converted to glucose 6-phosphate glucoseP via the Leloir pathway before entering glycolysis.
Similarly, glucose isomers, such as mannose and fructose, are converted to fructosephosphate fructoseP , which is further metabolized through glycolysis. The conversion of fructose to fructoseP is simple and requires one enzyme Hexokinase, EC 2. In contrast, mannose must be first phosphorylated by hexokinase EC 2. Once phosphorylated sugars enter glycolysis, they are metabolized to pyruvate.
While S. In the first stage, glucose is converted to glyceraldehydephosphate glyceraldehydeP , which is further metabolized to pyruvate in the second stage Figures 3 A,B. Figure 3. The production of ethanol from glucose by Embden—Meyerhof—Parnas pathway in Saccharomyces cerevisiae and Entner—Doudoroff pathway in Zymomonas mobilis. The catalytic enzymes are denoted with numbers. A Glycolysis first stage. B Glycolysis second stage. C Alcoholic fermentation.
In the next step, the enzyme 6-phosphofructokinase EC 2. The first stage of the ED pathway begins with phosphorylation of glucose by hexokinase EC 2.
Once oxidized, 6-phosphogluconolactone is hydrolyzed by the enzyme 6-phosphogluconolactonase EC 3. In contrast to the first stage of EMP and ED pathways, the second stage of these two pathways is identical.
During the second stage, glyceraldehydeP is converted to pyruvate summarized in Figure 3 B. Following glycolysis, pyruvate is converted to ethanol primarily by a two-step alcoholic fermentation Figure 3 C.
In the first step, pyruvate is converted by pyruvate decarboxylase EC 4. In addition to galactose, the enzymatic hydrolysis of agarose and carrageenan produces the non-fermentable sugars, 3,6-anhydro- l -galactose l -AnG and 3,6-anhydro- d -galactose d -AnG , respectively.
To increase bioethanol yield from algal biomass, l -AnG and d -AnG must be converted into fermentable sugar. Metabolic pathways for l -AnG and d -AnG are only common in agar- and carrageenan-degrading microorganisms.
In these organisms, l -AnG and d -AnG are converted to 2-ketodeoxyphospho- d -galactonate d -KDPGal through six and four enzyme-catalyzed reactions, respectively [see Figure 5 in Lee et al. Glyceraldehydephosphate is then converted by glycolysis to pyruvate, which is metabolized to ethanol by alcoholic fermentation Figures 3 B,C.
Several microorganisms, which can metabolize alginate, have been identified [reviewed by Wong et al. In these microbes, the hydrolysis of alginate results in DEHU. However, the commercial production of ethanol from alginate is challenged by the lack of robust microorganisms that can simultaneously digest, metabolize, and ferment alginate to ethanol at the industrial level. While alginolytic microbes lack the robustness for the production of ethanol at large scale, major ethanologenic microbes, such as S.
To overcome this challenge, strains that can simultaneously degrade, metabolize, and ferment alginate to ethanol were engineered. The molecular engineering of alginate and mannitol metabolic pathways in S. The strain, which expresses the DEHU transporter gene from the marine fungus Asteromyces cruciatus and alginate metabolism genes from bacterial origin, is capable of degrading alginate to uronic acid monomers. These monomers are then converted to ethanol Enquist-Newman et al.
Similarly, a plasmid-based E. Ethanol productivity of this strain was further enhanced using recombinase-assisted genome engineering RAGE. Postgraduate courses How to apply Fees and funding Frequently asked questions.
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