2010-05-02, 04:34 PM
Microalgae are photosynthetic organisms that are made up of a variety of individual species that are uni-, multi-cellular and pro-, eukaryotes in nature (1). Major groupings include cyanobacteria, green algae, diatoms, yellow-green algae, golden algae, red algae, brown algae, dinoflagellates and ‘pico-plankton’ (2). Notable biological features of microalgae are their high growth rate all year round (1, 3), higher photon conversion efficiency (3, 4), tolerance to adverse environmental conditions (2, 3), and versatility in fuel production (1, 2, 3, 4). The following traits suggest that microalgae would make an ideal biofuel in terms of ecological sustenance and economic efficiency.
A key focus has been placed on biodiesel as a suitable application for the properties of microalgae. Biodiesel remains a relatively untapped market as provisions from oil crops and animal fats only contributed to .3% of the 2008 demand for transport fuels (3). Li (1) in addition to Schenk (3) share the calculative consensus that the pervading issue is the improbability that biomass from current biodiesel products can handle mainstream demand. Second generation biofuels are tasked with the responsibility of finding a solution to the issue of sustainability. Plants such as switchgrass have ideal growth qualities of rapid growth, high biomass density, and low nutrient requirements (4). Algae shares these qualities as they are able to produce 15-300 times more biodiesel oil than traditional food crops (5). In addition to having high throughput, microalgae have a short harvesting cycle that usually lasts 1-10 days, allowing for continuous harvests (5). Schenk (3) remarks that even if all arable land on Earth (approximately 13% of total land) was used to grow current oil-producing crops for biofuels, it would not even amount to half of the energy consumed in 2008. Microalgae has been predicted to be able to keep up with the harsh demands of the economic market in terms of growing at a sustainable quantity for world consumption (3, 5, 6).
It is important to identify biological and biochemical elements of algae that explain its superior quality when it comes to biofuel production. A notable trait of microalgae is its high photosynthetic efficiency of approximately 10-20% compared to switchgrass which has an efficiency of .5% or less (1). A major focus of this paper will be on photosynthesis and how it is connected to bioenergy. The first process of photosynthesis involves the photolysis of water. Light harvesting complexes known as LHCI and LHCII gather photon energy from sunlight and transport it to photosystem II where it splits water into protons, electrons and oxygen: H20 2H+ + 2e- + 1/2O2 (3). In aerobic photosynthetic organisms, the hydrogen is transported into the thylakoid membrane where it is used to generate ATP while the electrons in conjunction with the hydrogen proton reduce NADP+ to produce NADPH (3). The energy molecules help drive carbon fixation during the Calvin Cycle and ATP is involved in Glycolysis. The primary relationship between substrate and product is therefore: (a) 2H+ --> ATP and (b) 2e- --> NADPH.
Studies on producing biohydrogen fuels have dealt with the manipulation of photolysis. In the case of green algae, Chlamydomonas reinhardtii undergoes a second reaction when it is exposed to anaerobic conditions: 2H+ + 2e- --> H2 (7). The environmental conditions result in the activiation of reversible hydrogenase, an enzyme that combines hydrogen protons and electrons in order to form molecular hydrogen (7). Schenk (3) explains that due to anaerobic conditions, NADPH and ATP are unable to be synthesized. Therefore, he views the enzymatic activity as a defense mechanism arguing that the second substrate 2e- is dangerous to the cell as it can cause over-reduction. In order to prevent this from occurring, the cell metabolizes stored starch in order to drive the hydrogenase enzyme which results in producing hydrogen gas.
The Calvin cycle is a mechanism that synthesizes glucose from the substrates of carbon dioxide and water. The process is powered by the light dependent reactions which supply ATP and NADPH in order to drive the reaction. According to Berg (8), the fixation step of the carbon cycle is a condensation reaction involving a molecule of CO2 and ribulose 1,5 – biphosphate. Even though a six carbon molecule is formed, the stereochemistry of the molecule and the adjoining phosphates result in it being an unstable compound and breaking in half to form two copies of 3 – phosphoglycerate. Berg notes that the second step is reduction as ATP and NADPH are added to the molecule in order to produce the main product of glyceraldehyde 3-phosphate (8). Glyceraldehyde 3-phosphate is an important intermediate molecule that can be used to synthesize glucose, starch, and cellulose.
Improving fuel efficiency via bioalcohol is important given the short-comings of ethanol. In order to improve output, cellulose is being experimented as a favorable alternative to reliance on sugar and starch. Cellulosic biomass consists of cellulose, hemicellulose, and lignin (4). As mentioned previously, glyceraldehyde 3-phosphate is produced from the Calvin cycle and can pair up with an identical molecule in order to form glucose. The energy consumed in this endergonic reaction is the phosphate on each molecule resulting in the reduction of ADP to ATP in order to stabilize the structure of the hexose, which will become glucose. The cell wall of algae and most plants is often carbon rich as cellulose is a polymer chain of glucose molecules that are bound by glycosidic bonds (4).
Cellulose is composed of glucose monomers attached in a β (1-4) configuration that can be broken down into individual glucose monomers in order to provide fuel. The difficulty lies in the heterogeneity of the compound (4). Sticklen (9) explains that in regards to bioalcohol in algae, cellulose and hemicellulose are the contributors to biofuel while lignin is an unnecessary component that cannot be processed. Lignin itself is composed of phenylpropanoid units which attribute to its compressive strength and the stiffness of the cell wall (4). It is not derived from sugar or starch and therefore cannot easily be used to produce bioalcohol.
Another type of variation exists within cellulose and hemicellulose which provide another hurdle in using algae as a bioalcohol. Hemicellulose contains large amounts of pentose sugars such as D – xylose and L – arabinose which cannot be fermented by popular micro-organisms such as Saccharomyces cerevisiae (4). Therefore algae is not efficiently converted into fuel during this process and may cause potential problems when it comes to actual use in transportation.
Another problem of using cellulose as a biofuel are cellulose degradation and biomass degradation (4). Cellulose degradation requires the application of acid in order to deconstruct the cell wall and hot temperatures are necessary in order to get rid of excess moisture from plants that impede on their fuel efficiency based on volume (10). In order to ferment cellulose under these abiotic stress factors (acid and heat), microorganisms such as Thermoanaerobacter BG1 have been selectively mutated in order to enhance their ability to metabolize carbohydrates (11). Adapting ideal micro-organisms for algae fermentation via biotechnological engineering is an area of recent research in order to solve the problems posed by unwanted heterogenous material.
The main focus on algae has been applying the organism to be used as a biodiesel renewable fuel source. As biodiesel uses oil as a substrate, it is highly desirable to use algae with high lipid content. Hu (2), notes that green algae represent ideal candidates for lipid production as they are ubiquitous, easy to isolate, and have rapid growth in laboratory conditions. Algae were primarily monitored for their production of triacylglycerol (TAG), an esteric compound that consists of a glycerol bound to three fatty acids.
Fatty acids are the source of energy when it comes to fuel from TAG. At the head of a molecule is an acidic carboxyl group (-COOH) that contains a long hydrocarbon chain attached to the end. As mentioned previously, algae have high photosynthetic efficiency. In terms of TAG production, high photosynthetic efficiency is ideal as de novo synthesis of fatty acids occurs in the chloroplast (2). A major component in lipid synthesis is acetyl CoA, a derivative of pyruvate. The Calvin cycle has particular importance in this process as it produces glyceraldehyde 3-phosphate (G3P) which is an intermediate compound in the glycolytic mechanism. Based on the needs of algae, it is possible to either proceed endergonically with the substrate and produce glucose or to exergonically produce pyruvate.
It has been noted (2, 5) that lipid accumulation is triggered by a depletion of an important nutrient or environmental stress. Hu (2) notes that under optimal conditions of growth, algae only synthesize 5-20% of their dry weight. This value is much lower than lipid production under unfavorable conditions which result in 20-50% of their dry weight. The study analyzed the variation in lipds under both circumstances. Under good growth conditions, structural lipids were synthesized for use in the membrane. But when it came to bad growth conditions, storage lipids such as TAG were synthesized resulting in more concentrated content. The glycolytic pathway gives us a potential explanation to this phenomenon. When algae has the required nutrients, it invests energy on endergonic reactions that will improve the organism. But when algae finds itself in a resource scarce environment, it will proceed with exergonic reactions in order to supply itself with the energy necessary in order to survive. Glucose is a primary component used in the construction of the cell wall and pyruvate is a derivative of acetyl CoA responsible for TAG production from G3P (6).
The first step in the formation of fatty acids is the formation of malonyl CoA from acetyl CoA after being catalyzed by acetyl CoA carboxylase (2). It is also possible that G3P and acetyl CoA can be synthesized to eventually form the TAG product (2). Photosynthesis therefore has importance in the production of biofuels as it provides G3P substituents necessary to drive the reaction and produce fatty acids essential for TAG.
Fatty acids can exist as a saturated (a) and an unsaturated compound (b). The criteria for these compounds are dependent on the nature of the hydrocarbon chain that follows from the carboxyl group. Saturated compounds (a) contain no double bonds whereas unsaturated compounds (b) are distinguished by the prevalence of a single double bond. The existence or lack of existence of a double bond determines different chemical properties of the fatty acid (8). In addition, the length of the hydrocarbon chain also distinguishes fatty acids apart from one another (8). It is important to mention the different possibilities in fatty acid composition as they make a difference in the nature of the biodiesel that is produced.
According to Hu (2), saturated fats lead to biodiesel with high oxidative stability and cetane number, but have poor temperature tolerance. Unsaturated fats have desirable cold-flow properties but are particularly sensitive when it comes to oxidation. Fatty acids primarily effect the type of biodisel that is produced and not the actual throughput.
Algae productivity under stress environments (particularly lacking nitrogen (2)), result in algae having the potential to produce 8-24 times more lipids than the best land plants. Durrett (6) notes that biodiesel has a 25% higher energy content per volume when it comes to using TAG lipids which results in greater supplies of fuel for the economy. Also important is that the highly oxygenated state of biodiesel results in lower levels of carbon monoxide (CO) and the emission of greenhouse gases (6).
In conclusion, algae has been established to be an ideal biofuel crop for its high growth rate all year round and its biochemical properties. Even though it is important to meet supply demands of fossil fuel, it is also important to consider the ecological impact that algae could have on the environment. A major problem with the production of ethanol, a bioalcohol, was the fact that it would use up arable land and would cost the price of the food crop being used to become more expensive in the market (10). In addition, the high quantity of crops resulted in a heavy demand for freshwater, a resource that is also important for human sustenance (5). Algae is an ideal second generation biofuel as it does not need to contend for arable land space. Open-pond systems that utilize algae can be located in the desert as long as it is within the quantity of desiccation tolerance for the species being used (3). Another type of energy plan is the use of algae in wastewater environments. Algae can consume nitrogen and phosphorous from contaminated water, therefore allowing freshwater to be used for crop production and human consumption (1).
Even though algae appear to be an ideal species, there is no firm way to gauge whether or not it will succeed in reality. Many critics argue that it would be better to invest more energy on increasing the productivity of ethanol, rather than spend time on an alternative solution. In the case of bioalcohol this holds true as many requirements need to be fulfilled in order to utilize algae as a biofuel (4). In cases such as biodiesel, algae appears to have several strong characteristics that make it a viable solution (3, 5, 6, 9). It is important to get algae out on the market in order to gauge its strengths and weaknesses. Assessments can then be made on its realistic application as a biorenewable fuel source.
(1) Li, Y., Horsman M., Wu N., Lan C., and Calero-D. (2008) Biofuels from Microalgae. Biotechnology Progress. Volume 24 Issue 4: 815-820.
(2) Hu, Q., Sommerfeld, M., Jarvis, E., Ghirardi, M., Posewitz, M., Seibert, M., and Darzins, A. (2008) Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances. The Plant Journal 54: 621-639.
(3) Schenk, P., Thomas-Hall, S., Stephens, E., Marx, U., Mussgnug, J., Posten, C., Kruse, O., and Hankamer, B. (2008) Second Generation Biofuels: High – Efficiency Microalgae for Biodiesel Production. Bioenerg. Res. 1:20-43.
(4) Rubin, E. (2008) Genomics of cellulosic biofuels. Nature Volume 454 Issue 14: 841-845.
(5) Chisti, Y. (2007) Biodiesel from microalgae. Biotechnology Advances Volume 25 Issue 3: 294-306.
(6) Durrett, T., Benning, C., Ohlrogge, J. (2008) Plant triacylglycerols as feedstocks for the production of biofuels. The Plant Journal 54: 593-607.
(7) Ghirardi, M., Zhang, L., Lee, J., Flynn, T., Seibert, M., Greenbaum, E., and Melis, A. (2000) Microalgae: a green source of renewable H2. TIBTECH Vol. 18: 506-511.
(8) Berg, J., Tymockzko, J., and Stryer, L. (2002, 2007) Biochemistry Sixth Edition: 433-450.
(9) Sticklen, M. (2008) Plant Genetic Engineering for biofuel production: towards affordable cellulosic ethanol. Nature reviews genetics 9: 433-443.
(10) Miller, J. (2002) Biosynthesis of Major Food Products: 9-10.
(11) Georgieva, T., Mikkelsen, M., and Ahring, B. (2007) High ethanol tolerance of the thermophilic anaerobic ethanol producer Thermoanaerobacter BG1L1. Central European Journal of Biology: 364-377.
(12) The University of Arizona Department of Chemistry and Biochemistry. <http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/LIPIDS/Fig11_1abFattyAcids.GIF> Date accessed: April 22, 2010.
A key focus has been placed on biodiesel as a suitable application for the properties of microalgae. Biodiesel remains a relatively untapped market as provisions from oil crops and animal fats only contributed to .3% of the 2008 demand for transport fuels (3). Li (1) in addition to Schenk (3) share the calculative consensus that the pervading issue is the improbability that biomass from current biodiesel products can handle mainstream demand. Second generation biofuels are tasked with the responsibility of finding a solution to the issue of sustainability. Plants such as switchgrass have ideal growth qualities of rapid growth, high biomass density, and low nutrient requirements (4). Algae shares these qualities as they are able to produce 15-300 times more biodiesel oil than traditional food crops (5). In addition to having high throughput, microalgae have a short harvesting cycle that usually lasts 1-10 days, allowing for continuous harvests (5). Schenk (3) remarks that even if all arable land on Earth (approximately 13% of total land) was used to grow current oil-producing crops for biofuels, it would not even amount to half of the energy consumed in 2008. Microalgae has been predicted to be able to keep up with the harsh demands of the economic market in terms of growing at a sustainable quantity for world consumption (3, 5, 6).
It is important to identify biological and biochemical elements of algae that explain its superior quality when it comes to biofuel production. A notable trait of microalgae is its high photosynthetic efficiency of approximately 10-20% compared to switchgrass which has an efficiency of .5% or less (1). A major focus of this paper will be on photosynthesis and how it is connected to bioenergy. The first process of photosynthesis involves the photolysis of water. Light harvesting complexes known as LHCI and LHCII gather photon energy from sunlight and transport it to photosystem II where it splits water into protons, electrons and oxygen: H20 2H+ + 2e- + 1/2O2 (3). In aerobic photosynthetic organisms, the hydrogen is transported into the thylakoid membrane where it is used to generate ATP while the electrons in conjunction with the hydrogen proton reduce NADP+ to produce NADPH (3). The energy molecules help drive carbon fixation during the Calvin Cycle and ATP is involved in Glycolysis. The primary relationship between substrate and product is therefore: (a) 2H+ --> ATP and (b) 2e- --> NADPH.
Studies on producing biohydrogen fuels have dealt with the manipulation of photolysis. In the case of green algae, Chlamydomonas reinhardtii undergoes a second reaction when it is exposed to anaerobic conditions: 2H+ + 2e- --> H2 (7). The environmental conditions result in the activiation of reversible hydrogenase, an enzyme that combines hydrogen protons and electrons in order to form molecular hydrogen (7). Schenk (3) explains that due to anaerobic conditions, NADPH and ATP are unable to be synthesized. Therefore, he views the enzymatic activity as a defense mechanism arguing that the second substrate 2e- is dangerous to the cell as it can cause over-reduction. In order to prevent this from occurring, the cell metabolizes stored starch in order to drive the hydrogenase enzyme which results in producing hydrogen gas.
The Calvin cycle is a mechanism that synthesizes glucose from the substrates of carbon dioxide and water. The process is powered by the light dependent reactions which supply ATP and NADPH in order to drive the reaction. According to Berg (8), the fixation step of the carbon cycle is a condensation reaction involving a molecule of CO2 and ribulose 1,5 – biphosphate. Even though a six carbon molecule is formed, the stereochemistry of the molecule and the adjoining phosphates result in it being an unstable compound and breaking in half to form two copies of 3 – phosphoglycerate. Berg notes that the second step is reduction as ATP and NADPH are added to the molecule in order to produce the main product of glyceraldehyde 3-phosphate (8). Glyceraldehyde 3-phosphate is an important intermediate molecule that can be used to synthesize glucose, starch, and cellulose.
Improving fuel efficiency via bioalcohol is important given the short-comings of ethanol. In order to improve output, cellulose is being experimented as a favorable alternative to reliance on sugar and starch. Cellulosic biomass consists of cellulose, hemicellulose, and lignin (4). As mentioned previously, glyceraldehyde 3-phosphate is produced from the Calvin cycle and can pair up with an identical molecule in order to form glucose. The energy consumed in this endergonic reaction is the phosphate on each molecule resulting in the reduction of ADP to ATP in order to stabilize the structure of the hexose, which will become glucose. The cell wall of algae and most plants is often carbon rich as cellulose is a polymer chain of glucose molecules that are bound by glycosidic bonds (4).
Cellulose is composed of glucose monomers attached in a β (1-4) configuration that can be broken down into individual glucose monomers in order to provide fuel. The difficulty lies in the heterogeneity of the compound (4). Sticklen (9) explains that in regards to bioalcohol in algae, cellulose and hemicellulose are the contributors to biofuel while lignin is an unnecessary component that cannot be processed. Lignin itself is composed of phenylpropanoid units which attribute to its compressive strength and the stiffness of the cell wall (4). It is not derived from sugar or starch and therefore cannot easily be used to produce bioalcohol.
Another type of variation exists within cellulose and hemicellulose which provide another hurdle in using algae as a bioalcohol. Hemicellulose contains large amounts of pentose sugars such as D – xylose and L – arabinose which cannot be fermented by popular micro-organisms such as Saccharomyces cerevisiae (4). Therefore algae is not efficiently converted into fuel during this process and may cause potential problems when it comes to actual use in transportation.
Another problem of using cellulose as a biofuel are cellulose degradation and biomass degradation (4). Cellulose degradation requires the application of acid in order to deconstruct the cell wall and hot temperatures are necessary in order to get rid of excess moisture from plants that impede on their fuel efficiency based on volume (10). In order to ferment cellulose under these abiotic stress factors (acid and heat), microorganisms such as Thermoanaerobacter BG1 have been selectively mutated in order to enhance their ability to metabolize carbohydrates (11). Adapting ideal micro-organisms for algae fermentation via biotechnological engineering is an area of recent research in order to solve the problems posed by unwanted heterogenous material.
The main focus on algae has been applying the organism to be used as a biodiesel renewable fuel source. As biodiesel uses oil as a substrate, it is highly desirable to use algae with high lipid content. Hu (2), notes that green algae represent ideal candidates for lipid production as they are ubiquitous, easy to isolate, and have rapid growth in laboratory conditions. Algae were primarily monitored for their production of triacylglycerol (TAG), an esteric compound that consists of a glycerol bound to three fatty acids.
Fatty acids are the source of energy when it comes to fuel from TAG. At the head of a molecule is an acidic carboxyl group (-COOH) that contains a long hydrocarbon chain attached to the end. As mentioned previously, algae have high photosynthetic efficiency. In terms of TAG production, high photosynthetic efficiency is ideal as de novo synthesis of fatty acids occurs in the chloroplast (2). A major component in lipid synthesis is acetyl CoA, a derivative of pyruvate. The Calvin cycle has particular importance in this process as it produces glyceraldehyde 3-phosphate (G3P) which is an intermediate compound in the glycolytic mechanism. Based on the needs of algae, it is possible to either proceed endergonically with the substrate and produce glucose or to exergonically produce pyruvate.
It has been noted (2, 5) that lipid accumulation is triggered by a depletion of an important nutrient or environmental stress. Hu (2) notes that under optimal conditions of growth, algae only synthesize 5-20% of their dry weight. This value is much lower than lipid production under unfavorable conditions which result in 20-50% of their dry weight. The study analyzed the variation in lipds under both circumstances. Under good growth conditions, structural lipids were synthesized for use in the membrane. But when it came to bad growth conditions, storage lipids such as TAG were synthesized resulting in more concentrated content. The glycolytic pathway gives us a potential explanation to this phenomenon. When algae has the required nutrients, it invests energy on endergonic reactions that will improve the organism. But when algae finds itself in a resource scarce environment, it will proceed with exergonic reactions in order to supply itself with the energy necessary in order to survive. Glucose is a primary component used in the construction of the cell wall and pyruvate is a derivative of acetyl CoA responsible for TAG production from G3P (6).
The first step in the formation of fatty acids is the formation of malonyl CoA from acetyl CoA after being catalyzed by acetyl CoA carboxylase (2). It is also possible that G3P and acetyl CoA can be synthesized to eventually form the TAG product (2). Photosynthesis therefore has importance in the production of biofuels as it provides G3P substituents necessary to drive the reaction and produce fatty acids essential for TAG.
Fatty acids can exist as a saturated (a) and an unsaturated compound (b). The criteria for these compounds are dependent on the nature of the hydrocarbon chain that follows from the carboxyl group. Saturated compounds (a) contain no double bonds whereas unsaturated compounds (b) are distinguished by the prevalence of a single double bond. The existence or lack of existence of a double bond determines different chemical properties of the fatty acid (8). In addition, the length of the hydrocarbon chain also distinguishes fatty acids apart from one another (8). It is important to mention the different possibilities in fatty acid composition as they make a difference in the nature of the biodiesel that is produced.
According to Hu (2), saturated fats lead to biodiesel with high oxidative stability and cetane number, but have poor temperature tolerance. Unsaturated fats have desirable cold-flow properties but are particularly sensitive when it comes to oxidation. Fatty acids primarily effect the type of biodisel that is produced and not the actual throughput.
Algae productivity under stress environments (particularly lacking nitrogen (2)), result in algae having the potential to produce 8-24 times more lipids than the best land plants. Durrett (6) notes that biodiesel has a 25% higher energy content per volume when it comes to using TAG lipids which results in greater supplies of fuel for the economy. Also important is that the highly oxygenated state of biodiesel results in lower levels of carbon monoxide (CO) and the emission of greenhouse gases (6).
In conclusion, algae has been established to be an ideal biofuel crop for its high growth rate all year round and its biochemical properties. Even though it is important to meet supply demands of fossil fuel, it is also important to consider the ecological impact that algae could have on the environment. A major problem with the production of ethanol, a bioalcohol, was the fact that it would use up arable land and would cost the price of the food crop being used to become more expensive in the market (10). In addition, the high quantity of crops resulted in a heavy demand for freshwater, a resource that is also important for human sustenance (5). Algae is an ideal second generation biofuel as it does not need to contend for arable land space. Open-pond systems that utilize algae can be located in the desert as long as it is within the quantity of desiccation tolerance for the species being used (3). Another type of energy plan is the use of algae in wastewater environments. Algae can consume nitrogen and phosphorous from contaminated water, therefore allowing freshwater to be used for crop production and human consumption (1).
Even though algae appear to be an ideal species, there is no firm way to gauge whether or not it will succeed in reality. Many critics argue that it would be better to invest more energy on increasing the productivity of ethanol, rather than spend time on an alternative solution. In the case of bioalcohol this holds true as many requirements need to be fulfilled in order to utilize algae as a biofuel (4). In cases such as biodiesel, algae appears to have several strong characteristics that make it a viable solution (3, 5, 6, 9). It is important to get algae out on the market in order to gauge its strengths and weaknesses. Assessments can then be made on its realistic application as a biorenewable fuel source.
(1) Li, Y., Horsman M., Wu N., Lan C., and Calero-D. (2008) Biofuels from Microalgae. Biotechnology Progress. Volume 24 Issue 4: 815-820.
(2) Hu, Q., Sommerfeld, M., Jarvis, E., Ghirardi, M., Posewitz, M., Seibert, M., and Darzins, A. (2008) Microalgal triacylglycerols as feedstocks for biofuel production: perspectives and advances. The Plant Journal 54: 621-639.
(3) Schenk, P., Thomas-Hall, S., Stephens, E., Marx, U., Mussgnug, J., Posten, C., Kruse, O., and Hankamer, B. (2008) Second Generation Biofuels: High – Efficiency Microalgae for Biodiesel Production. Bioenerg. Res. 1:20-43.
(4) Rubin, E. (2008) Genomics of cellulosic biofuels. Nature Volume 454 Issue 14: 841-845.
(5) Chisti, Y. (2007) Biodiesel from microalgae. Biotechnology Advances Volume 25 Issue 3: 294-306.
(6) Durrett, T., Benning, C., Ohlrogge, J. (2008) Plant triacylglycerols as feedstocks for the production of biofuels. The Plant Journal 54: 593-607.
(7) Ghirardi, M., Zhang, L., Lee, J., Flynn, T., Seibert, M., Greenbaum, E., and Melis, A. (2000) Microalgae: a green source of renewable H2. TIBTECH Vol. 18: 506-511.
(8) Berg, J., Tymockzko, J., and Stryer, L. (2002, 2007) Biochemistry Sixth Edition: 433-450.
(9) Sticklen, M. (2008) Plant Genetic Engineering for biofuel production: towards affordable cellulosic ethanol. Nature reviews genetics 9: 433-443.
(10) Miller, J. (2002) Biosynthesis of Major Food Products: 9-10.
(11) Georgieva, T., Mikkelsen, M., and Ahring, B. (2007) High ethanol tolerance of the thermophilic anaerobic ethanol producer Thermoanaerobacter BG1L1. Central European Journal of Biology: 364-377.
(12) The University of Arizona Department of Chemistry and Biochemistry. <http://www.biochem.arizona.edu/classes/bioc462/462a/NOTES/LIPIDS/Fig11_1abFattyAcids.GIF> Date accessed: April 22, 2010.
![[Image: Algae2.jpg]](http://igutek.scripts.mit.edu/terrascope/Algae2.jpg)