Illustration of cellulosic biomass processing today: 1) biomass harvested; 2) enzyme treatment; 3) enzymes break down cellulose chains into sugars; 4) microbes ferment sugars into ethanol; 5) ethanol is purified. To make the process more cost-effective process, the BESC project would use biotechnology approaches to combine steps 3 and 4. Diagram: Department of Energy BioEnergy Science Center (BESC), led by Oak Ridge National Laboratory in Oak Ridge, Tennessee.
Biofuel or bioenergy?
Although biofuel is the most familiar term when the public considers alternatives to fossil fuels, bioenergy is more encompassing. “The term bioenergy includes all useful forms of energy that can be extracted from living organisms or their residues, such as crop residues and wastes.”1 The challenge is how to meet the needs of larger-scale production of biofuel, because many believe that replacing petroleum fuel with biofuel might lead to deforestation, among other possible negative environmental impacts, which can also include the loss of wildlife habitat.1 It may be feasible, nonetheless, to produce biofuel on a larger scale by putting more effort into the production of bioenergy from natural sources that do not require large tracts of new land or the use of existing trees. Developers could use crop residues and waste, or they could cultivate fast-growing trees specifically for harvest, for example.
Are there alternatives to corn seed ethanol?
In 2007 and 2008, the ethanol biofuel industry used about 16% of the U.S. corn crop for ethanol production.2 As of May 2008, there were 134 ethanol plants in operation in the U.S., with 77 additional plants under construction.3 These enterprises convert the starch of corn seeds into fermentable sugar using a microbial amylase enzyme and then ferment the sugar into biofuel ethanol. This method converts food (starch) into fuel. Producing ethanol from corn seeds is not new—a blend of corn gasoline (10% ethanol) was commercially produced and sold at a gas station in the U.S. as early as 1933.4
Today, better alternative sources for biofuels can come from:
- agronomic crop residues such as leaves, stems, corncobs, and rice husks, and
- fast-growing perennial grasses, such as switchgrass, giant reeds, and Miscanthus (Miscanthus floridulus).5
Biofuel of this type is called cellulosic biofuel, and it is a way of turning cellulose-containing plant material, including agricultural byproducts, into bioenergy. Cellulose is the most common organic compound on the planet,6 which means that making ethanol from cellulose expands the range of natural resources available for the production of biofuels.
As mentioned earlier, the production of cellulosic biofuel has been around for a long time. In his September 1925, interview with the New York Times, Henry Ford said that there is energy for biofuels in any vegetation that grows wild or domestically.6 Unfortunately, the current costs [economical and environmental] to produce cellulosic biofuel are still relatively high.
Harnessing plant bioenergy
Currently, bioethanol can be produced from the seeds of sorghum, the number two bioethanol crop in the USA, but scientists are experimenting with using the whole plant. Photo: University of Georgia, Bioenergy Program.
Plants for use in bioenergy can be considered green bioreactors. A functionally active gene normally produces a protein. If the sequence of a gene from one living organism produces a protein with a useful function, and the gene is then transferred from this donor organism to another living organism (host organism), the host should be able to produce the same useful protein that is normally produced in the donor. This is how transgenic plants [plants that have received one or more transferred genes] become green bioreactors that can be used for the inexpensive production of valuable compounds. For example, scientists have already engineered plants for the production of cellulases—major biofuel industrial enzymes—that can make biofuel production cost effective.5 Organisms that have been engineered in this way are called recombinant.
The technology for transfer of foreign genes into a plant’s genome is very similar to the technology that has been used for decades to produce biotech drugs. Instead of using bacterial cells that take up the donor genes, plant biotechnologists either use Agrobacterium tumefaciens,9 a species of gram-negative bacteria that naturally transfers genes into plants, or they use the gene-gun method. Transferring genes using the gene-gun method (also called the biolistic method) involves firing at plant cells pellets of metal (usually tungsten powder or gold powder) coated with the desirable DNA.
What are cellulosic biofuels?
Plant cell walls contain soft fibers and hard fibers. The soft fibers mostly consist of cellulose—a chain of tightly bonded glucose molecules. The hard fibers are mostly lignins, and these bind to the cellulose and strengthen the cell walls. The glucose elements of cellulose can be separated from each other to yield single glucose molecules, which can then be fermented into biofuel.
Currently, microbial reactors produce a mixture of cellulases commercially (see Figure 1). These are utilized for the manufacturing of fermentable sugars for cellulosic biofuel. Together with pretreatment processes that break apart the cell walls and remove lignin,10 this method adds to the overall cost of biofuel production greatly. To reduce the expenses associated with pretreatment processes, producers could
reduce the amount of lignin in bioenergy crops via crop genetic engineering in a way that would not interfere with the crop’s structural integrity and resistance to biological stress factors such as diseases and insects;
change lignin’s chemical structure to reduce the need for expensive pretreatment processes;5,11
or, produce the required cellulase enzymes within the leaves and stems of bioenergy crops via crop genetic engineering. In fact, these enzymes have already been produced in corn and rice leaves and stems (not in seeds, roots, or flowers), and these plant-produced cellulases can convert pretreated corn stover into fermentable sugars for biofuel effectively.12,13,14,15,16,17
Can the industry be sustainable?
Even if researchers can engineer recombinant bioenergy crops with modified/reduced lignin content and produce all of the required cellulases, and figuring in government subsidies to the industry, the cost of cellulosic ethanol would still be at least as high as the current price of petroleum. This is not good enough to create sustainable cellulosic biofuel commercially or to reduce the U.S. dependence on foreign oil.
The petroleum industry makes its profit not only from its main products, including gasoline, ethane, kerosene, and natural gas, but also from petroleum derivatives, or co-products, such as lubricants, asphalt, petroleum coke, and paraffin wax.18 To make cellulosic biofuel profitable and competitive with corn ethanol (corn seed starch ethanol), co-producing recombinant high-value products such as biodegradable plastics can increase returns from bioenergy crop farming and the biofuel industry.11,12
POET, the largest bioethanol producer in the world, was the first cellulosic ethanol company in the United States to produce 25% cellulosic ethanol and 75% corn ethanol (the company began production in the third quarter of 2009).19 Other cellulosic biofuel plants are planned in the U.S. in the near future. For example, switchgrass is a relatively fast-growing perennial grass that is adaptable to almost all parts of the U.S., and it has great potential to produce cellulosic ethanol and butanol (an alcohol that is chemically related to ethanol and has potential as a biofuel).20 It is likely, nonetheless, that the producers will not profit from their cellulosic ethanol unless the U.S. government subsidizes them heavily, or they use genetically engineered cellulosic bioenergy crops.
Development cycle for the Sticklen group’s designer corn variety, Spartan Corn III, which is used in cellulosic biofuels production. Artwork by Michigan State University State News.
What are some other possible sources for bioenergy?
Engineering designer crops to produce molecules is over a decade old.5,7,11,19,21 This technology is called molecular farming, and it is used to manufacture several products, including biodegradable plastics.14,15,20,22,23 Plant scientists use molecular tactics to modify the plant genome to produce these products. For example, the Sticklen research team at the University of Michigan has programmed transgenic plants to store recombinant products in specific leaf and stem subcellular compartments so that they can be harvested later.13,14,15
Designer corn: The Sticklen team has also developed a designer corn variety—Spartan Corn III—for use in cellulosic biofuel production, by firing a gene of an anaerobic microbe, called Butyrivibrio fibrisolvens H17C, into corn cells using a gene gun and growing whole transgenic corn plants from the cells.24 The microbe lives naturally in the rumen, or second stomach, of cattle, where it helps the animals digest silage by producing an enzyme. Spartan Corn III can produce the same enzyme in its cells. In Spartan III, the recombinant enzyme is stored inside a subcellular compartment in the recombinant corn, called a vacuole, until harvest time (see Figure 2).
Rice components: It is possible to use molecular farming with rice straw or rice hull. In most parts of the world, rice straw is burned as waste, which can create environmental and health problems, such as asthma.17 Producing high-value co-products along with the conversion of rice straw to biofuel would make it economically feasible to decrease environmental contaminants such as smoke and improve the health conditions of rice farmers. Using rice straw and rice hull for biofuel could become very important in the future because these two crop residues make up 50% of the total of the world’s agronomic waste.17
Is biobutanol the future industry?
Many producers view corn bioethanol technology as a transitional operation until cellulosic ethanol technology can become economically competitive with corn ethanol; however, cellulosic bioethanol technology might soon be viewed as transitional, too. Scientists are researching systems that will bring the cost of production down for biobutanol production. While bioethanol is produced by fermenting glucose from yeast, biobutanol is produced by the fermentation of glucose using the Clostridium acetobutylicum bacterium.
Biobutanol has the following advantages over ethanol, and indeed, over petroleum fuels.25 Biobutanol
- evaporates six-fold less than ethanol and even less than gasoline;
- does not corrode like bioethanol, which is corrosive, and at high concentrations can destroy engine parts;
- can be transported through existing gasoline pipelines, unlike bioethanol (85% E), which must be moved by tankers;
- has energy content almost equal to the energy content of gasoline—the same cannot be said for bioethanol; and
- can be blended as an additive to diesel fuel, which then reduces soot emissions—the same is not possible with bioethanol.
Issues to consider
Converting food into fuel: The conversion of food and feed into fuel can increase the price of these two important commodities. In addition, land used to grow food competes with land used to grow fuel products. This problem can be mitigated by using non-food and non-feed bioenergy crops, such as perennial grasses, and the non-food and non-feed portions of agricultural crops, such as crop residues, for production of cellulosic biofuel.
Monoculture of bioenergy crops: When a bioenergy crop such as switchgrass or Miscanthus is farmed on a large scale, it will be farmed as a monoculture crop (large-scale cultivation of one crop year after year). Monoculture farming takes a toll on the environment because pests, diseases, and weeds remain for years in the fields. Farmers must apply pesticides, fungicides, and herbicides, which can contaminate soil and water. To solve this problem, a large percentage of monocultured agronomic crops today are genetically engineered to improve resistance to harmful biological and chemical factors. A bioenergy crop, switchgrass or Miscanthus for example, could still be used in monoculture, but it may first need to be genetically transformed to improve resistance to insects, diseases, and herbicides.12
Pollen grain flow: Another concern about genetically engineered crops is the possible spread of pollen grains from one field to another.26 Scientists are looking into solutions. One solution under study, for example, is to produce biotech products only in crop leaves and stems—not in the flowers or seeds.16
Soil microbe populations: Soil microbes need to feed on sufficient crop residues in soil. Biofuel producers must consider methods that do not cause a problem by removing too much residue. For example, a suggestion has been made to keep at least two-thirds of plant residues on the soil to ensure the sustainability of a microbial population.27 Another report suggests the development and use of advanced cropping systems or producing and removing of biomass from soil without “undermining crop and soil productivity.”28 The U.S. National Renewable Laboratory (NREL) has also been heavily involved with the effect of corn stover removal on emissions of carbon from soil.29
Although there are many possibilities for producing biofuel from the agricultural sector, and therefore decreasing our dependence on petroleum fuels, it is important for scientists, communities, and governments to examine the potential side effects and consequences of utilizing plants and agricultural lands for this purpose.
Acknowledgment: The author’s research was financially supported by the Consortium for Plant Biotechnology Research (CPBR), Corn Marketing Program of Michigan (CMPM), National Corn Growers Association(NCGA), and a grant from USDA to Iowa State University (PI: Prof. Jeffery Wolt; Co-PI: Miriam Sticklen). The author wishes to thank Dr. Russ Freed for editing this article.
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