Numbering some 70, species , microalgae play an outsize role in balancing marine ecosystems and regulating global nutrient cycles. To transform these tiny plants into fuel, researchers typically grow microalgae in large, open ponds or in enclosed photobioreactors, although at scale they would likely need to be grown in the open air. Most research funding has been shuttled into improving these processes, minimizing inputs and maximizing outputs with the aim of making large-scale algae production commercially viable.
ExxonMobil claimed to have made a big step toward this goal in Working with biotech company Synthetic Genomics — which ExxonMobil helped to found in — ExxonMobil announced it had created a genetically modified strain of microalgae that could produce double the lipids without significantly inhibiting growth.
This innovation could theoretically allow the company to afford an enormous scale-up of its biofuel production. In fact, ExxonMobil was confident enough in its breakthrough that it claimed gene editing could allow the production of a startling 10, barrels of algae biofuel per day by Yet while ExxonMobil celebrates these supposed breakthroughs in genetic engineering, other energy companies have long since given up on algae biofuels.
For example, Algenol shifted to carbon capture and fresh water creation in , and Chevron-backed Solazyme announced in that it would discontinue its biofuels program altogether. It seems that only ExxonMobil retains the dizzied excitement over algae biofuels of decades past. Despite industry optimism, decades of research seem to have converged upon a disappointing reality: The economic and biological limitations of algae make it an unrealistic fuel alternative for the future.
It requires too much fertilizer, too much water, and too much energy to produce at scale. To grow enough algae to meet 5 percent of the U. And the industrial processes needed to actually convert microalgae into fuel could actually cause a net energy loss: Algae could take up to 53 percent more energy to produce than it would offer as a biofuel.
With these considerations in mind, think tanks and research bodies around the world are advising governments against intensifying algae biofuel research, especially when funding could compete with more promising research into renewables like solar and wind.
What are Algae? History as Fuel The idea of using algae as a source of food, feed and energy goes back more than half a century. Video Industry exec Dr. Margaret McCormick reviews historical uses of algae.
Test Your Knowledge! Considering the theoretical fermentation yields on biomass sugars and the energy content of ethanol, this projection also establishes the theoretical maximum production of bio-based gasoline equivalents at close to 96 billion gallons. Since the United States uses approximately billion gallons of gasoline, 40 billion gallons of road diesel and 20 billion gallons of jet fuel all derived from crude oil per year, it is clear that biofuels based on terrestrial feedstocks can never meet that demand.
At the National Renewable Energy Laboratory NREL , where we conduct our research, we concluded the same thing when the original billion-ton study was released. That prompted us to rebuild the Aquatic Species Program, previously funded from to by the U. Department of Energy, to evaluate the potential of algae-based biofuels. Figure 1. Above are two green algae types, the star-shaped Pediastrum duplex and the water- bug-shaped Scenedesmus sp. The relatively high lipid content found in algal biomass holds great promise as a possible biofuel feedstock.
But multiple challenges stand between today and a time when algae-derived fuels are routinely pumped into fuel tanks. We are confident that lipids derived from algae hold great promise as a supplemental biofuel feedstock. Algae have many inherent advantages in this context, with the high-lipid content found in some species being a fundamental edge. Also, since microalgae are not a common food source, algal cultivation for fuel is unlikely to interfere with food production at the levels that cultivation of other feedstocks, such as corn, might.
Because algae grow in many different environments, it could be produced on acreage that is not agriculturally productive. Algae farming could also make use of multiple types of water: fresh, brackish, saline and wastewater. It is widely believed—though research is needed to confirm this—that the use of algal-based fuel would result in a tiny fraction of the net greenhouse gases that can be traced to fossil-fuel use today. And scaling up algae farming could lead to yields of other commercially viable products besides fuel.
All this promise is conditional, of course. Highly productivity algal strains must be identified. New and reliable algae-farming methods must be developed.
Hyper-efficient systems for extracting lipids and any other commercial products grown in algae must be invented too. If all that can be accomplished, there is still one potential deal breaker. All of this must be done at a cost that makes algae-derived biofuel competitive with petroleum-based fuels.
Research at NREL is attempting to address some of these challenges. Macroalgae, the seaweeds, grow in open waters, both fresh and marine. These aquatic plants are made up mainly of carbohydrates and have been harvested for centuries as food, including the nori used to wrap sushi, and thickening agents such as agar. The Aquatic Species Program explored the potential of macroalgae as fuel but dropped that project due to the significant challenges related to harvesting costs and fuel conversion.
Microalgae, on the other hand, are unicellular photosynthetic microorganisms. They are ubiquitous in nature, found in freshwater, seawater, hypersaline lakes and even in deserts and arctic ecosystems. They can be further subdivided into two main categories: eukaryotic algae, possessing defined organelles such as nuclei, chloroplasts, mitochondria and so on, and prokaryotic algae cyanobacteria or blue-green algae , possessing the simpler cellular structure of bacteria.
Although the relatedness of cyanobacteria to nonphotosynthetic bacteria allows for exploitation of genetic-engineering technologies and makes them an attractive starting point for biofuels research, they lack one very important thing that eukaryotic microalgae can possess in abundance—neutral lipids, which are rich in triacylglycerols TAGs.
Figure 2. Above are photomicrographs of algal cells that Lee Elliott, a doctoral candidate at the Colorado School of Mines and a National Renewable Energy Laboratory researcher, isolated from water samples collected in the southwestern United States.
BODIPY fluoresces green when dissolved in lipid droplets, and thus can be used to indicate high lipid content in algal cells. Here, chlorophyll fluoresces red. Photomicrographs courtesy of Lee Elliott. Of the eukaryotic microalgae, green algae are the taxonomic group most often referred to as oleaginous, or oil-rich, microalgae.
They are ubiquitous in a variety of habitats and grow faster than species from other taxa, and as much as 60 percent of their cell dry weight can be oils. However, the composition of the oils is highly dependent on the species and the conditions in which the algae grow.
Oils that are rich in neutral lipids are desirable in a biofuel context because of their potential high fuel yield.
Because TAGs are made up of three molecules of fatty acids that are esterified—or altered—to one molecule of glycerol, close to percent of their weight can be converted into fuels. With polar lipids, on the other hand, only one or two fatty-acid molecules are esterified to glycerol and the remaining components e.
As a result, these types of lipids generate lower fuel yields. Figure 3. Only a small portion of algae species have been screened for their lipid content and only a share of those screened are considered oleaginous, meaning that lipids comprise 20 percent or more of their dry weight.
Among those screened so far, green algae species produce the most lipids, mostly in the form of triacyglycerol TAG , which functions as carbon and energy storage in algae cells. Studies have shown that some algal lipid production increases in stressed environments.
Above are the percentages of cellular lipid contents found in different types of microalage and cynobacteria under normal green and stressed red growing conditions, including nitrogen depletion or other deficits. Illustration by Barbara Aulicino. The Plant Journal. Fatty acids, the building blocks for lipids, are synthesized by enzymes in the chloroplast, of which acetyl-CoA carboxylase ACCase is key in regulating the synthesis rates. When cells are actively growing, their metabolic focus is on photosynthesis and the production of biomass.
The fatty acids produced are mostly found in polar-membrane lipids, such as phospho- and glycolipids, which are invaluable to photosynthesis. Unfortunately only about 30 to 50 percent of polar lipids can be converted into fuel molecules. But when the cells experience metabolic stress, such as a lack of essential nutrients, including nitrogen, cell metabolism is redirected to reduce the growth rate and favor the production of carbon-storage compounds, mainly carbohydrates and TAGs. Little is known about the regulation of TAG formation at the molecular and cellular level, but greater understanding could lead to the engineering of algae with higher ratios of neutral lipids.
Organic solvents can extract oils from actively growing cells. But oils extracted from stressed cells yield more fuel. In Chlorella vulgaris , a strain that our laboratory has studied extensively, the extracted oil content amounts to about 30 to 50 percent of the biomass under both active-growth and nutrient-limited conditions.
However, the fatty-acid content, reflecting the potential fuel yield, can vary from 10 to 50 percent of the biomass over the growth cycle.
This illustrates the big discrepancies often seen between the extracted algal oils and the actual fuel-yield potential. Unlike typical terrestrial oil-producing plants, in which specialized cells yield oils, every algal cell can produce oils. Algal oils, just like oils produced by soy, canola, palm and the less-known jatropha plants, can be made good biodiesel feedstocks through transesterification.
In that process, a catalyst creates a biodiesel fuel consisting of fatty acid methyl esters by hydrolyzing and methylating fatty acids in the oils. Refining the mixture is typically the next step and involves removing the non—fatty-acid components—such as glycerol, polar lipids and residual pigments—from the fuel. If the scientists could keep the algae multiplying and pull the "lipid trigger" anyway, they'd be in fat city.
But their understanding of the biology was incomplete, and the task wasn't easy. It would take some time and effort to know if and when their the process would become cheap enough to compete with crude. Another challenge was getting the algae to keep growing without injecting a lot of energy into the system. They installed large open ponds near Roswell, New Mexico, and began trying to produce tiny algae at oil tanker scales.
It worked, but there were problems. Again, it would take some time and effort to know if and when everything would work together. The program did not get time or the money to find out.
By the time Bill Clinton took office, funding for the program had dwindled to a trickle, and in , the Department of Energy abandoned the program to focus all its biofuel efforts on ethanol. A dark decade fell upon the field of algal biofuel. There wasn't even money available to take care of the algal collection that had been so painstakingly created. In an effort to salvage some of the science, a few hundred strains of algae were sent to the University of Hawaii, but the refuge proved less than ideal.
When a National Science Foundation grant ran out in , it became difficult to continue the laborious work of maintaining the collection.
The organisms sit in rows of test tubes living and reproducing. Every two months, they have to be transferred, "passaged," to a new nutrient-rich tube.
Random genetic mutations can enter a population and lead to permanent genetic changes. The algae can die. It's not exactly clear how it happened, but a review released earlier this year found that more than half the genetic legacy.
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