March 2011
The alternative energy industry is poised for rapid global growth as attempts to reduce greenhouse gas emissions and dependence on nonrenewable fossil fuels gain traction. These efforts will be particularly intense in the transportation sector, where alternatives for car, truck, train, marine, and aviation fuels may still be decades away.1 Companies are therefore investing substantially in the conversion of plant products into renewable biofuels. The early diesel engine that inventor Rudolf Diesel demonstrated at the 1900 World’s Fair was powered by pure peanut oil, and it was soon discovered that plant oils can be modified to create better-performing fuels using simple chemical reactions (transesterification) that transform them into biodiesel.2 Diesel fuel already has a dominant position in the refined petroleum market, even in countries where standard gasoline is the primary liquid fuel, and diesel-powered vehicles dominate commerce and transportation worldwide.3
Although conventional biofuel production relies primarily on land plants as a feedstock, many researchers believe that biofuel production on the scale needed to compete with petroleum-based fuels on the open market will require the use of microscopic algae,4 which grow abundantly and naturally in the world’s surface waters and can be converted into multiple kinds of biofuel. Algae have the potential to produce sufficient quantities of biofuel to satisfy the world’s growing energy demands, even considering predicted limitations on the availability of land and water resources.5
What are algae, and how can they be used to produce biofuels?
Figure 1. Pediastrum, a colonial green alga
Algae are tiny, plantlike organisms that include the green alga Pediastrum (Figure 1). The algae used in biofuel production are freshwater algae, comprising both prokaryotic and eukaryotic species, that grow naturally in every freshwater creek, river, pond, lake, and reservoir on the Earth’s surface. Algae are also aquatic biomass production systems that have both a higher fuel yield potential and lower water demand than terrestrial plants, and they generate cellular products such as oils, starch, protein, and other marketable compounds.6 Like land plants, microalgae derive their energy from the biochemical process of photosynthesis, which captures the sun’s radiant energy and converts atmospheric carbon dioxide (CO2) into new cellular biomass. When measured in standard calorie units, the energy content of algae per unit weight does not differ significantly from that of land plants. In addition, although there can be considerable variation in their cellular oil content,7 all species of algae contain oil that can be extracted for use in biofuel production. To date, more than $1 billion in private sector funding has been committed to the development of algae-based fuels.8
The idea of using algae as a feedstock for biofuel production dates back at least 50 years, to when William J. Oswald and Clarence G. Golueke first proposed the use of “raceway ponds” to cultivate large quantities of algal biomass for fermentation to create methane gas.9 In such ponds, growing algae are moved along with paddles and then removed at the downstream end. Soon after the proposed use of raceway ponds, research in algae-derived bioenergy focused on the production of liquid fuels that can be combusted directly in standard internal combustion and jet engines,7 and large oil companies and research institutions have recently joined forces in commercial ventures to produce biodiesel from algae. However, algae can also be used to produce other renewable energy products, such as biohydrogen, hydrocarbons, and bioethanol; in addition, as noted above, the algal biomass itself can be processed to generate biogas.6,10
Alternatively, dried algae can be combusted directly, much like the burning of crop residues, wood, coal, or peat. This use of algae is important because the direct combustion of plant biomass is a sector of bioenergy already in development that takes advantage of existing commodity supply chains.1 Just like any energy commodity that can used for direct combustion, however, algal biomass would need to consistently meet several key criteria with respect to its energy, moisture, and undesirable pollutant content.1
Algae offer numerous significant benefits relative to their soil-grown counterparts:11,12
Algal cells can exhibit extremely rapid growth rates, doubling one to three times per day, and they can be grown abundantly in waters of widely varying chemical composition.
Algal cells can synthesize and accumulate large quantities of bioproducts (e.g., oil), that can be harvested and marketed to offset the costs of biofuel production.
Cultivating algae rather than land plants, such as corn, for bioenergy could reduce the diversion of agricultural crops away from vitally needed food production.
The land “footprint” needed to produce a given amount of bioenergy is much smaller for algae than for terrestrial biofuel crops.
Algae can be grown using effluents from domestic wastewater treatment plants and other sources of nontoxic liquid waste, which provide an abundant source of water and mineral nutrients that are required for algal growth.
If grown in wastewater streams, the water “footprint” needed to produce a given amount of bioenergy is much smaller for algae than for terrestrial biofuel crops.
Algae can provide an important ecosystem service by removing nitrogen, phosphorus, and other contaminants from wastewater feeds.
Algae can also be used to remove carbon dioxide from high-CO2 gas streams, such as flue gases and flaring gases, that can be piped to algal biofuel production facilities from nearby energy generation plants.
Algal biomass yields can be optimally maintained by modifying harvesting rates.
The ability of algae to grow continuously in many climates may help reduce the strong seasonality of biomass yields currently seen with terrestrial biofuel crops.
Treating algal biofuel production units as engineered ecosystems
Humans have had centuries of experience in the successful mass cultivation of land plants. In contrast, the large-scale cultivation and marketing of algae as a major commodity is still in its infancy. The individuals, consortia, and companies who become involved in the commercial production of biofuels from algae will want to maximize the annual yield of algal biomass that can be profitably harvested and converted into a marketable bioenergy product. A recent life-cycle assessment comparing algal cultivation in open raceway ponds to that in closed photobioreactors that use air to circulate the growing cells suggests that raceway-based biodiesel production is likely to have a lower overall global warming potential and fossil-energy requirement.13 Moreover, maintaining stable, single-species algal cultures at full commercial production scale would likely be exceptionally challenging for closed photobioreactor cultivation systems, which could require millions of gallons of water flow per day. Mass sterilization of such large volumes of inflowing water would be difficult, if not economically impractical, and undesirable “infections” of photobioreactors by low-yield algal strains would present a major risk.
For the large-scale generation of biofuels from algae, open pond-based production facilities must be treated as complex, bioengineered ecosystems that obey the known principles of ecology,12 similar to the freshwater ecosystems in which algae naturally grow. One key ecological principle is the observation that joint cultivation of multiple plant species (polyculture) typically provides a greater total biomass yield than cultivation of a single type of crop, a phenomenon known as “overyielding.”14,15 As a result, high-yield algal biofuel production may be achieved using open ecological systems that cultivate the multispecies algal assemblages occurring naturally in the surrounding region (Figure 2).12
Figure 2. A natural, mixed-species
assemblage of algae and zooplankton.
Daphnia are the large, spined
animals in this photograph.
Another key concept that applies to commercial-scale open-pond production is that of predator-prey interactions. Outbreaks of plant-eating insects, for example, threaten agricultural crops and cost billions of dollars annually in pest control across the globe. Similarly, tiny, naturally occurring, plant-eating aquatic animals (herbivorous zooplankton) that graze on the microalgae growing in raceway pond water can invade and become established within any open cultivation system. Because these ponds ultimately develop diverse communities containing both algae and zooplankton, they tend to exhibit oscillations in population numbers (common in predator-prey dynamics and resembling those of insect pests that reduce crop yields in standard agriculture).
However, a solution to this dilemma comes from the now well-known concepts of biomanipulation and top-down ecological control.16,17 Most gardeners know that one way to reduce losses to insect pests is to introduce a healthy population of an insect predator, like the ladybird beetle, into their garden; ladybird beetles dramatically reduce the abundance of nuisance insect species such as aphids, scale insects, mealy bugs, and mites.
Similarly, open- pond systems are exposed to invasion by a wide variety of other organisms, some of which are undesirable because they can greatly reduce algal biomass yields. Having too many large herbivorous zooplankton, such as Daphnia (Figure 2), in the water is like having too many cattle grazing in a rancher’s field: They eat almost all of the available plants. Like the garden food web mentioned above, aquatic food webs can be modified to minimize losses of algae. For example, adding a carnivorous fish such as the mosquitofish (Gambusia affinis) to the system removes the offending zooplankton and thereby maximizes the pond-grown algal biomass that can be harvested for use in biofuel production chains.12
Future limitations and issues to be solved
In the future, mass-cultivated algae may prove to be one of the best options for the commercial-scale production of enough bioenergy to replace a substantial fraction of annual fossil fuel usage.18 However, the complex logistics of biomass production, harvesting, storage, processing into marketable biofuels, and subsequent transportation represent a very large problem requiring massive capital and intellectual investments to meet society’s ever-increasing energy needs.1 This will be particularly true in the case of algae, because significant scientific, technological, and engineering challenges remain to be solved before algae can achieve their full potential as a biofuel feedstock.4,5,19 Mass cultivation facilities such as raceway ponds may contribute to the viable and cost-effective generation of renewable energy in many regions of the world. However, this is a goal that requires a concerted, synthetic approach integrating concepts and tools from multiple disciplines, including engineering, social sciences, and natural sciences. Foremost among these are the principles of ecology, which have been shown to apply in other bioengineered ecosystems.20,21 In addition, integrating algal biofuel production facilities with allied industries could improve resource management and minimize the ecological footprint of algal biofuel production.22
Acknowledgments: The author’s research has been financially supported by the University of Kansas Transportation Research Institute, and by an EPSCoR grant from the National Science Foundation. The author thanks the members of KU’s Feedstock to Tailpipe® Initiative and, most especially, algal production team members Dr. J. deNoyelles and Dr. B. Sturm. He also thanks Dr. R.O. Megard and Dr. J. Shapiro (University of Minnesota) for the photographs that are included in this article; two anonymous reviewers for their comments; and S. Sheehan for inviting this article.
© 2011, American Institute of Biological Sciences. Educators have permission to reprint articles for classroom use; other users, please contact editor@actionbioscience.org for reprint permission. See reprint policy.
