By Jesse Moore
Microalgae have a large capacity for producing lipids for biodiesel and carbohydrates for bioethanol. The potential for microalgae as biofuel feedstocks is high because of their high rate of productivity, the potentially high percentage of biomass composed of lipids or carbohydrates, and because they lack lignin. The absence of lignin production in most algae is a benefit because processing lignin is currently a major impediment for bioethanol production. Microalgae, in addition to being utilized for biofuel production, make other compounds that can be use make high value products, such as animal feeds and dietary supplements. Microalgae are of course not without their potential problems.
Microalgae are receiving more and more attention as the search for sustainable and profitable biofuel feedstocks progresses. Much of the current research involving microalgae is focused on the production of biodiesel. Lipids from various sources can be converted to biodiesel through the process of transesterification (Chisti 2007). Microalgae provide an excellent source of lipids for two major reasons. First, microalgae productivity can be an order of magnitude greater than terrestrial vegetation used for biofuel feedstocks (see Table 1). Second, the lipid content of microalgae can exceed 70% of their dry mass, although algae with lipid content of around 30% is more common (Chisti 2007). High productivity combined with high lipid content results in a large amount of lipid that can be harvested annually for biodiesel production.
Biodiesel is not the only biofuel underdevelopment, nor is it the most popular. Currently the proportion of the total U.S. transportation fuel comprised of bioethanol is 200 times greater than that comprised of biodiesel (Ragauskas et al. 2006). A variety of feedstocks and different molecules can be used in the production of bioethanol. First generation bioethanol is produced from the fermentation of monsaccharides from crops such as sugar cane and sugar beets. Second generation bioethanol converts cellulose and hemicelluloses to ethanol. Feedstocks for second generation bioethanol include crop residue, slash from forestry operations, and fast-growing grass species. One major impediment to large-scale second generation bioethanol production revolves around lignin, a structural component of plants that help them support their own weight. Lignin is a recalcitrant substance (meaning that it is not easily degraded) and currently cannot be converted to bioethanol. The lignin content of bioethanol feedstocks is typically burned to produce heat and energy for the bioethanol production facility (Ragauskas et al. 2006). Lignin can comprise a significant portion of plant biomass, this is however not the case for microalgae (see Table 1).
Most microalgae do not contain lignin; in fact, lignin was only recently discovered in red algae called Calliarthron cheilosporioides (Martone et al. 2009). Microalgae do not require the structural support that lignin provides because their mass can be supported by the water they live in (this is incidentally related to the high lipid content of microalgae which add buoyancy and keep them from sinking too quickly away from sunlight need for photosynthesis). The fact that microalgae produce fermentable carbohydrates without containing lignin makes them an attractive feedstock for bioethanol production (Dismukes et al. 2008). This is especially true when the superior productivity of microalgae is taken into account.
Table 1 illustrates the differences in productivity of three terrestrial crops and three microalgal crops. The microalgal crops have considerably higher productivity (measured in megagrams of dry weight produced annually per hectare). The fermentable carbohydrate productivity and lignin productivity are the products of productivity and percent fermentable carbohydrate and percent lignin, respectively. It is clear that microalgae have the potential to produce more fermentable carbohydrates annually per hectare than terrestrial feedstocks, despite containing a lower percentage of fermentable carbohydrates, because of their high rates of productivity. Table 1 also shows that the terrestrial feedstocks produce a considerable amount of lignin annually, which inhibits their efficient conversion to bioethanol. Using microalgae for bioethanol production effectively avoids the predicament dealing with lignin.
Research on how to efficiently convert microalgal biomass to bioethanol is being conducted. Hirano et al. are investigating a particularly interesting method of microalgal bioethanol production. They have demonstrated that under dark, anaerobic conditions the starch within the cells of microalgae will be fermented. This essentially means that ethanol can be made within the microalgal cells. The potential for simple, low cost methods of bioethanol production are apparent. The next phase of this research should be developing methods of improving the efficiency of this intracellular ethanol production.
Understanding the biology of microalgae is important to maximizing the production of biofuels from these organisms. For example, microalgae respond to surrounding conditions by producing different chemicals. When physical conditions are stressful or certain nutrients are lacking, microalgae respond by producing more carbohydrates or lipids. Whether carbohydrates or lipids are produced depends on the species of microalgae being grown (Dismukes et al. 2008). It is therefore possible to control the composition of the microalgal feedstock being produced by adjusting physical or nutritional conditions to favor the production of lipid for biodiesel or carbohydrates for bioethanol. There is, of course, a downside to this manipulation. Altering the conditions under which microalgae are grown to produce stress or a nutrient limitation, results in reduced growth rates for the microalgae (U.C. San Diego, Scripps Institute of Oceanography 2009). This is potentially a real problem considering that high rates of productivity are one of the greatest traits of microalgae, in terms of biofuel production. Research is being conducted to better understand the mechanism by which microalgae increase lipid and carbohydrate production, and to determine whether or not this can be controlled without impacting the growth rate of the microalgae (U.C. San Diego, Scripps Institute of Oceanography 2009).
In addition to utilizing the lipids and carbohydrates from microalgal biomass for biofuel production, it is also possible to make use of the other compounds present in microalgal biomass. Microalgal proteins can be use for fish feeds in aquaculture or animal feeds for livestock and poultry (Meng et al. 2009). Algae are currently grown commercially to produce profitable chemicals, such as antioxidants and dietary supplements. These chemicals comprise only a minor component of the microalgae cells, but have a high enough value to make their production profitable (Rosenberg et al. 2008). The cost of producing bioethanol and biodiesel from microalgae can be reduced by exploiting every component of microalgae biomass.
Microalgal biofuel production may appear faultless on the surface, but there are significant impediments to the process and large potential unknown problems to arise. One aspect of microalgal production that is typically overlooked deals with the nutrient sources used in production. Agricultural fertilizers are made from petroleum products and have caused a great deal of environmental damage, which calls into question the sustainability of biofuel production in general. There are potential solutions to dealing with the issue of nutrient sources, including using nutrient-rich wastewater to grow the microalgae or growing cyanobacteria, which are able to fix nitrogen from the atmosphere.
Another problem that arises in microalgae production is that the algae-culture systems can easily and quickly be contaminated with other organisms (Zittelli et al. 2006). Species of algae other than the target species can be introduced to the production system and compete with the target species. This presents a problem because the overall productivity of the system can be reduced. Other organisms, such as grazers, can also enter the system and reduce the microalgae productivity through herbivory. One way to avoid the issue of contamination is to grow microalgae that flourish under extreme conditions. For example, Arthrospira maxima grow at very high pH (9.5-11), while Tertraselmis grow in extremely saline waters (Dismukes et al. 2008). Another method for avoiding contamination is to grow the microalgae in a closed system, under very controlled conditions. There, however, is no guarantee that contamination will not occur.
The potential benefits of large-scale production of microalgae for biofuels and other products far outweigh the existing and potential issues associated with microalgae production. At the moment, microalgae appear to be the best option for biofuel feedstocks because of their tremendous productivity, ability to use waste products in their production, and the valuable byproducts that can be produced. Research dealing with improving production systems, identifying ideal microalgae for biofuel production, and investigating the sustainability of microalgal biofuel is necessary before large-scale microalgal biofuel operations are established.
Chisti, Yusuf. Biodiesel from microalgae. Biotechnology Advances 25 (2007) 294-306.
Dismukes, G Charles, Damian Carrieri, Nicholas Bennette, Gennady M Ananyey, Matthew C Posewitz.
Aquatic phototrophs: efficient alternatives to land-based crops for biofuels. Current Opinion in Biotechnology 19 (2008) 235-240.
Hirano, Atsushi, Ryohei Ueda, Shin Hirayama, Yasuyuki Ogushi. CO2 Fixation and Ethanol Production
with Microalgal Photosynthesis and Intracellular Anaerobic Fermentation. Energy 22: 2/3 (1997) 137-142.
Kristensen, Erik. Characterization of Biogenic Organic Matter by Stepwise Thermogravimetry (STG).
Biogeochemistry 9: 2 (1990) 135-159.
Martone, Patrick T., José M. Estevez, Fachuang Lu, Katia Ruel, Mark W. Denny, Chris Somerville, John
Ralph. Discovery of Lignin in Seaweed Reveals Convergent Evolution of Cell-Wall Architecture. Current Biology 19 (2009) 169–175.
Meng, Xin, Jianming Yang, Xin Xu, Lei Zhang, Qingjuan Nie, Mo Xian. Biodiesel production from
oleaginous microorganisms. Renewable Energy 34 (2009) 1-5.
Ragauskas, Arthur J., Charlotte K. Williams, Brian H. Davison, George Britovsek, John Cairney, Charles A.
Eckert, William J. Frederick Jr., Jason P. Hallett, David J. Leak, Charles L. Liotta, Jonathan R. Mielenz, Richard Murphy, Richard Templer, Timothy Tschaplinski. The Path Forward for Biofuels and Biomaterials. Science 311 (2006) 484-489.
Rosenberg, Julian N, George A Oyler, Loy Wilkinson, Michael J Betenbaugh. A green light for engineered
algae: redirecting metabolism to fuel a biotechnology revolution. Current Opinion in Biotechnology 19 (2008) 430-436.
Sánchez, Carmen. Lignocellulosic residues: Biodegradation and bioconversion by fungi. Biotechnology
Advances 27 (2009) 185–194.
University of California, San Diego, Scripps Institution of Oceanography. “Biofuel Development Shifting
From Soil To Sea, Specifically To Marine Algae.” ScienceDaily 4 January 2009. 19 March 2009 <http://www.sciencedaily.com¬ /releases/2008/12/081220084424.htm>.
Zittelli, Graziella Chini, Liliana Rodolfi, Natascia Biondi, Mario R. Tredici. Productivity and photosynthetic
efficiency of outdoor cultures of Tetraselmis suecica in annular columns. Aquaculture 261 (2006) 932-943.