|Fig. 1: An algal open pond design. (Source: Wikimedia Commons)|
In 2012, fossil fuels, encompassing coal, natural gas, and oil, constituted 87% of the world's energy consumption and caused 98% of carbon emissions.  While fossil fuels remain an abundant and relatively stable energy source, their future use is constrained by global measures to reduce CO2 emissions and curb global warming. Among current renewable alternatives, algae based biofuels are gaining traction as an effective replacement for fossil fuels.
Algae cells harvest energy directly from sunlight through photosynthesis. Through this process, sunlight, water, and CO2 are efficiently converted into oxygen and algal biomass, which in many species is approximately 60% energy-rich lipids.  Extraction of this energy-dense oil allows for the creation of non-petroleum based, renewable biofuel. The remaining harvested biomass can be easily used as high protein feed for livestock.
Compared to other forms of renewable energy, such as solar and wind power, algae biomass is easily converted to liquid fuel, allowing it to easily replace current petroleum fuels, as demonstrated by the growing promise for the use of algae-based jet fuels.  Compared to alternative biofuel systems, the use of algae presents several advantages. For instance, algae strains boast higher photosynthetic efficiencies than land plants, permitting more efficient elimination of greenhouse gasses.  Powerplants are even beginning to use algae to combat these emissions by directing smokestack gasses directly into ponds of algae, which use the carbon to fuel their own growth. 
In the past, the high price of production has greatly limited algae biofuel's economic feasibility. However, once optimized and expanded to a larger scale, current technology holds the potential to produce 47,000-308,000 liters/hectare of oil from algae annually. This would reduce the price of algae biofuel per barrel to only $20, nearly one-fifth the price of oil.  According to the U.S. Energy Department, replacing petroleum use with algae biofuel would require around 15,000 square miles of land.  Since algae thrive in typically uninhabitable environments, there would be little competition with agriculture crops for fertile land.
Currently, most algae biofuel is produced in small-scale facilities. But in order to become a viable alternative energy source, current technology must reach commercial scales. Several challenges must be overcome before this possibility can become reality. Algae's photosystems harvest more light energy than is needed for their own photosynthetic pathway to outcompete surrounding plants, releasing excess energy as heat. This dramatically hinders the creation of large open ponds (Fig. 1) as sunlight only reaches a depth of a few centimeters.  Consequently, increasing the size of algae ponds would necessitate increasing their surface area rather than their volume and thus require larger areas of land. Expanding the size of open ponds also poses several risks to the production process by increasing the frequency of pond contaminations and making it more difficult to maintain an even and constant environment throughout the pond.  In addition, the waste produced by large facilities could pose hazardous risks to the surrounding community, including human toxicity and loss of natural biodiversity.
Biofuel created from algae presents a promising alternative to fossil fuels, with the potential to help curb harmful carbon emissions. However, current technology is not sufficient to ensure commercial, low-cost production. Although algae-based biofuels are growing in popularity worldwide as an alternative fuel, if current technology fails to advance, oil will likely hold its place as the world's primary energy resource.
© 2015 Alyssa Noll. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
 A. Demirbas and M. F. Demirbas, "Importance of Algae Oil as a Source of Biodiesel," Energ. Convers. Manage. 52, 163 (2011).
 S. Kumar, "Algae Fuels," Physics 240, Stanford University, Fall 2012.
 A. Melis, "Solar Energy Conversion Efficiencies in Photosynthesis: Minimizing the Chlorophyll Antennae to Maximize Efficiency," Plant Sci. 177, 272 (2009).
 E. Hartman, "A Promising Oil Alternative: Algae Energy," Washington Post, 6 Jan 08.
 S. A. Scott et al., "Biodiesel from Algae: Challenges and Prospects," Curr. Opin. Biotech. 21, 277 (2010).