D. Ryan Georgianna1 & Stephen P Mayfield1 .
Exploiting diversity and synthetic biology for the production of algal biofuels
Modern life is intimately linked to the availability of fossil fuels, which continue to meet the world’s growing energy needs even though their use drives climate change, exhausts finite reserves and contributes to global political strife. Biofuels made from renewable resources could be a more sustainable alternative, particularly if sourced from organisms, such as algae, that can be farmed without using valuable arable land. Strain development and process engineering are needed to make algal biofuels practical and economically viable.
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Microalgae accumulate oil as nonpolar storage lipids, such as triacylglycerides (TAGs)1. The photosynthetic and cellular membranes of algae also contain polar lipids, such as glycolipids, phospholipids and sterols. Oils from algae can yield biodiesel through transesterification2, and gasoline (petrol) or jet fuel through distillation and cracking3. Biofuel can be produced from various sources, but yield estimates are significantly higher for algae than for any other crop. This has considerable implications for land-area requirements: algae cultivated on only 30 million hectares and yielding biofuel at a conservative estimate of 40,000 litres per hectare per year is sufficient to replace the 1,200 billion litres of petroleum used by the world’s largest consumer of petroleum, the United States (Fig. 1). The area is similar to that used for soya planting in the United States (about 29 million hectares) and roughly twice that used for the US production of corn ethanol (about 14 million hectares was used to produce almost 64 billion litres of ethanol in 2011), or an area about the size of New Mexico4–6. Furthermore, algal cultivation can use the large amount of non-arable land available for development without displacing food production, and its relatively high demand for water can be met by using low-quality sources such as waste or salt water7. Algae are almost ideal as organisms for developing the highly productive and robust crop strains that are essential for economically viable biofuel production. The search for these strains can exploit the vast diversity of algae, ranging from giant multicellular kelps to single-celled microalgae. Microalgae have been the focus of intense biofuel production efforts because of the ease, scale and speed at which they can be
The underlying technologies and optimization strategies for producing biofuel through agricultural and industrial commercialization are very different. In agricultural production, microalgae are cultivated in open ponds, with sunlight driving photosynthetic growth. This method uses non-arable land, consumes large amounts of CO2 during the biomass production phase and, in principle, is extraordinarily scalable — limited only by space and capital costs. But growing algae efficiently and sustainably in fully exposed outdoor ponds is difficult, and suitable cultivation systems and practices are still under development. Industrial production uses similar processes to industrial microbial fermentation in which yeast or bacteria are used as biorefining agents to produce food, beverages or high-value biotechnology products. For algae, production is driven by sunlight (in a photobioreactor) or reduced carbon sources such as sugar (in a fermentation reactor). Fermentation occurs in complete darkness and is identical to that of bacteria or yeast, which, as mature industrial technologies, have many of the systems and processes needed for algal fermentation already in place. However, fermentation has,...