Overcoming biological constraints to enable the exploitation of microalgae for biofuels John G. Day ⇑ , Stephen P. Slocombe, Michele S. Stanley Scottish Association for Marine Science, Scottish Marine Institute, Oban, Argyll PA37 1QA, UK article info Article history: Received 31 January 2011 Received in revised form 9 May 2011 Accepted 13 May 2011 Available online 27 May 2011 Keywords: Algal biofuels CO 2 sequestration Intelligent screening Lipids Photosynthesis abstract Microalgae have significant potential to form the basis of the next biofuel revolution. They have high growth and solar energy conversion rates. Furthermore, their osmotolerance, metabolic diversity and capacity to produce large amounts of lipids have attracted considerable interest. Although there are a handful of commercially successful examples of the photoautotrophic mass-culture of algae, these have focused on the production of higher value products (pigments, health-foods etc.). The technical and com- mercial challenges to develop an economically viable process for biofuels are considerable and it will require much further R&D. In this paper the biological constraints, with a particular focus on strain selec- tion are discussed. Ó 2011 Elsevier Ltd. All rights reserved. 1. Introduction Anthropogenic climate change, population growth and reduc- tion in the remaining easily exploitable oil reserves have been amongst the most potent drivers to find novel energy supplies. Global developments of renewables (wind, wave, tidal and geo- thermal) have massive potential, but transport fuels, specifically for air travel, will require liquid fuels. Although biofuels have the potential to supply transport fuels, first generation plant-crop de- rived fuels are confined to agriculturally productive areas and can depend on freshwater supplies for irrigation. It is clear that fur- ther intensification of existing biofuel crop production could place unsustainable demands on these resources with significant impli- cations for food production. Additional strategies are needed if biofuel production is to be sustainable at large-scale. Microalgae have some clear advantages over ‘‘higher’’ plants, these include: their very high growth rates, their capacity to utilise a large fraction of the solar energy (in theory 10% of the total so- lar energy can be fixed into biomass) and can grow in conditions that are not favourable for terrestrial biomass growth (Carlsson et al., 2007). Future large-scale algal production facilities could also exploit abundant supplies of brackish and salt water by using mar- ine micro-algae. Set against the high oil yields observed with algae are the economic costs for growth, harvesting and extraction of oil, which are high compared with biofuel crops (Brennan and Owende, 2010). Moreover, as with agriculture, nutrient supply in the form of fertilizers or organic waste would still be required for algal culture. To date, the largest algal production facilities have focused on extremophiles, either on the basis of pH; Arthrospira/Spirulina (Benemann, 2003) for dietary supplements, or salinity for b- carotene production using Dunaliella salina (Ben-Amotz, 2004). In the above examples the product value is such that relatively low yields are commercially viable. However, in the case of biofuel a dry weight biomass yield in the region of 100 tonnes/ha/yr would be required for an open pond system to be economically viable at the time of writing (Carlsson et al., 2007). Although this is a major challenge, it is realistic, as yields of 60 tonnes/ha/yr have already been achieved for Pleurochrysis carterae and D. salina in field-scale production systems (Navid and Borowitzk, 2006). The reported oil yield for Pleurochrysis was 21.9 tonnes/ha/year, which compares favourably with the 4–5 tonnes/ha/yr obtained from oil palm, the highest yielding oil crop plant (Navid and Borowitzk, 2006; Sumathi et al., 2008). It is worth noting that this productivity from algae could meet annual US petroleum consumption with a land area of 427,000 square kilometres, close to that of California (US petroleum consumption was 18.8 million barrels per day for 2009, equivalent to approximately 935 million tonnes per year: US Energy Information Administration website, http://www. eia.doe.gov/energyexplained/index.cfm?page=oil_home#tab2). There is clearly a great deal of potential and over the past few years a large number of academics and commercial groups have started to work in the field, virtually creating a new biotechnolog- ical sector. However, if algal biofuels are to compete against 0960-8524/$ - see front matter Ó 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.biortech.2011.05.033 ⇑ Corresponding author. Tel.: +44 1631 559349; fax: +44 1631 559001. E-mail address: jgd@sams.ac.uk (J.G. Day). Bioresource Technology 109 (2012) 245–251 Contents lists available at ScienceDirect Bioresource Technology journal homepage: www.elsevier.com/locate/biortech