(ha) per year. Such reports have spurred investment in intensive research on algal biofuel production. However, such high productivity projections have yet to be obtained in largescale, long-term experiments. Serious barriers remain for reproducing optimal growth and productivity conditions at a commercial scale. They include maintaining the stability of the culture and delivering the required nutrients and other resources in an efficient manner at such scales. Current yields from large-scale operations range from 40-60 tonnes dry weight of algal biomass production per ha per year, and conservative projections anticipate up to 100 tonnes dry weight of biomass, or 30 tonnes of biodiesel per ha per year in subtropical or tropical, sunny climates (Scott et al., 2010). Estimated yields from a variety of cultivation systems are discussed later in the chapter.
2.1.1 Strain Diversity
The choice of strains for biomass production depends on the desired product and technology to be used for fuel production, the source, and the type of cultivation facility (open versus closed). Initial efforts using outdoor ponds focused on production of biodiesel by the transesterification of TAGs to produce fatty-acid methyl esters (FAME).1 Therefore, strains that accumulate TAGs were selected. Five groups of microalgae were classified as high priority for biofuel production by the U.S. Aquatic Species Program (Sheehan et al., 1998): diatoms (Bacillariophyceae), green algae (Chlorophyceae), golden-brown algae (Chrysophyceae), prymnesiophytes or haptophytes (including Prymnesiophyceae), and eustigmatophytes (Eustigmatophyceae). Many strains and genera of eukaryotic microalgae are potential high-oil producers for large-scale culture (Sheehan et al., 1998; Rodolfi et al., 2009). These include species of Tetraselmis, Dunaliella, Chlorococcum, Scenedesmus, and Chlorella, and particularly Neochloris oleoabundans and Botryococcus braunii from Chlorophyta; the genera of Amphora, Amphiprora, Cylindrotheca, and Navicula, and the species of Nitzschia dissipata, Phaeodactylum tricornutum, and Chaetoceros muelleri from Bacillariophyta; the species of Nannochloropsis ocalata and N. salina from Eustigmatophyceae; and the genera of Isochrysis and Pavlova from Haptophyta.
Improvements of technologies that convert total biomass to yield drop-in fuels—such as those being pursued by companies such as Inventure (Inventure, 2012), Xtrudx (Xtrudx Technologies, 2012), and Solvent Rescue Limited (Solvent Rescue Limited, 2012) and academic institutions such as Old Dominion University (Hatcher, 2011)—are changing the scope of organisms that are being considered for biofuel production. All categories of algae are rich in polar lipids that can be recovered by such processes, and they have cellulose or other polysaccharide cell walls composed of sugars. Cyanobacteria store excess carbon as glycogen rather than TAGs, and cyanobacteria and macroalgae accumulate quantities of other complex polysaccharides. These and other macromolecules are all potential carbon sources for producing drop-in fuels if appropriate processing technologies are available. In addition, algal carbohydrate potentially can be a feedstock for fermentative fuel production processes that are based on heterotrophic organisms, such as those used by LS9, Inc. (LS9 Inc., 2011) and Solazyme (Solazyme, 2012). Cyanobacteria are used directly for ethanol production by Algenol (Chance et al., 2011a; Algenol Biofuels, 2012). As of 2012, a number of marine macroalgal species are being considered for biofuel production in India. An example
1 As Chapter 3 discusses, algal triacylglycerols are reacted with methanol to form fatty-acid methyl esters (FAME). Due to its higher viscosity compared to conventional liquid transportation fuels, FAME cannot be used as a drop-in fuel, but can be blended with conventional diesel.