Key parameters determining the economic feasibility of algae biofuels include biomass productivity, lipid content, and lipid productivity. Microalgae produce a variety of lipids, tri — and diglycerides, phospholipids, glycolipids, alkenes, and pigments such as the carotenoids. Reports of total lipid content for specific strains (i. e., compounds soluble in organic solvents per dry weight, as originally described by Bligh and Dyer 1959) vary in the literature (Griffiths and Harrison 2009). This is due, in part, to variations in the sequence and polarity of solvent systems used for extraction (Guckert et al. 1988). Because of the complexity of lipid compounds in algae and that the fractions of each class can vary with environmental conditions, lipid quantification, which is essential to the development of production models for algae biofuels, needs refinement. The biodiesel industry is currently based on transesterification of plant triglycerides forming alkyl esters of the fatty acid moiety. The fate of other cellular lipid compounds in the tranesterification process, potentially a large fraction of lipoidal extracts, will require more attention.

Perhaps a more important consideration of reported variations in lipid content, even within a specific species, is the physiological responses in lipid metabolism due to culture conditions including temperature, salinity, growth phase, nutrient deprivation, and the diurnal light cycle, all of which have a strong influence on lipid content (Roessler 1988, 1990). Unfortunately, biomass productivity is often inversely correlated with overall lipid productivity. High lipid and carotenoid content is usually produced under stress conditions, especially nutrient limitation which prevents cell growth and division resulting in excess photosynthate shunted toward triglyceride accumulation (Griffiths and Harrison 2009; Illman et al. 2000; Jakobsen et al. 2008; Lv et al. 2010; Rodolfi et al. 2009).

Lipid productivity is the product of lipid content and productivity. A survey of the literature on growth rates and lipid content under nutrient-replete and nutrient — deficient conditions showed a stronger correlation between biomass and lipid productivity rather than simply lipid content (Griffiths and Harrison 2009). In continuous ponding operations, selection of fast-growing strains increases yield and decreases the cost of harvesting and extraction (Borowitzka 1997) and reduces competition by invading strains. High productivity is also advantageous in a two — stage process, as described above, with the first stage designed to optimize biomass production and nutrient removal from wastewater followed by a second phase to induce hyper-lipid production.