Depending on the primary biofuel target, a broad group of naturally occurring lipids remain in algal spent biomass; these include fats; waxes; sterols; fat-soluble vitamins (e. g., A, D, E and K); mono-, di-, and triacylglycerols; diglycerides, and phospholipids (Williams and Laurens, 2010). Of particular importance are polyunsaturated fatty acids (PUFA); interest has emerged in recent years owing to their potential therapeutic uses in addition to nutritional applications derived from physiological roles in actual cells. PUFAs have been thoroughly studied, especially o3 long-chain PUFA (LC-PUFA), in regard to docosahexaenoic acid (DHA), eicosapentaenoic acid (EPA), and a-linolenic acid (ALA). Their importance to human health has backed up market demand for them (Guedes et al., 2011a).

The predominant PUFA in various marine algae is EPA at concentrations as high as 50% of its total fatty acid content (Murata and Nakazoe, 2001; Dawczynski, Schubert et al., 2007). Marine algae also contain 18:4 n-3, which is hardly found in other organisms; notably, red alga species contain significant quantities of EPA and arachidonic acid (20:4), whereas green algae are unique in their content of 16:4, varying from 4.9% to 23.1% of the total fatty acids, besides 16:0, 18:1, and 18:3 acids. Unsaturated fatty acids predominate in all brown algae and saturated fatty acids in red algae, both groups being balanced sources of n-3 and n-6 acids (Mabeau and Fleurence, 1993; Sanchez-Machado, Lopez-Hermndez et al., 2004).

The main effects of n-3 fatty acids on human health can be classified into three categories: (1) structural components of cell and organelle membranes, (2) significant role in lowering blood lipids, and (3) precursors in mediating biochemical and physiological responses. Human beings have to include ALA, EPA, and/or DHA in their daily diet, especially via inclusion of marine products. However, algae exhibit competitive advantages as sources of PUFAs: Fish (the most common source) have typically lower contents (on a mass basis) and are subjected to seasonal variations in fatty acid profile, besides their being proven to be contaminated by heavy metals (Guil-Guerrero, Navarro-Juoirez et al., 2004). Fur­thermore, they have a limited capacity to synthesize PUFA, so most of them are simply accumulated from their microalgal diet (Guedes et al., 2011a). Algae are indeed a good source of EPA (Plaza, Cifuentes et al., 2008) and an important source of n-3 PUFAs (Murata and Nakazoe, 2001). Besides the well-accepted effect of 18:4 n-3 on the immune system in humans (Ishihara, Murata et al., 1998 ), several other bioactivities have been reported, as tabulated in Table 10.4.

The relative composition of algal lipids depends greatly on the species as well as available nutrients and prevailing environmental conditions during cell culture and harvest. For instance, it has been shown that the composition of algal lipids varies considerably with the growth cycle, under nutrient limitation and a diurnal light/dark cycle (Ekman A, Bulow L et al., 2007; Greenwell, Laurens et al., 2010). Many algal species can be induced to accumulate substantial contents of lipids; although average lipid contents vary between 1% and 70%, some species may reach 90% (w/wDW) (Guedes et al., 2011a). Concerning their extraction, several methods can be applied, but the most common are expeller/oil pressing, liquid-liquid extraction (solvent extraction), supercritical fluid extraction (SFE), and ultrasound techniques, all of which bear advantages and limitations, as discussed elsewhere (Singh and Gu, 2010).