Microalgae produce tri — and di-glycerols, phospho- and glycol-lipids, and hydrocar­bons (Chisti, 2007). Although claims regarding yield per acre are often exaggerated, third-generation microalgal biomass could yield 58,700 L biodiesel ha-1y-1, or even 90,000 L ha-1y-1, comparable to 53,200 L ha-1y-1 (Weyer et al., 2010), an order of magnitude greater than the yields from first-generation biofuel crops (Chisti, 2007).

The mode of cultivation of algae is reflected in biomass and lipid yield (Table 13.3). Lipids as a percent of dry cell weight ranged between 1.9 and 75, and Botryococcus braunii yielded the highest percent (Malcata, 2010). Pienkos et al. (2011) summarized lipids (% of DW) in the range of 9.8% in cyanobacteria, 22.7% to 37.8% in diatoms, 25.5% to 45.7% in green microalgae, and 27.1% to 44.6% in other eukaryotic algae. Lipid production in autotrophic algae ranged from 0 to 2500 mg L-1d-1, and the high­est was in Chlorella protothecoides (Chen et al., 2011). Areal production ranged from 0.57 to 38 g m-2d-1, and Dunaliella salina was the most productive (Mata et al., 2010).

Heterotrophy promotes faster growth and lipid accumulation. Compared to pho­totrophic cultures, cultures of Chlorella protothecoides grown heterotrophically had higher values of biomass productivity (1.7 to 7.4 g L-1d-1) and lipid productiv­ity (732.7 to 3,701.1 mg L-1d-1), with lipid as percent dry cell weight ranging from 43% to 57.8% DW. C. protothecoides, when grown under heterotrophic conditions, yielded 55% lipid per cell dry weight (Xu et al., 2006). In mixotrophic cultures of C. protothecoides using glucose/acetate, higher levels of biomass (4.76 ± 1.50 g L-1d-1), biomass productivity (1.59 ± 0.50 g L-1d-1), and lipid productivity (0.25 g L-1d-1) were obtained; but because the cost of the raw materials was unacceptable, glycerol and acetate were used as carbon sources (Heredia-Arroyo et al., 2010). With glyc­erol, the corresponding values were 3.97, 0.93, and 0.19 g L-1d-1 (Heredia-Arroyo et al., 2010). However, in phototrophic cultures of C. protothecoides, corresponding

TABLE 13.3 Summary of Inter-specific and Intra-specific Variations in Microalgal Lipid Criteria Example Min. Max. Remarks3 Ref. Lipid Chlorella protothecoides 43 57.8 Heterotrophic cultivation Chen et al., 2011 (% of cell dry weight) 11 23 Chen et al., 2011 32 species 5 67.8 Chen et al., 2011 44 species 5 63 Mata et al., 2010 Several species 5 63 Chisti, 2007; Khan et al., 2009; Harun et al., 2010; Pitman et al., 2011; Chen et al., 2011 Scenedesmus dimorphus 6 Gouveia and Oliveira, 2009 Botryococcus braunii 25 75 Chisti, 2007 Schizochrtrium sp. 77 Chisti, 2007 Scenedesmus obliquus 11 55 Gouveia and Oliveira, 2009 Chlorella vulgaris 14 55 Gouveia and Oliveira, 2009 Chlorella protothecoides 23 55 Gouveia and Oliveira, 2009 Neochloris oleoabundans 35 65 Gouveia and Oliveira, 2009 Nannochloropsis oculata 31 Log phase Chiu et al., 2009 Nannochloropsis oculata 40 Early stationary phase Nannochloropsis oculata 50 Stationary phase Nannochloropsis sp. 22 60 Rodolphi et al., 2009 Lipid production Chlorella protothecoides 733 3701.1 Heterotrophic cultivation Chen et al., 2011 (mg L-1d-1) Chlorella protothecoides 0 5.4 Chen et al., 2011 32 species 0 178.8 Chen et al., 2011 Chlorella protothecoides 0 1214 Huerlimann et al., 2010 a All values are for autotrophic cultivation unless specified otherwise.

TABLE 13.3 (Continued) Summary of Inter-specific and Intra-specific Variations in Microalgal Lipid Criteria Example Min. Max. Remarks3 Ref. 27 species 0 133 Huerlimann et al., 2011 Several species 3 2500 Chisti, 2007; Khan et al.. 2009; Harun et al„ 2010; Pitman et al., 2011; Chen et al., 2011 Nannochloropsis sp. 30 86.3 Chiu et al., 2009 44 species 10.3 142 Mata et al„ 2010 pg lipid cell-1 4 species 0 29.108 Huerlimann et al., 2010 All values are for autotrophic cultivation unless specified otherwise.

values were 0.002 to 0.02 g L-1d-1 biomass, 0.2 to 5.4 mg L-1d-1 lipid, and 11% to 23% lipid dry cell weight (Chen et al., 2011). Twenty-one other phototrophic spe­cies had a range of biomass production rates from 0.02 to 0.53 g L-1d-1, 0.2 to 178.8 mg L-1d-1 lipid, and 5.1% to 67.8% lipid dry cell weight. Calculation of lipids on a cell basis also varied from 0.068 to 29.11 pg cell-1 (Huerlimann et al., 2010).

The data on lipid variations given by Chisti (2007), Khan et al. (2009), Harun et al. (2010), Basova (2005), Huerlimann et al. (2010), Mata et al. (2010), Malcata

(2010) , and Chen (2011) summarized in Table 13.3 show that wide inter — and intra­specific variations in lipid levels as percent dry cell weight exist. For example, the lowest (6%) was in Scenedesmus dimorphus (Gouveia and Oliveira, 2009), com­pared to 75% in Botryococcus braunii (Chisti, 2007) and 77% in Schizochrtrium sp. (Chisti, 2007). Gouveia and Oliveira (2009) reported a wide range of values within the same species: 11% to 55% in Scenedesmus obliquus, 14% to 56% in Chlorella vulgaris, 23% to 55% in Chlorella protothecoides, and 35% to 65% in Neochloris oleoabundans. Chisti (2007) reported 25% to 75% in Botryococcus braunii. These variations could be attributed to variations in the physiological state of the cells; marked differences in the lipid were noticed in cultures har­vested in logarithmic, late logarithmic, and stationary phases of Nannochloropsis sp., Isochrysis sp., Tetraselmis sp., and Rhodomonas sp. (Huerlimann et al., 2010). Results of Chiu et al. (2009) corroborate that lipids vary with the phase of growth of the alga Nannochloropsis oculata; lipids were 30.8% in log phase cultures, 39.7% in early stationary phase, and 50.4% in stationary phase cells (Chiu et al., 2009). In Nannochloropsis sp., the lipid as percent dry cell weight ranged from

21.6 to 60 (Rodolfi et al., 2008; Chiu et al., 2009), and their production rates correspond to 30 mg L-1d-1 and 86.3 mg L-1d-1, respectively. Sturm and Lamer

(2011) , based on energy evaluation from wastewater algal biomass production, concluded that if the lipid in dry biomass from the field is less than 10%, com­pared to 50% to 60% in laboratory-scale reactors, it would be better to use the biomass as a combustible source of viable energy.