The most common procedure for cultivation of microalgae is autotrophic mode. Microalgae in photoautotrophic nutrition mode use sunlight as the energy source and inor­ganic carbon (CO2) as the carbon source to form biochemical energy through photosynthesis (Huang et al., 2010). This is one of the most prevailing environmental conditions for the usual growth of microalgae (Chen et al., 2011). In photoautotrophic nutritional mode, photosynthet­ically fixed CO2 in the form of glucose serves as a sole energy source for all metabolic activities (Figure 8.6). The simpler form of photosynthate, such as simpler carbohydrates, serves as sole energy source for carrying out the metabolic activities of the algal cells (Chang et al., 2011). These carbohydrates, under nutrient-limiting and stress conditions, will favor the lipid biosynthesis, which also helps to cope — up with the stress (Gouveia and Oliveira, 2009). Lipid productivity greatly depends on the photosynthetic activity in terms of atmospheric CO2 fix­ation and microalgae species. Large variations in lipid productivity, ranging from 5% to 68%, were reported under varying operating conditions and species diversity (Murata and Siegenthaler, 2004; Ohlroggeav and Browseb, 1995; Chen et al., 2011; Mata et al., 2010). A major advantage of the autotrophic nutritional mode is the algal oil production at the expense of atmospheric CO2. Large scale microalgae cultivation systems (such as open/raceway ponds) are usually operated under photoautotrophic conditions (Mata et al., 2010). Autotro­phic nutritional mode also has fewer contamination problems compared with other

Autotrophic Nutrition

Calvin

Cycle

FIGURE 8.6 Autotrophic mode of nutrition in microalgal cells towards CO2 fixation and lipid biosynthesis

nutritional modes. Under autotrophic nutrition, the photosynthates also get consumed dur­ing respiration associated with the biomass growth, and hence the lipid productivity repre­sents the combined effects of oil content and biomass production (Chiu et al., 2008).