1.3.1 Carbon Sources

Carbon sources are usually the most critical factors for the growth of microalgae. In gen­eral, microalgae can be grown under photoautotrophic, heterotrophic, and mixotrophic con­ditions using diversified carbon sources, such as carbon dioxide, methanol, acetate, glucose, or other organic compounds (Xu et al., 2006). Photoautotrophic cultivation means that microalgae use inorganic carbon (e. g., carbon dioxide or bicarbonates) as the carbon source to form chemical energy through photosynthesis (Ren et al., 2010). Some microalgae species can directly use organic carbon as the carbon source in the presence or absence of a light sup­ply. This is called heterotrophic cultivation (Chojnacka and Noworyta, 2004). However, the most commonly used carbon source for microalgae growth and biofuels production is still carbon dioxide or bicarbonates, since using organic carbon sources would be too expensive for producing low-price products such as biofuels.

In addition, from the aspect of CO2 emissions reduction, a net-zero CO2 emission could be achieved when the biofuels are directly converted from using CO2 as the substrate. In partic­ular, photoautotrophic growth of microalgae represents an ideal model of reutilization of CO2 coming from flue gas of power plants and industrial activities (Packer, 2009), as microalgae biomass can be further utilized to produce biofuels or other value-added products (Hsueh et al., 2007; Raoof et al., 2006). In addition, most microalgae have much higher cell growth and CO2 fixation rates than terrestrial plants (around 10-50 times higher), which demon­strates another advantage of direct conversion of photoautotrophic growth of microalgae.

Therefore, it seems more reasonable from the perspectives of economic feasibility and environmental protection that microalgae-based biofuels should be produced via photoautotro­phic growth of microalgae. However, another thought is to produce biofuels from microalgae grown under heterotrophic conditions using organic carbon sources (e. g., sugars) derived from biomass. In this way, biofuel productivity could be markedly enhanced, since heterotrophic growth of microalgae is usually faster than autotrophic growth (Chen, 1996). Nevertheless, again, the high cost of obtaining the organic carbon sources from raw biomass is still a great concern.