The type of light source is known to be a critical factor affecting the growth of microalgae due mainly to the difference in the coverage of wavelength range (Terry, 1986). In addition to the type of light source, the light intensity is also very important for microalgae growth (Grobbelaar et al., 1996; Sanchez et al., 2008; Ugwu et al., 2008; Yoon et al., 2008). In general, the effect of light intensity on the photoautotrophic growth of microalgae could be classified into several phases, such as the light-limitation phase, the light-saturation phase, and the light- inhibition phase (Ogbonna and Tanaka, 2000). To maximize biomass productivity, the satura­tion light intensity needs to be distributed throughout the entire microalgae cultivation system. However, this is impossible in practical cultivation systems, since the light distribution inside the photobioreactor normally decreases significantly along with the distance due to the light shading effects (see Figure 2.1), especially when the cell concentration gets very high or when

FIGURE 2.1 Effect of light intensity on specific growth rate of microalgae under phototrophic cultivation (Ogbonna and Tanaka, 2000).

significant biofilm formation on the surface of the reactor vessel occurs (Chen et al., 2008). Im­proving the mixing of the cells can reduce the effects of light shading or photoinhibition at dif­ferent zones of the photobioreactor. Some literature describes the effect of light intensity on the lipid content of microalgae. Lv et al. (2010) demonstrated that in comparing low and high light intensity (i. e., light-limitation and light-saturation conditions), using a light intensity of 60 gmol/m2/s led to an increase in biomass concentration and lipid content of Chlorella vulgaris, along with changes in pH, NADPH, and Mg2+ concentration (Lv et al., 2010).